10.25.2010

Roof and Attic Ventilation

Roof Ventilation Update

The construction industry's leading researcher explains why what we think is true often isn't, and how some of our best hunches, based on observation of field performance, have paid off with problem-free attic assemblies

by William B. Rose

I've gotten many calls over the years about attics and attic ventilation; almost invariably the caller is confused, having heard different things from different people. In this article, I'll discuss the performance of attic assemblies and try to shed light on why there are so many points of view about roof ventilation.

The temperature of a northern-climate roof we monitored throughout the 1990s is shown below (Figure 1). Here is a summary of the study: The roof gets cold at night and is hot during the day. It gets hotter on a sunny day than on a cloudy day. Attic assemblies with openings to the outdoors ("vented" attics) stay a bit cooler during the daytime than unvented assemblies. They also stay slightly warmer at night.

Figure 1. Sheathing temperatures are affected somewhat by roof ventilation, but many other factors play a bigger role.

Many factors influence the temperature on the roof. A prioritized list might include hour of day, outdoor air temperature, cloud cover, color of the roof, roof orientation, where the measurement is taken (sheathing or shingles, top or bottom), latitude, wind speed, rain or snow on the roof, heat conduction across attic insulation, roof framing type (truss or cathedral), and attic ventilation to the outdoors. As you can see, ventilation falls pretty far down the list.

To better understand how wind affects roof ventilation, Canadian researchers T.W. Forest and I.S. Walker measured the air exchange rate in attic assemblies using tracer gases. The graph below (Figure 2) gives us a feel for what they found. That is, air-change rates in the attic tended to increase with wind speed, but the amount of air change at a given wind speed was unpredictable. In fact, even with specific information about climate, construction type, and wind speed and direction, the resulting air-change rates may vary by a factor of 10 or more. Whether air flows out through a roof opening or in through that opening, and whether this airflow induces flow from indoors into the attic or helps dilute and remove moist air from the attic, can never be pinned down very well, except to say that wind is a more powerful factor than buoyancy (the "stack effect").

Figure 2. While higher wind speeds tend to increase attic ventilation, the relationship is a weak one: Ventilation rates at a given wind speed can vary by a factor of 10.

For the most part, roof assemblies behave like any wood structure — they are wetter when cold and drier when warm. Roof assemblies tend to be hot, thanks to the sun, so they tend to be dry. Of course, if the roof leaks, that becomes the biggest source of wetness. High moisture levels indoors or in basements or crawlspaces can also increase moisture levels in the roof. Roof members can become particularly wet or covered with frost near holes in the ceiling or leaks in attic ductwork, where humid air enters the attic. It was the formation of local frost "walnuts" like those shown on the next page (Figure 3) that led researchers in the late 1930s to recommend attic ventilation. (If only they had offered to seal up the ceiling instead!)

Figure 3. Moist interior air leaking through a hole in the ceiling can produce moldy sheathing or frost on a roof truss. This photo by the author shows results from the Attic Performance Project.

Many attic assemblies are built with vents to the outdoors on the presumption that outdoor air will enter the attic and dilute moisture coming from indoors or from the foundation. The further presumption is that indoor air is wet and outdoor air is dry. Both of these assumptions are often false. If there are openings in the ceiling, then air movement in the attic can induce airflow from below, or dilute air from below, or do nothing, in ways that are just plain unpredictable no matter how much research is done. Attic air movement can also induce flow into the living space below, which is a nasty problem when the air conditioning is running.

Suppose that the picture of attic ventilation provided by physics, described above, doesn't quite cut it. Too many qualifications; nothing pinned down. Then we can go to our own observations and experiences, subjective and incomplete as they may be. Here's my main finding: Attic assemblies built over the last 15 years or so are pretty good. They may be a crapshoot in building-physics terms, but the crapshoot is heavily biased toward good performance.

Let's look at attic assemblies by component:

Truss construction seems to do quite well. There are disasters that occur during construction. Truss uplift continues to be a problem requiring cosmetic fixes. The industry has, for the most part, discontinued the use of fire-retardant treatment of truss members, thereby avoiding what was a serious concern for several years. The truss heels in many cases still fail to provide the height necessary for good insulation. Attics have become a forest of truss webs, and thus are less usable for attic storage space. But the overall picture is good (at least by my observations).

Gypsum wallboard ceilings have shown improvement. The message seems to have gotten out that ceilings must be airtight — there is no justification, summer or winter, for allowing indoor air or foundation air to pass into attic cavities. The common culprits, such as framed soffits over kitchen cabinets, open oversized plumbing or mechanical chases, and leaky can lights, are going away in most construction where the word has gotten out. Weatherization of existing buildings has kept the focus on closing off any ceiling bypasses. In my experience, most truss-framed attics do fine without special vapor-barrier membranes in the ceiling, but in cold locations, cathedral ceilings may need vapor protection just as walls do.

Insulation. Regarding insulation, most areas of the country have healthy amounts in the attic — R-30 in general and R-38 in northern areas. Cellulose provides good insulation and helps block airflow. Fiberglass, in sufficient density and with good installation, also provides good thermal insulation. Foam insulation is being used more commonly, and has become the material of choice for residential air-sealing. Structural insulated panels (SIPs) work fine, as long as the airflow problem at joints is addressed. Foam insulation has been sprayed on the underside of board and wood-panel sheathing with great success. Insulated panels (often polyisocyanurate) make for good roof-deck assemblies, as we know from commercial low-slope construction, where the foam insulation is often sandwiched between the structural roof deck and the roofing membrane. (All foam needs fire protection, of course.) Open-cell foams such as Icynene may need more vapor protection than closed-cell foams, which have greater resistance to vapor flow.

Vapor barriers still cause squabbling, but most builders know that moisture flow from below comes mostly through holes in the ceiling. Cathedral ceilings require special care in insulation placement and vapor protection. But the new code provisions should encourage insulated sheathing materials or insulated "sandwich" assemblies that resist moisture transport and heat flow as a package. With these roof assemblies (I call them "insulated vapor retarders" or "fat vapor retarders"), the inside surface stays close to indoor conditions, the outside surface stays close to outdoor conditions, and nothing bad happens in the middle. Our laboratory has had such an assembly in place for more than 15 years, with one inch of foil-faced polyisocyanurate insulation directly beneath the OSB decking; the sheathing gets hot during the day, but the OSB above the foam insulation is the driest sheathing of all. Remember: Hot means dry.

Ductwork in unconditioned attic assemblies is not ideal. It's best to place all ductwork in conditioned spaces.

OSB has become the universal sheathing material, by economic and environmental necessity. But we still know too little about the moisture performance of this material, such as under what conditions it will begin to fail. In my laboratory, we have seen the material swell by 50 percent or more under extreme conditions. Will it begin to show signs of sagging between trusses, or will workers be putting their feet through it at the time of reroofing? I don't know, but the absence of signs of product failure in the field — at least to my drive-by observations — is reassuring. Nevertheless, I look forward to the day when the marketplace provides a product with more clearly established performance characteristics. I'll be a strong supporter.

Shingles. I'm reviewing the condition of the shingles installed on our research laboratory in 1989; after 18 years, signs of aging are appearing. We hope to conduct laboratory tests to pin down and better quantify the shingle performance and the factors that influence it. The aging we see shows some temperature effect: The white shingles are in better shape than the dark, and a few of the most aged-looking shingles are found on the hottest bay, the one with foam directly on the underside of the sheathing. Without the numbers to go by, we must rely on observation, and our observations suggest that performance depends on other factors besides the presence or absence of ventilation and whether the assembly is truss-framed or cathedral ceiling.

