Consider a bump-out breakfast room facing east, surrounded by tall windows to capture views and morning sun. Specifying triple-pane, low-E windows with a marginal SHGC rating (perhaps 0.40 or higher, depending on climate) for that particular circumstance, says Gary Gaiser, an architect with Pella’s Architectural Support Services division, boosts the windows’ ability to retain heat and create a warmer interior surface (thanks mostly to the third pane) while also benefiting from some solar heat gain to achieve a comfortable morning environment, perhaps without the need to jack up the thermostat.

That spec also enhances comfort by mitigating the formation of a “convective current”—essentially, warm room air chasing air cooled by the window’s surface, creating an air stream that most people perceive as a draft from a leaky window. The warmer interior surface of the glass also alleviates radiant heat loss from the occupants themselves—again, a dynamic of warmth trying to heat cooler surfaces or air.

As a result of all that applied science, the home’s heating system doesn’t have to work as hard (if at all) to warm up the breakfast room or take the chill off the homeowners; it may also cut down on the home’s lighting demand. “With the number of glazing options available, you can pick and choose applications for different parts of the house,” says Gaiser. “You get the most [economic] value by specifying the same [high-performance] glazing across the board, except where you need a different spec to address comfort.”

That kind of “spot specing” of windows, and the resulting impact on other systems and comfort, also may allow builders to balance the premium cost of a few triple-pane windows in a breakfast nook against the savings of right-sized (smaller) heating equipment and distribution schemes (fewer and shorter ducts) and perhaps fewer lighting fixtures, not to mention the energy those systems consume. “The initial cost savings could be in the hundreds of dollars,” Vilhauer says.

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.

RULES OF THUMB

For a 2,815-square-foot, passive solar house, Ft. Worth, Texas, custom builder Don Ferrier’s client, Morgan Bodie, used a software module within AutoCAD to simulate seasonal sun paths to calculate and confirm his window specs in light of other building practices, including an 8-inch-thick insulated concrete slab (stained and sealed as a finished floor) to serve as a thermal mass and structural insulated panels for the home’s walls and roof to reduce thermal bridging.

Under those conditions, and given his ability to orient the house along an optimum east-west axis, Bodie specified dual-pane windows with no low-E coating on the home’s south elevation, shaded by 4-foot roof overhangs to mitigate solar gain in the summer. “It was actually hard to order windows without a low-E coating,” Ferrier recalls.

Meanwhile, the limited number of windows along the west exposure feature low-E coatings on the interior faces of both panes and also are shaded by roof overhangs and trees. Bodie took advantage of the home’s best view to the north by specifying standard, low-SHGC-rated insulating windows for high visual light transmittance without an energy penalty or premium cost. “If you don’t pay attention to this stuff,” says Ferrier, “it’s energy efficiency suicide.”

A preliminary audit of the house, scheduled for completion this summer, estimates a HERS rating of 48 and a reduction in annual carbon dioxide emissions of more than 14,000 pounds. –R.B.