ABOUT INSULATION
Thicker is better In cold weather, a puffy parka holds in your body heat. Insulation does the same thing for a house. The thicker the insulation, the better it works to reduce heat flow from the inside of a home to the outside during winter, and from outside to inside during summer.The thermal barrier of a home should consist of a continuous layer of insulation on all sides—including the lowest floor, the exterior walls, and the ceiling or roof.
Doubling the thickness of insulation will double the insulation's R-value, cutting heat loss in half. Each time the insulation layer is doubled in thickness, this rule applies. The energy saved per year by doubling insulation from R-10 to R-20, however, will be considerably more than the energy saved by doubling insulation from R-20 to R-40, because of the law of diminishing returns. In some cases, like an attic, it's worth piling on more insulation because there is plenty of room. It's much more expensive to add that much insulation to exterior walls.
It takes more than just insulation to slow heat Stopping air leaks is just as important—maybe more important—than adding insulation. Unless builders prevent air from leaking through walls and ceilings, insulation alone won't do much good. Not only are drafts uncomfortable, but air moving through insulated cavities can cut the efficiency of the insulation by as much as 50%.
Some insulation types make good air barriers, and some don't. In all cases, it's best to keep the insulation tight to the air barrier.
THERMAL BRIDGING IS CONDUCTION IN ACTION
When there is no insulation in a roof or wall, the framing is the most insulated part of the assembly. It has the highest R-value. Softwood lumber has an R-value of 1.25 per inch, so a 2x6 stud has an R-value of almost 7. As soon as you put insulation between the studs or rafters above R-7, however, the framing becomes the weak thermal link. If the framing cavities are filled with closed-cell spray foam insulation, the insulation has an R-value of about 36. At that point, the studs or rafters become a glaring weakness in the design.
If you look for it, thermal bridging can sometimes be seen from either inside or outside. Inside, it can cause a problem called ghosting, or cold stripes behind the drywall during winter. These cold stripes can encourage condensation that leads to the accumulation of dust particles on the drywall; eventually, visible vertical stripes may form. Outside, you can see the effect of thermal bridging in snow-melt patterns on roofs and drying patterns on walls.
A continuous layer of rigid foam installed on the inside or outside of a wall or roof drastically reduces thermal bridging through the framing.
R-VALUE MEASURES HOW WELL INSULATION WORKS
Heat flows from hot to cold; it can't be stopped, but it can be slowed If we measure the rate at which heat flows through a building material or building assembly—for example, a wall or a roof—we can calculate a number (the R-value) to indicate its insulating ability. The higher a material's R-value, the better the material is at resisting heat flow through conduction, convection, and radiation (outlined above). Insulation manufacturers report R-values determined by tests following ASTMstandards (for example, ASTM C518).Common insulation types and their R-values Residential insulation materials have R-values that range from about 3 to 7 per inch. The amount of insulation installed in any given building assembly depends on the climate, the part of the house being insulated, the project budget, and local code requirements.
- Batts and blankets: R-3.1 to R-4.1 per in.
- Blown-in and loose-fill insulation: R-2.6 to R-4.2 per in.
- Rigid foam: R-3.6 to R-6.8 per in.
- Closed-cell spray foam: R-6 to R-6.8 per in.
- Open-cell spray foam: R-3.5 to R-3.6 per in.
In some parts of the country, minimum code requirements for insulation already (or may soon) exceed these DOE recommendations. For example, the 2009 International Residential Code requires cold-climate builders to include a minimum of R-20 wall insulation and R-15 basement wall insulation.
DOE-Department of Energy-recommended R-values for various parts of a house
| Zone | Attic | Wall | Floor | Slab edge | Basement wall (framing cavity insulation) | Basement wall (continuous rigid insulation) |
|---|---|---|---|---|---|---|
| 1 | R-30 to R-49 | R-13 to R-15 | R-13 | R-4 | R-11 | R-10 |
| 2-3 | R-30 to R-60 | R-13 to R-15 | R-13 to R-25 | R-8 | R-11 | R-10 |
| 4 | R-38 to R-60 | R-16 to R-21 | R-25 to R-30 | R-8 | R-11 | R-4 |
| 5 | R-38 to R-60 | R-16 to R-27 | R-25 to R-30 | R-8 | R-11 to R-19 | R-10 to R-15 |
| 6-8 | R-49 to R-60 | R-18 to R-27 | R-25 to R-30 | R-8 | R-11 to R-19 | R-10 to R-15 |
In any case, green builders almost always exceed minimum code requirements for insulation thickness. Many energy consultants, including Betsy Pettit and Joseph Lstiburek, now recommend that cold-climate homes include R-60 ceilings, R-40 above-grade walls, R-20 basement walls, and R-10 basement slabs.
Some builders go further; for example, an Illinois home designed to meet the rigorous German Passivhaus standard is insulated to nearly R-60 on every side—even under the slab.
AIR AND MOISTURE ARE PART OF THE PICTURE
Insulation can't work in a wind tunnel No matter what type of insulation you choose, it will perform poorly if installed in a house that is riddled with air leaks. Because many types of insulation (like loose fill and batts) work by trapping air, leaky walls, roofs, and floors mean poor thermal performance. For this reason, building scientists are fanatical about air-sealing. To get the most out of batts and blown insulation, every house needs an air barrier adjacent to or contiguous with the insulation layer.Some types of insulation are fairly effective at stopping air infiltration. For example, when rigid foam is used as wall sheathing, it can be an effective barrier, as long as the seams are taped. Spray polyurethane foam creates a very effective air barrier.