Of course, natural weathering tests that began 18 years ago say little about shingles that are made today. I sense that the shingle industry is currently producing dimension shingles that seem to lie quite flat, resist wind uplift, and hang on to their UV-protecting granules. I don't know how to reroof over dimension shingles, and it does seem unfriendly to the landfill to have that much more mass in the shingle. Nevertheless, my drive-by observations show a lot of good-performing shingles going on roofs over the last couple of decades, and that is very reassuring.

Roof vents. Many years ago, we measured the "net free area" of about a dozen ridge-vent materials. (We used an apparatus that measures the pressure drop across a vent device with great accuracy.) We found that ridge vents with large openings (minimum opening dimension around 1/4 inch) had an equivalent net free area very close to their rated capacity. Vent devices with small openings — or with filter fabrics, or scrims — performed much worse, as much as 75 percent less than their rated area. (If you want to know how restrictive a vent device is, use your imagination — if it looks like air would have a hard time moving through, it probably does.)

This discrepancy would be a big deal, I suppose, for someone who felt that vent regulations were critical to attic performance. I don't, so for me, having vent devices with less airflow than advertised is not a cause for concern.

You — and your building code inspectors — may be unaware that the 2006 version of the IRC for one- and two-family dwellings permits attic construction with no ventilation of the attic cavity. This new provision, R806.4, is largely due to the efforts of Joseph Lstiburek, Armin Rudd, and their colleagues. In brief, unvented conditioned attic assemblies are permitted when an air-impermeable insulation such as rigid foam is applied in direct contact to the underside/interior of the structural roof deck, with sufficient thickness — given the climate — to prevent condensation on the underside (see "Insulating Unvented Attics With Spray Foam," 3/07).

This new provision is a direct challenge to the rule of thumb that has been in place for 50 years, which says that you have to vent a steep-roof attic so the ratio of net free vent area to the projected roof area is 1-to-300 (or 1-to-150 when using "cross ventilation" rather than soffit and ridge vents). This ratio arose from observations of frost on protruding nail points in Wisconsin homes by researchers at the Forest Products Laboratory in 1937, and frost on aluminum plates in research "doghouses" at the University of Minnesota in 1938, under "outdoor" conditions of -13°F.

The Federal Housing Authority turned these findings into the famous 1-300 ratio in 1942, to be applied as a minimum building requirement for the small homes in its financing program. The requirements were picked up by model codes and others following World War II, and the rest, as they say, is history. Shingle manufacturers did not begin piggybacking their warranties on venting regulations until reports of shingle problems began piling up following the change in asphalt sources in the early 1980s.

Every designer and builder should be able to produce good attic and roof assemblies, both with and without ventilation — or anything in between — with just part of a conventional ventilation system. For example, from our studies, roof assemblies that have holes but not necessarily straight airflow paths (one gable end vent, or soffit-only) should also be candidates for good performance. And although unvented roof assemblies can perform well, there are still good reasons to vent: The truss-framed, steep-roof attic with an insulated ceiling has been the workhorse of single-family construction, and ventilation works well with this construction, at least in the northern United States.

In some cases, there are also good reasons not to vent: in wildfire areas, in complex cathedral ceiling assemblies, in existing and historic buildings that have never had ventilation, in shed roofs beneath clerestory windows, with foam insulation (foam and ventilation do not go together — think fire), and in complex roof assemblies that combine steep and low-slope construction. I've also heard persuasive arguments against venting in hurricane-prone regions, but I'm not an expert in that area. In short, since critical performance doesn't hinge on ventilation, then either vent, no-vent, or an in-between "kinda"-vent can be taken as the starting point. Whether the choice works or not depends mostly on other factors.

So you should vent where venting is appropriate and not vent where it is not appropriate. As it turns out, the worst-performing, most mold-ridden attics I have seen were vented — with a flooded crawlspace and a direct path for air movement from the crawlspace to the attic. You can mess up a vented attic by allowing such airflow. You can mess up an unvented attic as well, usually by not providing vapor protection appropriate to the climate and indoor moisture levels. Tight ceilings would be a great first step toward moisture control, summer and winter.

The father of a colleague of mine says that when the word "ventilation" comes out, people stop using their heads. Vented assemblies often perform well, but not always. Sometimes roofs appear to be vented but actually aren't. Still, we can take comfort in the observation, based on years of experience, that our attic assemblies are pretty darn good, and — in my opinion — they're getting better. We need to constantly be on the lookout for new conditions and new problems, as they crop up.

Those of you working in the trenches should continue to build in a way that complies with code and that you know works for your climate. For more information about ice damming, summer cooling load, shingle service life, and moisture issues, visit www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf (TenWolde and Rose, "Issues Related to Venting of Attics and Cathedral Ceilings"). For all four of these concerns, ventilation makes a contribution that is generally more positive than negative, but it hardly ever makes the difference between success and failure.

For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. The major causes of moisture problems in attics and roofs are holes in the ceiling and paths for unwanted airflow from basements and crawlspaces. People should focus first on preventing air and moisture from leaking into the attic. Once this is accomplished, roof ventilation becomes pretty much a nonissue.

William B. Rose is a research architect with the Building Research Council at the University of Illinois at Urbana-Champaign, and the author of Water in Buildings: An Architect's Guide to Moisture and Mold. This article was adapted from The JLC Guide to Moisture Control.

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Scott's Contracting
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Moisture Content and Wood Movement

Understanding Moisture Content and Wood Movement

by Carl Hagstrom on September 3, 2010

(with Gene Wengert, The Wood Doctor.)

Expected movement can be accurately predicted, which means avoiding potential problems down the road.

In this article, we'll explain the importance of understanding wood movement, how to use a moisture meter to measure the moisture content (MC) of trim, how to decide when a load of trim should be rejected, and how to accurately estimate how much trim will move after it's installed.

Most finish carpenters are aware that seasonal changes in humidity cause trim and flooring to shrink in the winter and expand in the summer. But few realize that the expected movement can be accurately predicted and potential problems avoided. It's our premise that with a moisture meter and an understanding of wood movement, most wood movement problems can be avoided. Plus, with this data, finish carpenters can accurately predict how trim and flooring will behave after it's installed.

Wood Movement — You Can't Stop It

Wood is hygroscopic, which means its MC will fluctuate based on the relative humidity (RH) of the surrounding air. As humidity increases, the MC increases, and the wood expands, and as the humidity decreases, MC decreases, and the wood shrinks. This relationship is referred to as Equilibrium Moisture Content (EMC), and can be accurately predicted.

Understanding Equilibrium Moisture Content

The moisture content of wood is tied directly to the relative humidity of the surrounding air. The higher the relative humidity, the higher the MC of the wood. Period. If you're installing wood that's recently been transported, or installed on a job, it might take a little while for the material to reach its equilibrium moisture content (EMC) with the air—in other words, for the wood to accommodate to the humidity level for the climate around the wood: the wood may take on more moisture or it may dry out. For example, if wood at 10% MC is exposed to 25% RH, the wood will dry to 5% MC (and shrink as it dries).

The EMC helps us understand the response wood will have to relative humidity, whether it will shrink or expand. For woodworkers and carpenters, the EMC is more helpful than RH. The simplified chart to the right shows the EMC values of wood when stored at the humidity and temperatures indicated.

Complete EMC levels for wood stored in unheated structures in your area of the country can be found HERE.

How Wood Moves

If the MC of the wood you install is too high, excessive shrinkage may occur, along with the risk of problems of unacceptable gaps and cracks in the wood itself. When the MC is too low, the wood may expand, and may buckle, bow, and distort surrounding material.

There are six key areas finish carpenters should be aware of when it comes to wood movement.

1. Width of material

The wider the board, the more movement will occur (the term "board" technically refers to wood 1 1/2 in. thick or less, but for this article its use will refer to wood typically used by finish carpenters). It's a direct proportion: an 8-in. board will move twice as much as a 4-in. board, and a 12-in. board will move 3 times the amount as a 4-in. board. And it's important to keep in mind that a glued-up panel behaves basically as one wide piece of lumber.

