Optimizing Windows
Here’s how window design, placement, and performance can make a greater impact on efficiency and comfort without busting the budget.
By:Rich Binsacca http://www.ecohomemagazine.com/green-products/optimizing-windows.aspx?page=2
Scott's Contracting GREEN BUILDER, St Louis "Renewable Energy" Missouri.http://www.stlouisrenewableenergy.com, contact scotty@stlouisrenewableenergy.com for additional information
Rules of Thumb for Selecting Windows
Though windows are only one target among several products and practices toward a top-notch thermal envelope, there are still rules of thumb to follow to optimize their impact.
Southern Exposure: An all-day exposure, per the sun’s path. In heating (i.e., cold) climates, leverage it with a better U-factor (ideally 0.20 or less) but less-efficient SHGC (perhaps 0.50 or higher) to boost heat gain in the winter and offset heating energy; use overhangs or other shading devices to cut down gain in the summer, when the sun is higher in the sky. In cooling (i.e., hot) climates, spec windows with U-factors and SHGC ratings of 0.30 or better and use shading tactics. “If I have to choose between blocking the summer sun and some solar gain in the winter, I’ll elect to block it,” says Texas custom builder Don Ferrier.
Western Exposure: Solar gain mostly in the late afternoon. Bob Saxler, architectural marketing manager at Andersen Windows, advises builders to focus on this elevation first, as it is the most difficult to control. If possible, orient the house and floor plan away from this exposure, such as situating utility areas, bathrooms, and, ideally, the garage on that side, and specify small and fewer operable (ideally casement) windows with efficient U-factors and SHGC ratings to mitigate solar gain and provide some measure of passive ventilation. If you have a view to the west, he says, boost the SHGC even more and look for multiple shading opportunities inside and out.
Northern Exposure: In this hemisphere, the least opportunity for solar gain. A dual-pane window with a standard low-E coating on the inner face of the outside pane (cold climate) or the outer face of the inside pane (hot climate), is sufficient. “We always recommend a low-E window for north-facing windows for its insulating value alone,” says Val Brushaber, director of product management, certification, and architectural development for Hurd Windows & Doors. The number and size of windows can be dictated by views, exterior aesthetics, and floor plan as much as thermal efficiency, though fewer windows is always better in that regard. North is also notorious for prevailing winds, so think about air infiltration and passive ventilation through casement windows (instead of hung units) or fixed windows to lessen leakage.
Eastern Exposure: Rich in daylight, but far cooler than its opposite exposure. You can dial up the SHGC rating to 0.40 or more, especially in heating or mixed climates, while a U-factor of 0.30 is plenty to retard thermal transfer through the window.
AN INTEGRATED APPROACH
The breakfast room scenario, and even more dramatic examples of homes that use windows of varying U-factor and SHGC values to take advantage of passive solar heating, cut down on heat gain from a western elevation, or save costs on a northern exposure, underscores the merits of a building science (or integrated) approach to energy efficiency and thermal comfort.
“In reality, everything is related [to thermal performance and comfort],” says Saxler. “You need a total home evaluation if you want to fine-tune your windows.” An energy audit by a local rater certified by the Residential Energy Services Network (www.natresnet.org) is a good place to start, while most window manufacturers and/or their dealers employ software programs to determine ideal specifications based on other, whole-house factors, such as wall insulation values.
Gaiser is even more blunt about the role of windows in lowering a home’s energy demand. “If you want to save energy, put in fewer windows,” he says. Upgraded insulation costs far less than high-performance windows, he says, and results in a better thermal envelope and a faster return on investment. “Windows rarely pay back as well as other energy-efficiency measures. Put them in for light and views, but not for energy efficiency.”
Which takes us back to the original question of whether specifying window performance for each orientation is worth the effort, at least in terms of thermal value. For builders, the decision may come down to marketability. “It’s an opportunity to stay one step ahead of the competition, to offer a house that’s more comfortable and efficient than the next guy,” says Saxler. “If you do the building science, you’re in the top 10% of all builders.”
Contact
Scotty@stlouisrenewableenergy.com for your next Window Order
RESOURCES:
The Efficient Windows Collaborative:
http://www.efficientwindows.org/
Energy Star (Qualified Windows):
www.energystar.gov/windows
Lawrence Berkeley Laboratory, Windows & Daylighting Division:
http://windows.lbl.gov/
National Fenestration Rating Council:
http://www.nfrc.org/
Residential Windows, A Guide to New Technologies
and Energy Performance
by John Carmody, et. al., available at http://www.wwnorton.com/
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