But neither rigid foam nor spray foam addresses air leaks at the seams where different components meet, such as under the bottom plates of walls. An air barrier is only effective if all of these seams and intersections are addressed with gaskets, glues, or sealants.
Of all available insulation materials, fiberglass batts are the most permeable to air leakage—so permeable that fiberglass is used to make furnace air filters. Because it doesn't restrict air flow, fiberglass is often singled out and derided for its poor performance.
In fact, much of the criticism of fiberglass insulation is unwarranted. As long as fiberglass is installed in a house with an adequate air barrier, it will perform well. Fiberglass performs best when installed in a framing cavity (for example, a stud bay or joist bay) with an air barrier on all six sides.
Installation details for high-quality fiberglass batts have been incorporated into the insulation installation guidelines established by the home raters from the Residential Energy Services Network (RESNET).
For every location in a house, there are always several ways to create an effective air barrier. However, not all methods are equally easy to achieve. In many locations, including rim-joist areas, spray polyurethane foam is so much faster than alternative methods that its use has become almost universal among builders of high-performance homes.
Moisture can piggyback on air There's another benefit to stopping air: less moisture in roofs and walls. That's because most moisture problems in walls and roofs are caused by moisture transported by air. Vapor diffusionis a much smaller problem.
Moisture can accumulate in a wall or ceiling when warm, humid interior air leaks through cracks in the shell. When this exfiltrating air encounters a cold surface—for example, OSB wall sheathing—the moisture in the air can condense into liquid and puddle in the wall cavity. The same thing can happen in summer, when warm, humid outdoor air leaks through cracks in the wall. If the home is air-conditioned, the moisture in this infiltrating air can condense when it reaches any cool surface—drywall, ductwork, etc. The best way to limit this type of moisture migration is to install an effective air barrier. If air isn't leaking through cracks in a home's walls and ceilings, the problem is nipped in the bud.
Insulation can stop air Some insulation types act as air barriers, while others act like air filters. If you choose an insulation that doesn't stop air flow, it's important to install an adjacent air barrier material.
Best to worst at stopping airflow: Spray foam Rigid foam Cellulose Blown-in fiberglass Fiberglass batts
SHOULD INSULATION STOP VAPOR?
Vapor permeability can be a good thing or a bad thing — vapor retarders slow wetting, but they also slow drying, which may be more important. As long as you design a roof, wall, or floor assembly with these concepts in mind, then almost any type of insulation can work.Least to most vapor permeable: Foil-faced polyisocyanurate Closed-cell spray foam XPS EPS Open-cell spray foam Cellulose Blown-in fiberglass Fiberglass batts
More on the vapor permance of insulation materials at BuildingScience.com.
INSULATE OUTSIDE THE BOX
Although residential wall insulation is traditionally installed in stud cavities, the best place to locate wall insulation is outside the frame. This exterior insulation reduces the thermal-bridging effect that studs have in a wall, because each piece of framing can act as a thermal bridge through the cavity insulation. These thermal bridges seriously degrade the performance of the wall.The thermal-bridging effect can be partially addressed by using rigid foam sheathing—usually 1 in. or 2 in. of XPS or polyisocyanurate. Even better are wall designs that place all the insulation—6 in. to 10 in. of rigid foam—outside the framing.
When insulation is outside the frame, framing materials stay warm and dry. When stud bays are not filled with insulation, the work of electricians and plumbers is greatly simplified.
Houses with foam sheathing should not include an interior polyethylene vapor retarder.
OTHER CONSIDERATIONS
Insulation can stop air Some insulation types act as air barriers, while others act like air filters. If you choose an insulation that doesn't stop air flow, it's important to install an adjacent air barrier material.Best to worst at stopping airflow: Spray foam Rigid foam Cellulose Blown-in fiberglass Fiberglass batts
SHOULD INSULATION STOP VAPOR?
Vapor permeability can be a good thing or a bad thing — vapor retarders slow wetting, but they also slow drying, which may be more important. As long as you design a roof, wall, or floor assembly with these concepts in mind, then almost any type of insulation can work.Least to most vapor permeable: Foil-faced polyisocyanurate Closed-cell spray foam XPS EPS Open-cell spray foam Cellulose Blown-in fiberglass Fiberglass batts
More on the vapor permance of insulation materials at BuildingScience.com.
INSULATE OUTSIDE THE BOX
Although residential wall insulation is traditionally installed in stud cavities, the best place to locate wall insulation is outside the frame. This exterior insulation reduces the thermal-bridging effect that studs have in a wall, because each piece of framing can act as a thermal bridge through the cavity insulation. These thermal bridges seriously degrade the performance of the wall.The thermal-bridging effect can be partially addressed by using rigid foam sheathing—usually 1 in. or 2 in. of XPS or polyisocyanurate. Even better are wall designs that place all the insulation—6 in. to 10 in. of rigid foam—outside the framing.
When insulation is outside the frame, framing materials stay warm and dry. When stud bays are not filled with insulation, the work of electricians and plumbers is greatly simplified.
Houses with foam sheathing should not include an interior polyethylene vapor retarder.
OTHER THERMAL BRIDGES
Uninsulated slab edges Window frames Wall and roof penetrations
--contact for additional details Scott's Contracting scottscontracting@gmail.com
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