2. Grain orientation matters

Boards are characterized as being either "flat sawn" or "quarter sawn." Quarter sawn lumber (also referred to as "rift sawn" or "vertical grain") shrinks and expands roughly half as much as flat sawn. Most over-the-counter finish material is flat sawn, and you should assume flat sawn values unless you're sure your material is quarter sawn. Quarter sawn lumber has annular rings that are oriented between 45 and 90 degrees to the board's face. Flat sawn grain orientation falls between 0 and 45 degrees to the board's face.

Wood Grain (Note: Click any image to enlarge. Hit "back" button to return to article")

3. Moisture content of the wood at delivery

The only way to accurately predict wood movement is to know the MC of the material when you receive it. Moisture content is measured using a moisture meter. Failure to check your delivered material means you have no chance of anticipating movement problems. Furthermore, material that measures outside of the acceptable MC level should be rejected.

4. Humidity inside and outside the structure

Homes in most of the U.S. that lack humidity control typically experience interior levels of humidity from 25% RH to 65% RH. This range of humidity will cause a 6% change in the MC of the wood. This change in MC will cause a 12-in. wide maple board to change 1/4 in.

When material is installed that was delivered at an unacceptable MC, or the humidity range in the structure exceeds typical values, the amount of wood movement increases—and can cause problems even in well-designed trim details. It's worth noting that panel material (plywood, MDF, composite materials) move at about 1/10th the rate of solid wood.

In most of North America, exterior humidity levels range from 60% RH to 70% RH in summer and winter, but are lower in the Southwest, and higher near large bodies of water. If the material is delivered at 6 to 8% MC, it can experience more than a 2% change in size as it adjusts to the EMC.

5. Species affects the amount of movement

Wood movement depends in part on the species. A 12-in. wide western red cedar board will fluctuate 1/8 in. while the same size maple board will fluctuate 1/4 in. The formula for calculating wood movement is complex and extremely accurate, but tedious.

One simple rule of thumb serves as an approximate guide to predicting wood movement: "Most species of flat grain material will change size 1% for every 4% change in MC." Applying this formula to a situation where the seasonal EMC ranges from 6% to 10%, a 12-in. wide board will change dimension 1/8 in.

I've put together a rough chart (see below, click to enlarge) that offers approximate movement values for various widths and commonly used species of wood. These values are based on flat sawn lumber, and offer a general idea of anticipated annual in-service movement. The movement values for quarter sawn lumber are approximately 1/2 the flat sawn values.

If you want to know exactly how much the material you're using is going to shrink or expand, use this online shrinkage calculator. Simply enter the high and low MC values and the width and species of the board.

6. Applied finish does not stop movement

While it's true a high quality finish will slow the rate of moisture exchange, it will not stop it. Material finished on all surfaces will expand or contract at a slower rate than raw wood, but make no mistake—finished wood will eventually acclimate to EMC levels.

Events That Increase Movement Risks

There are many events that can contribute to excessive wood movement issues. Nearly all of them can be prevented before they cause a problem if—and only if—you measure the MC of the wood as soon as it's delivered, and avoid using wood that is too wet or too dry for the expected in-use EMC. The moment the wood is delivered, it begins to acclimate to the surrounding environment. At the very least, it's important that you document the delivered MC, just in case wood movement becomes an issue. But responsible carpentry can't be accomplished without reading the delivered moisture content of the wood and planning for wood movement during and after acclimation.

Excessive MC in delivered material

Optimum MC for interior millwork is 6-8%. In the real world, your material may arrive around 9-10%. For installations in unheated areas, the preferred readings are in the 12-14% range, assuming an area is protected from the weather. In most cases, you can deal with material that's a couple of points high, but keep in mind that the wider the stock, the greater the movement. Ideally, the moisture content of wood should not change more than 2% when put into use.

Think through your trim details and consider how they will react when the wider assemblies shrink. With wide glued-up material, slightly higher MC levels may not be acceptable. If you'll be installing wide material, it's a good idea to be upfront with your supplier and let them know that the material's MC must be within the range you specify. As a last resort, you may choose to dry the wood in your shop if the shop's EMC is low, and have any shrinkage problems show up before the wood is installed.

Delivered material that's too dry

This is seldom an issue for interior trim, but can be a real issue for exterior trim. Material delivered at 6% MC, and installed outside, will acclimate at 12% in the more humid months, resulting in a 6 point MC change. This swelling of the material can cause significant problems in situations where installation creates accumulated movement (more on this below).

Long-term storage of trim material

If you plan on storing trim material for any length of time in an unheated area, keep in mind that, in most parts of the US, the material will acclimate to roughly 11-12% MC. (See the humidity moisture content chart at the beginning of this article.)

If MC is too high, lower readings can be achieved by moving the material into a heated area. The amount will depend on the temperature and humidity of the storage area. The change in MC won't happen immediately, and the material in the center of a pile will change at a slower rate than the material at the edges. Spacing the material so all surfaces are exposed to the air helps, as does good air circulation throughout the pile. You'll need to take sample readings with your moisture meter to determine when the material reaches your intended MC.

Higher temperatures result in a more rapid change in MC when the humidity remains constant (roughly speaking, moisture moves twice as quickly for every increase in temperature of 20 degrees). And despite what you may think, moisture gain or loss does not stop when temperatures fall below freezing. The moisture in wood is chemically bound in the walls of the wood cells and cannot freeze.

Typical on-site humidity

At certain points during construction, such as when pouring concrete, plastering or drywalling, tremendous amounts of moisture are often added to the air, causing humidity spikes as high as 80-85% RH. If you are storing finish material on-site during these periods, be sure to keep them wrapped in a vapor impermeable material (like plastic) with as few gaps as possible. Wood stored in this manner will not pick up any appreciable moisture.

Interior trim should not be installed until the temporary construction humidity has subsided. Use an accurate digital hygrometer to measure RH (under $40). Generally speaking, interior trim should not be installed when the humidity is above 60%, or the material may climb above acceptable MC levels.

Humidity in un-heated areas fluctuates about 10%; therefore dry material (6% to 8% MC) installed in un-heated areas will swell significantly. It's important that the MC of exterior trim be within 2-3 points of the EMC values for the area before it is installed.

In-service low humidity issues

In heating climates, older, drafty homes may see humidity drop, measuring 20% RH in the winter. The EMC in this environment will vary nearly 8% wintertime to summertime. Homes with wood stoves and no humidity control can see EMC swings of up to 11%. In extreme environments, consider using cabinet grade plywood for wide panel application instead of solid wood.

In-service high humidity issues

Typically, high humidity (constant levels above 60%) is not an issue. But if you find yourself working on a project that includes a room with a spa, heated pool, or damp crawl space, proceed with serious caution—85% RH means an 18% EMC. A 12-in. wide piece of birch installed at 8% MC in such a room will swell in width over 3/8 in. Letting your material acclimate to the high MC levels before installing is one approach, but keep in mind that if there is ever a period where the pool is drained for a significant time, and the humidity drops to typical levels, the trim material will experience severe shrinkage. A carefully-worded disclaimer regarding wood movement would seem to be in order.

Understanding Accumulated Wood Movement

Glued-up solid wood panels behave as though they were one wide board—a 24-in. wide panel will shrink and swell four times as much as a 6-in. board. But what about a series of boards installed side by side (T&G flooring, for example)? While it's true that each board can move independently, accumulated movement can cause significant problems, typically when the newly installed material gains moisture. (See photo, right)

If the material in non-glued assemblies (flooring, for example) is installed "tight", and there's no gap to absorb expansion as the material gains moisture, the increase in width of each floor board becomes cumulative, and causes the entire floor to "grow" buy the sum of each piece's individual movement. In cases of excessive shrinkage, unacceptable gaps can result between each floorboard.

For example, random width oak flooring is delivered at 8% MC. The width of the room is 12 feet, and the floor acclimates to a high level of 11% MC, the cumulative movement is about 1 3/8 in. In the real world, a lot of this expansion is "lost" as the fit tightens up, but in some cases the wood fibers compress, and fiber compression can cause grain ridges. By using a moisture meter, and predicting the movement, you can decide whether you should install the material "tight" or "loose" to absorb what you know will be an increase in material width.


Moisture content on exterior trim can range from 12% to 16% depending on the region, time of year, and location of the material. (Click images to enlarge)

Common Movement Issues

Paneled Passage Doors

Experienced door hangers know that a paneled passage door with a tight reveal will shrink in the winter and possibly stick in the summer. (Remember, if you live in California, the winters may be more humid than the summers!). But basing your door gap on the time of year you hang the door can be a mistake if you don't know the MC of the door.

The seasonal width changes of a door are controlled by the MC change in the door's stiles.

. . . . . . . . . .

If that fir door you're getting ready to hang in the winter has been stored for six months in an unheated building, the moisture of the 5-in. stiles may easily measure 12-13% MC. After that door is hung, the MC of those stiles will drop to 6%, and the door can easily shrink 3/16 in. Knowing the MC at the time of installation provides the needed guidance.

And keep in mind that the door panels in this example will shrink significantly after installation. This won't affect the fit of the door, but if the door finish is applied at the MC noted, there will likely be unfinished wood exposed as the door panels shrink to their in-service width. (See photo, left) This is particularly noticeable when a light wood is stained dark.

By measuring the MC of the door stiles, you can base your door gap on established movement values, not guesswork, and avoid callbacks when the fit becomes a problem.

Doors with horizontal battens

Unless you're setup to build these doors properly, avoid them. The typical horizontal batten door is built using T&G material for the door face, and then battens are fastened to the back of the door to hold things in place. As the seasonal MC of the T&G material rises and falls, the boards expand and contract, but the battens—with their grain running in the opposite direction—resist that movement, forcing the door to cup inward or outward depending on the direction of the movement.

The detail below is one method used for cabinet batten doors that successfully allows for seasonal wood movement.

Captive panels

Resist the temptation to "picture frame" a solid wood panel—the way some woodworkers new to the craft miter a nosing or a frame around a tabletop. The miter joint will always fail when the panel expands and contracts. Instead, use a breadboard nosing design so that the wide panel can shrink or swell without destroying the surrounding joinery. (See below)

Inside corner trim

When installing trim that covers an inside corner, fasten the trim through the corner and into the substrate so the adjoining finish material can move independently as its MC changes. A typical example is base shoe molding. The best practice is to nail base shoe to the plate, with a long nail that doesn't penetrate the baseboard or the flooring. But that's not practical on most jobs.

The second choice is to fasten the baseshoe to the baseboard. Yes, the baseboard will lift off the floor in the heating season, but rarely more than 1/16 in. A wide floor, on the other hand, moves more than a 6-in. piece of baseboard; if you nail the base shoe to the floor, the base shoe may separate significantly from the baseboard.

. . . . . . . . . .

Common Myths

Wood is stable at below freezing temperatures.

The moisture in wood is chemically bound in the walls of the wood cells and cannot freeze, and expansion and contraction continues at below freezing temperatures. Wood does acclimate more slowly at lower temperatures.

Wood will expand on warmer days and contract on colder days.

For all practical purposes, thermal expansion and contraction of wood is not an issue. That said, warmer temperatures speed the exchange of moisture within the wood. Moisture exchange will happen more rapidly at warmer temperatures, but there is no thermal movement of wood worth measuring.

It doesn't matter if lumber is kiln-dried.

Kiln-dried hardwood lumber typically leaves the kiln near 6% MC (softwoods at 10-12%). But all kiln-dried material will acclimate to the surrounding EMC levels. The significant advantages of kiln-dried material is that it is typically heated to at least 130 degrees in the kiln, which will stop any insect activity, and also "set" the sap in resinous softwoods (sap in resinous air dried material can bleed from the board after it's installed, especially when interior temperatures rise in the summer).

They don't make wood like they used to.

It's true that most of the old growth timber is gone, but properly dried vertical grain material has highly desirable movement characteristics. If you're seeking material that will move the least, choose one of the more stable species, and specify vertical grain (and be sure to check your wallet before ordering!).

But most importantly, owning and using a moisture meter and knowing the in-use EMCs is an inexpensive way for carpenters to predict and avoid wood movement problems that could require costly repairs.

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AUTHOR BIOS

Carl Hagstrom is a partner in Woodweb, the leading online resource for professional woodworking. He has an extensive background in residential construction and architectural woodworking. He is also a contributing editor at the Journal of Light Construction, and a Certified Professional Building Designer.

Gene Wengert—having taken an interest in woodworking since 7th grade shop classes—was employed by the US Forest Products Lab as a college student starting in 1961. He worked in solar lumber drying, as well as discoloration of wood due to UV light. He then worked on the weathering of wood with the Lab and received a BS degree in meteorology from the University of Wisconsin. He continued to work on moisture related issues and developed expertise in processing northern and Rocky Mountain aspen, going from environmental benefits of the species through sawing, drying, and marketing. (Aspen is splinterless, did you know?) He worked at Virginia Tech as a wood specialist for the extension service, consulting with the wood industry daily. He also managed Tech's sawmill and dry kiln.

For fun, Gene has taken up long-distance bicycle riding (at age 55) and has done two trips from the Pacific to Atlantic Ocean and three from the Gulf to Minneapolis.

Dr. Gene Wengert is Professor Emeritus in Wood Processing, Department of Forestry, at the University of Wisconsin (Madison). He is also a technical advisor at Woodwebs's Sawing and Drying Forum, and Commercial Kiln Drying Forum. He frequently contributes to trade journals serving the primary lumber processing industry, and is president of The Wood Doctor's Rx, LLC, through which he provides educational and consulting services to lumber processing firms.

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Scott's Contracting
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Wall RValue, Configuring Wall RValues, Wall RValue Testing

Wall R-Values that Tell It Like It Is

by Jeffrey E. Christian and Jan Kosny
Jeffrey E. Christian is the manager of the DOE Building Envelope Systems and Materials Program at the Oak Ridge National Laboratory, Oak Ridge, Tennessee, and Jan Kosny is a research engineer at the University of Tennessee in Knoxville.

There's a lot more to most walls than meets the eye, and the R-value of a whole wall can be considerably lower than the R-value of the insulation that fills it. At DOE's Buildings Technology Center, scientists have developed a system for measuring whole-wall R-value, and have already tested several types of wall system.

DOE's rotatable guarded hot box is the workhorse behind the whole-wall rating label system. Sample wall sections are placed in the box, where their thermal properties can be tested in a controlled environment.

Several new wall systems are gaining popularity, due to increasing interest in energy efficiency, alternatives to dimensional wood framing, and building sustainable structures. Steel framing, insulating concrete forms, autoclave cellular concretes, structural insulated core panels, engineered wood wall framing, and a variety of hybrid wall systems are a few of the new types. But accurately comparing the thermal performance of these systems has been difficult.

How Wall R-Value Is Usually Calculated

Currently, most wall R-value calculation procedures are based on calculations developed for conventional wood frame construction, and they don't factor in all of the effects of additional structural members at windows, doors, and exterior wall corners. Thus they tend to overestimate the actual field thermal performance of the whole wall system.
In these common procedures, the user enters a framing factor (ratio of stud area to whole opaque exterior wall area). The framing factor is usually estimated, is seldom verified against actual site construction, and is frequently underestimated (see "Is an R-19 Wall Really R-19?" HE Mar/Apr '95, p. 5). Framing factors range from 15% to 40% of the opaque exterior wall area, yet lower values are commonly used. Unfortunately, the wall's energy efficiency is usually marketed solely by the misleading clear-wall R-value (Rcw).

Clear-wall R-value accounts for the exterior wall area that contains only insulation and necessary framing materials for a clear section. This means a section with no windows, doors, corners, or connections with roofs and foundations. Even worse is the center-of-cavity R-value, an R-value estimation at the point in the wall containing the most insulation. This converts to a 0% framing factor and does not account for any of the thermal short circuits through the framing.

The consequences of poorly selected connections between envelope components are severe. These interface details can affect more than half of the overall opaque wall area (see Figure 1). For some conventional wall systems, the whole-wall R-value (Rww) is as much as 40% less than the clear-wall value. Poor interface details may also cause excessive moisture condensation and lead to stains and dust markings on the interior finish, which reveal envelope thermal shorts in an unsightly manner. This moist surface area can encourage the growth of molds and mildews, leading to poor indoor air quality.

Metal-framed walls are particularly vulnerable to thermal shorts. Unfortunately, builders often attempt to solve metal wall problems by making thicker walls and adding more insulation in the cavity between the metal studs. In fact, the thicker walls have an even higher percentage difference between clear-wall and whole-wall R-value.

Figure 1. Interface details for metal and wood framing.

Measuring Whole-Wall R-values

To compare wall systems more accurately, we have developed a procedure for estimating the Rww for various system types and construction materials (see "Wall R-Value Terms"). The methodology is based on laboratory measurements and simulations of heat flow in a variety of wood, metal, and masonry systems (see "How We Evaluate Wall Performance"). The whole-wall R-value includes the thermal performance not only of the clear-wall area, with its insulation and structural elements, but also of typical envelope interface details. These details include wall/wall (corner), wall/roof, wall/floor, wall/door, and wall/window connections.

Table 1. Clear-Wall and Whole-Wall R-Values for Tested Wall Systems
No.	System Description	Clear Wall R-Value (Rcw)	Whole Wall R-Value (Rww)	(Rww/Rcw) x 100%
1.	12-in two-core insulating units concrete 120lb/ft³, EPS inserts 1 7/8-in thick, grout fillings 24 in o.c.	3.7	3.6	97%
2.	12-in two-core insulating units wood concrete 40lb/ft³, EPS inserts 1 7/8-in thick, grout fillings 24 in o.c.	9.4	8.6	92%
3.	12-in cut-web insulating units concrete 120lb/ft³, EPS inserts 2 1/2 in thick, grout fillings 16 in o.c.	4.7	4.1	88%
4.	12-in cut-web insulating units wood concrete 40lb/ft³, EPS inserts 2 1/2 in thick, grout fillings 16 in o.c.	10.7	9.2	86%
5.	12-in multicore insulating units polystyrene beads concrete 30lb/ft³, EPS inserts in all cores	19.2	14.7	77%
6.	EPS block forms poured in place with concrete, block walls 1 7/8 in thick	15.2	15.7	103%
7.	2 x 4 wood stud wall 16 in o.c., R-11 batts, 1/2-in plywood exterior, 1/2-in gypsum board interior	10.6	9.6	91%
8.	2 x 4 wood stud wall 24 in o.c., R-11 batts, 1/2-in plywood exterior, 1/2-in gypsum board interior	10.8	9.9	91%
9.	2 x 6 wood stud wall 24 in o.c., R-19 batts, 1/2-in plywood exterior, 1/2-in gypsum board interior	16.4	13.7	84%
10.	Larsen truss walls 2 x 4 wood stud wall 16 in o.c., R-11 batts + 8-in-thick Larsen trusses insulated by 8-in-thick batts, 1/2-in plywood exterior, 1/2-in gypsum board interior	40.4	38.5	95%
11.	Stressed-skin panel wall, 6-in-thick foam core + 1/2-in oriented strand board (OSB) boards, 1/2-in plywood exterior, 1/2-in gypsum board interior	24.7	21.6	88%
12.	4-in metal stud wall 24 in o.c., R-11 batts, 1/2-in plywood exterior + 1-in EPS sheathing + 1/2-in wood siding, 1/2-in gypsum board interior. NAHB Energy Conservation House Details.	14.8	10.9	74%
13.	3 1/2-in metal stud wall 16 in o.c., R-11 batts, 1/2-in plywood exterior + 1/2-in wood siding, 1/2-in gypsum board interior	7.4	6.1	83%
14.	3 1/2-in metal stud wall 16 in o.c., R-11 batts, 1/2-in plywood exterior + 1/2-in EPS sheathing + 1/2-in wood siding, 1/2-in gypsum board interior. AISI Manual details	9.9	8.0	81%
15.	3 1/2-in metal stud wall 16 in o.c., R-11 batts, 1/2-in plywood exterior + 1-in EPS sheathing + 1/2-in wood siding, 1/2-in gypsum board interior. AISI Manual details	11.8	9.5	81%
16.	3 1/2-in metal stud wall 24 in o.c., R-11 batts, 1/2-in plywood exterior + 1/2-in wood siding, 1/2-in gypsum board interior. AISI Manual details	9.4	7.1	75%
17.	3 1/2-in metal stud wall 24 in o.c., R-11 batts, 1/2-in plywood exterior + 1/2-in EPS sheathing + 1/2-in wood siding, 1/2-in gypsum board interior. AISI Manual details	11.8	8.9	76%
18.	3 1/2-in metal stud wall 24 in o.c., R-11 batts, 1/2-in plywood exterior + 1-in EPS sheathing + 1/2-in wood siding, 1/2-in gypsum board interior. AISI Manual details	13.3	10.2	77%

We estimated whole-wall R-values for 18 wall systems, using a computer model. We validated the accuracy of the modeling using the results of 28 experimental tests on masonry, wood frame, and metal stud walls. The model was sufficiently accurate at reproducing the experimental data.
The whole-wall R-values estimated for the 18 wall systems are shown in Table 1 along with the clear-wall R-values. A reference building was used to establish the location and area weighing of all the interface details. The comparison of these two values gives a good overall perspective of the importance of wall interface details for conventional wood, metal, masonry, and several high-performance wall systems.
In general, construction details for the wall systems chosen come from the ASHRAE Handbook and from the respective manufacturers. In the case of the metal frame systems, the details come from the American Iron and Steel Institute and other common sources.
A wall's thermal performance is often simply described at the point of sale as the clear-wall value. The results shown in Table 1 indicate that the whole-wall value could be overstated by up to 26% for these systems. These differences can be even greater with interface details that are easier to construct but that may have more thermal shorts.

Whole-Wall versus Clear-Wall
Interesting comparisons can be made using the data in Table 1 to illustrate the importance of using a whole-wall value to select the most energy-efficient wall system. It could be argued that the difference between the clear wall and whole-wall R-value represents the energy savings potential of adopting the rating procedure proposed in this paper. Most building owners assume that they have the higher clear-wall value, rather than the more realistic whole-wall value.

An insulating concrete form with metal ties is prepared for testing at the Buildings Technology Center. Its whole-wall R-value and thermal mass will be measured.

Knowing whole-wall R-value could affect consumer choices. Systems 5 and 6 in Table 1 show two different high-performance masonry units. If one used the clear-wall data to choose the unit with the highest R-value, one would pick System 5, the low-density concrete multicore insulation unit, because its clear-wall value is 19.2 compared to 15.2 for System 6, expanded polystyrene (EPS) block forms. However, if one used the whole-wall data, one would choose just the opposite, because System 6 has the higher value--15.7 compared to 14.7 for System 5. Also, the whole-wall value of the foam form system is actually higher than the clear-wall value by more than 3%. This illustrates the effect of the high thermal resistance of the interface details.
Systems 7, 8, and 9 are all conventional wood frame systems. Note that the details affect the whole-wall R-value more for 2 x 6 walls than for 2 x 4 walls. The ratio of Rww to Rcw is about 90% for the 2 x 4 walls and 84% for the 2 x 6 wall.

Comparing System 11, the 6-inch stressed-skin panel wall, to System 9, the conventional 2 x 6 wood frame wall, shows that the Rcw for the former (R-24.7) is 51% higher than that for the latter (R-16.4). However, the figures for the Rww are R-21.6 to R-13.7 respectively, an improvement of 58%. As this example shows, advanced systems will generally benefit from a performance criterion that reflects whole-wall rather than clear-wall values.

How We Evaluate Wall Performance

To determine whole-wall R-value, we test a clear-wall section, 8 ft x 8 ft, in a guarded hot box. We compare experimental results with sophisticated heat conduction model predictions to get a calibrated model. Next, we make simulations of the clear-wall area with insulation, structural elements, and eight interface details--corner, wall/roof, wall/foundation, window header, windowsill, doorjamb, door header, and window jamb--that make up a representative residential whole-wall elevation. Results from these detailed computer simulations are combined into a single whole-wall steady-state R-value estimation. This estimation is compared with simplified calculation procedures and results from other wall systems. The user defines a reference wall elevation to weigh the impact of each interface detail.

For each wall system for which the whole-wall R-value is to be determined, all details commonly used and recommended (outside corner, wall/floor, wall/flat ceiling, wall/cathedral ceiling, doorjamb, window jamb, windowsill, and door header) must be included. The detail descriptions include drawings, with all physical dimensions, and thermal property data for all material components contained in the details.

Beyond R-Value

The R-value is only the first of five elements that are needed to compare whole-wall performance. The other four elements are thermal mass, airtightness, moisture tolerance, and sustainability. We are working on standard ways to measure thermal mass, airtightness, and moisture tolerance. For some systems all five factors are important; for others, only whole-wall R-value is relevant.

Thermal Mass Benefit

Wall systems with significant thermal mass have the potential--depending on the climate--to reduce annual heating and cooling energy requirements below those required by standard wood frame construction with similar steady-state R-value. The thermal mass benefit is a function of climate.
Effective R-values for massive walls are obtained by comparing the massive wall to light-weight wood frame walls. However this effective R-value is only a way to determine the link between the thermal mass of the wall and annual space heating and cooling loads, or a way to answer the question "what R-value would an identical house with wood frame walls need to obtain the same space heating and cooling loads as the massive walled house?" The term cannot be generally applied to a given wall type.

A procedure to account for thermal mass was used to create the generic tables found in the Model Energy Code (MEC) for all thermal mass walls with more than 6.0 Btu/ft² of wall thermal capacitance. The tables have been in use since 1988. Customized tables can be used to show code compliance with the prescriptive Uw requirements in the MEC that are based on wood frame construction.

Airtightness

Users of the DOE Buildings Technology Center follow a combination of ASTM Standards C236 or C976 (ASTM 1989) or E1424 and E283 (ASTM 1995) to measure air leakage and heat loss through clear-wall assemblies under simulated wind conditions ranging from 0 to 15 mph. Varying the differential pressures from 0 to 25-50 Pascals (Pa) simulates the extremes to which a wall is exposed in a real building. The test specimens contain one light switch and one duplex outlet connected with 14-gauge wiring that spans the width of the wall.

Because heat loss in a building can be as high as 40% due to infiltration, it is important to include this performance parameter, but the quality of workmanship on the construction site, as compared to a laboratory specimen, must be considered. A second complicating factor is that materials may shrink or crack over time, and this will change the leakage. We will never completely predict the impact of workmanship on energy loss in buildings. What is important is to establish a uniform baseline for all wall systems.

Moisture Tolerance

The wall's moisture behavior, like the benefit of thermal mass, is a function of climate and building operation. Annual moisture accumulation due to vapor diffusion of a particular wall system can be estimated by computer simulation. It is harder to estimate moisture accumulation due to air flow into the wall. It is important, in a long-lasting wall assembly, that the wall have the ability to dry itself out if it is built wet or picks up moisture due to a leak. The drying rate can be modeled and measured in the laboratory. The potential for moisture accumulation over specific full annual climatic cycles can also be modeled by heat and mass transfer codes such as MOIST (available from the National Institute of Standards and Technology, Special Publications 853, Release 2.1) and MATCH (available from Carston Rode, Technical University of Denmark, Department of Buildings and Energy, Building 188, DK-2800, Lyngby).

Systems 12 through 18 are all metal-framed. On average, the whole-wall value for these seven systems is 22% less than the clear-wall value. Metal can be used to build energy-efficient envelopes, but not by using techniques common to wood frame construction. The conventional metal residential systems reflected in Table 1 do not fare as well, compared to the other systems, when the whole-wall value is used as the reference. For example, if one is considering either System 6 (EPS block forms) or System 12 (a 4-inch metal stud wall), the clear-wall R-value is about the same--R-15. However, if the comparison is made using the whole-wall R-value, the EPS block form system has a 45% higher value--R-15.7 compared to R-10.9.

A standard metal frame wall section before insulation and drywall is installed.

Whole-Wall versus Center-of-Cavity
We also compared whole-wall R-values to center-of-cavity R-values. When a real estate agent or contractor states the R-value of insulation across the cavity to a potential home buyer, the implied whole-wall R-value is often overstated by 27% to 58%. If one compared metal (System 13) and wood (System 7) frames using center-of-cavity R-values, one would conclude that there was no difference, since both have center-of-cavity values of about R-14. However, the whole-wall value of the 2 x 4 wood wall system is 56% better than the whole-wall value for the metal system -- R-9.6 compared to R-6.1.

These comparisons are not meant to imply that one type of construction is always better than another. They are all based on representative details. Whole-wall R-values could change if certain key interface details were changed. The purpose of making these sample comparisons is simply to show the importance of having the whole-wall value available in the marketplace, to guide designers, manufacturers, and buyers to more energy-efficient systems.

An autoclave concrete wall is stuccoed in preparation for the hot box test.

Coming Soon: A Wall Rating Label?

A number of innovative wall systems offer advantages that will continue to gain acceptance as the cost of dimensional lumber rises, the quality of framing lumber declines, availability fluctuates, and consumers remain concerned about the environmental impact of the nonsustainable harvesting of wood. For instance, while common dimensional lumber systems historically represent about 90% of the market, metal framing manufacturers anticipate attaining 25% of the residential wall market by the year 2000. This projection may be a bit optimistic, but it is clear that cold form steel is set to make major inroads into the residential market.
Now that a growing wall database and an evaluation procedure are available, the building industry can develop a national whole-wall thermal performance rating label. This would establish in the marketplace a more realistic energy savings indicator for builders and homeowners faced with selecting a wall system for their buildings.

Labels could also help specific systems to gain the acceptance of code officials, building designers, builders, and building energy-rating programs such as Home Energy Rating Systems (HERS) and EPA Energy Star Buildings. The whole-wall R-value procedure has been proposed for adoption in the ASHRAE Standard 90.2, the Council of American Building Officials Model Energy Code, and U.S. Department of Energy's national voluntary guidelines for HERS. Many of the documents that are available to show builders how to comply with applicable codes, standards, and energy efficiency incentive programs would benefit by using the whole-wall R-value comparison procedure.
Ultimately, wall comparisons should include five elements: whole-wall R-value, thermal mass benefits, airtightness, moisture tolerance, and sustainability (see "Beyond R-Value"). Publication of this article was supported by the U.S. Department of Energy's Office of State and Community Programs, Energy Efficiency and Renewable Energy.

Wall R-Value Terms

Center-of-cavity R-value: R-value estimation at the point in the wall that contains the most insulation.
Clear-wall R-value (Rcw): R-value estimation for the exterior wall area that contains only insulation and necessary framing materials for a clear section, with no windows, doors, corners, or connections between other envelope elements, such as roofs and foundations.
Interface details: A set of common structural connections between the exterior wall and other envelope components--such as wall/wall (corners), wall/roof, wall/floor, window header, windowsill, doorjamb, door header, and window jamb--that make up a representative residential whole-wall elevation.
Whole-wall R-value (Rww): R-value estimation for the whole opaque wall, including the thermal performance of both the "clear wall" area and typical interface details.
Opaque wall area: The total wall area, not including windows and doors.

Continuing research is being cofunded by DOE's Office of Buildings Technology and Community Programs and by private industry to add more advanced wall systems to the database, and to address not only thermal shorts, but thermal mass benefits, airtightness, and moisture tolerance. Industry participants so far include American Polysteel, Integrated Building and Construction Solutions (IBACOS), Icynene Incorporated, Society for the Plastics Industry Spray Foam Contractors, Hebel USA L.P., Composite Technologies, Structural Insulated Panel Systems Association, LeRoy Landers Incorporated, Florida Solar Energy Center, American Society of Heating, Refrigerating and Air-Conditioning Engineers and Enermodal.

The database of advanced wall systems is available on the Internet (http://www.cad.ornl.gov/kch/demo.html). For more information, contact Jeffrey E. Christian at Oak Ridge National Laboratory, P. O. Box 2008, MS 6070 Oak Ridge, TN 37831-6070. Tel:(423) 574-4345; Fax:(423)574-9338; E-mail: jef@ornl.gov.

Radiation, Convection, Conduction-Warm to Cold

Convection is the movement of air in response to heat

Warm air rises, cool air sinks. Because walls and windows are usually cooler than the middle of a room, they spur convective loops that can feel like a draft. A similar thing can happen inside a wall cavity.

When air is heated, it expands, and therefore becomes less dense, so it rises. The rising warm air displaces cooler air, which sinks. When the motion is constant, it's called a convective loop.

Woodstoves and windows cause convective loops by heating or cooling (respectively) the air closest to them.

Even in homes with airtight walls and ceilings, convective loops can feel like a cool draft and be uncomfortable to the people in the room.

Convective loops can occur inside poorly insulated wall cavities, too, degrading the performance of the insulation.

Heat flows through materials by conduction

Wood is a better insulator than nothing at all. Snow melts off the uninsulated rafter bays more quickly than directly above the wooden trusses, which have an R-valuearound 1.1 per inch, or R-4 for a 2x4 top chord.

Conduction is the flow of heat energy by direct contact, through a single material or through materials that are touching.

Substances that conduct heat readily are called conductors, while substances that don't conduct heat readily are called insulators. Metal is a good conductor; foam is a good insulator. Wood falls somewhere in between.

Radiation heats objects, not air

Solar radiation. Solar heat radiates through the vacuum of space and warms the earth.

Radiation is the transfer of heat by electromagnetic waves that travel through a vacuum (like space) or air.

Radiation cannot pass through a solid object like plywood roof sheathing. When the sun shines on asphalt shingles, heat is transferred to the plywood sheathing by conduction. After the plywood has been warmed by conduction, it can radiate heat into the attic.

Radiant barriers are materials (for example, aluminum foil) with a low-emissivity (low-e) surface. Although radiant barriers have a few applications in residential construction—they are sometimes integrated with roof sheathing—they are rarely cost-effective when compared to conventional insulation options.

-- Scott's Contracting scottscontracting@gmail.com http://www.stlouisrenewableenergy.blogspot.com http://www.stlouisrenewableenergy.com scotty@stlouisrenewableenergy.com

Insulating Roofs, Walls, and Floors

ABOUT INSULATING ROOFS, WALLS, AND FLOORS

Its not unusual for a house to have three or four types of insulation: spray foam, loose fill, rigid foam, and/or batts. Each type has multiple uses, but most also have limitations on where they can be used.

The best insulation for each location depends on a number of factors, including cost, ease of installation, available space, and the material's resistance to moisture.
All insulation types perform best when they're installed well. Some (like batts and blankets) can lose significant R-valuewith even a slightly sloppy installation.

Grading installation quality

The Residential Energy Services Network (RESNET), a national association of home-energy raters, long struggled with the question of how to estimate the R-value of walls that vary widely in performance depending on the skill of the insulation installer. Eventually, RESNET developed a useful rating system for insulation installation quality. The system is described in an article published in the January/February 2005 issue of Home Energy magazine, "Insulation Inspections for Home Energy Ratings," by Bruce Harley. The RESNET rating system recognizes three levels of insulation installation quality: Grade I, Grade II, and Grade III.

Grade I is the best installation

"In order to qualify for a Grade I rating, insulation must … ﬁll each cavity side to side and top to bottom, with no substantial gaps or voids around obstructions (that is, blocking or bridging—as seen in the grade II photo below), and it must be split, or ﬁtted tightly, around wiring and other services in the cavity. In general, no exterior sheathing should be visible through gaps in the material," Harley wrote. "Compression or incomplete fill amounting to 2% or less of the surface area of insulation is acceptable for Grade 1, if the compression or missing fill spaces are less than 30% of the intended fill thickness (that is, 70% or more of the intended insulation thickness is present)."

The standard for a Grade II installation is somewhat lower

"A Grade II rating represents moderate to frequent defects: gaps around wiring, electrical outlets, plumbing, other intrusions; rounded edges or 'shoulders,' larger gaps, or more signiﬁcant compression. No more than 2% of the surface area of insulation missing is acceptable for Grade II."

Grade III installations are the worst
"A Grade III rating applies to any installation that is worse than Grade II." For further information on the RESNET grading system—including illustrations of good jobs and sloppy jobs—see "Assessing the Quality of Insulation Installed in New York Energy Star Labeled Homes."

ABOUT INSULATING FOUNDATIONS

Basements

Because foundations aren't really exposed to vast temperature swings, less insulation is needed there. Insulation in a basement should be chosen to do more than slow the flow of heat through these relatively stable environments; the best choices of basement insulation stop air and water, too. Basement walls and floors can be insulated on the inside or the outside, inside being the easier method for retrofits and outside being easier (in general) for new construction.

Exterior insulation choices should be moisture tolerant

Below-grade walls and floors should be insulated on the outside with, , spray foam, or rigid mineral wool. Because polyisocyanurate can absorb water, it should not be used under a slab or on the outside of a foundation. Polyisocyanurate performs well, however, when used on the inside wall of a basement or crawlspace.

The most common insulation under slabs is XPS, although EPS also works if its density is adequate and if it is rated for ground contact. If the insulated slab must bear heavy loads, XPS is usually a better choice than EPS.

Closed-cell spray polyurethane foam can also be used under a slab.
Basement walls can be insulated on the exterior or interior with EPS, XPS, spray polyurethane foam, or rigid mineral wool (for example, Roxul drainboard).

To insulate a basement wall from the inside, the foam should be applied directly to the concrete, in order to keep moist interior air away from the cool, damp surface and lower the risk of condensation. To allow any accumulated moisture to dry to the inside, a semipermeable foam (EPS or XPS) is the best choice. To meet code requirements for a thermal barrier, the foam will probably need to be protected with a layer of gypsum drywall; fiberglass-faced drywall is more moisture resistant than paper-faced drywall.

Under no circumstances should fiberglass batts be used to insulate basement walls. Because fiberglass batts are air-permeable, they are unable to prevent moist interior air from contacting colder basement walls. That's why fiberglass-insulated basement walls can easily become damp and moldy.

Crawlspaces

Although some builders insulate the floor above a crawlspace (the crawlspace ceiling), most building scientists recommend building a sealed, insulated crawlspace that includes wall insulation. It usually requires less insulation (and involves fewer tricky details) to cover a short wall around the perimeter than the whole floor.

Sealed crawlspaces should be built and insulated exactly like basements.

Of course, a well-detailed insulated crawlspace needs more than just insulation. Among the other critical details are careful air-sealing of the rim-joist area and (if the crawlspace has a dirt floor) installation of a ground cover.

Slabs on grade

Some builders insulate slab perimeters without insulating under the slab. In all but the warmest climates, however, it's better to install a continuous layer of EPS, XPS, or spray polyuyrethane foam under the entire slab. Some builders modify an ICF for use as a form for the slab that includes insulation.

If the home has in-floor radiant heat, it's especially important to include a thick layer of foam directly under the entire slab. Experts disagree on exactly how much foam to add, but they all agree that at least some is a good idea. Engineer John Straube of Building Science Corp. says that after about 4 in.—perhaps 6 in. if the slab includes radiant heat—the money is better spent elsewhere. However, Passivhaus builders sometimes install up to 14 in. of sub-slab insulation.

Soil has a measurable R-value, so it can insulate the bottom of the slab from the exterior air to some extent. But soil is also a nearly infinite heat sink. The average soil temperature varies depending on the climate and the soil depth; however, if the soil has an average temperature of 55°F and the interior of a house has an average temperature of 72°F, heat will always want to flow from the warm side of the slab toward the soil. That's why it's important to insulate under a slab.

ABOUT INSULATING ABOVE-GRADE WALLS

The strategy adopted for insulating a home's above-grade walls depends on the wall construction used.

Walls built from SIPs or ICFs already include insulation.
Concrete-block walls are best insulated from the exterior with rigid foam or spray polyurethane foam.
Wood-framed walls can be insulated with cavity insulation (fiberglass batts, sprayed-in-place fiberglass, cellulose, or spray polyurethane foam), on the interior (with rigid foam board), on the exterior (with rigid foam board or spray polyurethane foam), or with a combination of approaches (for example, some cavity insulation and exterior foam sheathing).

Thermal bridging
The effective R-value of a framed wall assembly with cavity insulation is always less than the R-value of the insulation alone, as thermal bridging through the studs degrades the performance of the wall. Thermal bridging can be reduced, and the thickness of the wall increased, by:

adding foam sheathing to the exterior of the wall;
adding a layer of rigid foam under the interior drywall; or
building a double-stud wall with staggered studs.

Foam sheathing

The performance of any wood-framed wall will be improved by installing exterior rigid foam sheathing; the usual choices are XPS or polyisocyanurate. Although EPS can be used, it is more fragile than the other two options.
Adding foam insulation to the outside of a wall affects the wall's ability to dry out when it gets wet. Different types of foam insulation have different permeance ratings, but after a few inches they're all pretty impermeable to moisture. Most foam-sheathed walls are designed to dry to the inside. This means that interior plastic vapor barriers should never be used on foam-sheathed walls.

According to Joseph Lstiburek and Peter Baker of Building Science Corp. (see link below), adding 1 in. of R-5 insulation to a 2x6 wall insulated with fiberglass batts increases the effective R-value of the wall from 14.4 to 19.4, a 35% gain with only a 15% increase in wall thickness.

Adding 2 in. of foam raises the R-value from 14.4 to 23.8, an improvement of 65%. A layer of insulating foam on the outside of walls also reduces the risk of condensation by raising the dew point of the surface where water vapor is likely to collect.

Thick foam sheathing is safer than thin foam sheathing. To learn more about determining a safe thickness for exterior foam, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

ABOUT INSULATING FLAT CEILINGS

Flat ceilings under unconditioned attics can be insulated with fiberglass batts, blown fiberglass, or blown cellulose, but cellulose works best—especially in very cold temperatures when convective loops can degrade the performance of fiberglass. Regardless of the type of insulation used, more is always better, and it's usually an inexpensive upgrade as space is less of a limiting factor than it would be for walls.

Spray polyurethane foam can also be used to insulate a flat ceiling, although at a much higher cost than cellulose. An advantage of spray foam is that it air-seals as it insulates. With all types of attic insulation, air-sealing before insulating is almost more important than type and depth of insulation.

Attic-floor insulation should extend over the top plates of perimeter walls. To provide enough room for the necessary depth of attic insulation, be sure to specify raised-heel roof trusses.

Locating insulation at the attic floor has several advantages over locating insulation along the slope of the roof:

It's cheaper, easier, and faster to install thick insulation at the attic floor.
Unconditioned attics are easier to vent than insulated rafter bays.
It's easier to detect and pinpoint roof leaks when the attic is unconditioned.

ABOUT INSULATING ROOFS

Sloped ceilings and roofs can be insulated from above (by installing rigid foam on top of the roof sheathing), by installing insulation between the rafters, from below (by installing rigid foam under the rafters), or by a combination of some or all three of these insulation methods. Any of these methods will work. Although installing insulation on top of the roof sheathing is more foolproof, it's also less common.
EPS
,or polyisocyanurate foam can be installed above roof sheathing. Two or more layers of rigid foam with staggered seams can be topped with eave-to-ridge 2x4s to create vent channels, followed by a second layer of roof sheathing. Exterior insulation like this with staggered seams disrupts conductive heat flow through the framing assembly.

Installing insulation in rafter bays is risky, as interior moisture can migrate through the insulation (either by diffusion or by piggybacking with exfiltrating air) and contact the cold roof sheathing, leading to condensation. This problem can be prevented by using closed-cell spray polyurethane foam, with or without a ventilation channel under the roof sheathing.

ABOUT RETROFITTING INSULATION

Although adding insulation to an existing home is always more challenging than insulating a new home, weatherization contractors have developed many cost-effective methods of improving existing insulation levels.

It's important to manage any moisture problems in a home before engaging in air-tightening measures or insulation improvements. Inspect the home to identify any leaks or high-moisture areas, and be sure that the home is equipped with adequate mechanical ventilation.

Among the tried-and-true methods used by experienced weatherization workers:

To insulate a basement floor, install a continuous layer of XPS foam on top of the concrete. Top the foam with 2x4 sleepers and a plywood subfloor. If a low ceiling makes every inch critical, the sleepers can be omitted; in that case the plywood subfloor should be mechanically fastened through the foam to the concrete.
Basement or crawlspace walls can be insulated with interior XPS, EPS, or closed-cell spray polyurethane foam. The foam should be protected with a thermal barrier (for example, 1/2-in. drywall).
Above-grade frame walls can be insulated by blowing dense-packed cellulose into stud cavities through holes drilled through the siding. When insulation is complete, the holes are plugged.
If siding is being replaced, rigid foam or spray polyurethane foam can be installed on top of the exterior sheathing. Exterior foam retrofit jobs require considerable trim work around windows and doors, however.
Flat ceilings under unconditioned attics are usually easy to insulate with blown-in cellulose.
Improving the insulation over a sloped ceiling is often easier from the exterior than the interior. Rigid foam insulation can be added above the roof sheathing in conjunction with new roofing.

After air-sealing and insulation work is complete, the renovated home should be tested for radon. Radon levels often increase after a home has been weatherized.
If a house is undergoing extensive remodeling, it's worth considering a deep energy retrofit.
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Scott's Contracting
scottscontracting@gmail.com
http://www.stlouisrenewableenergy.com
http://stlouisrenewableenergy.blogspot.com

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