Windows have come a long way from the single-pane, aluminum-framed units of 40 years ago. Today’s windows boast high-performance coatings, tints, gas fills, suspended films, spacers, and frame material options, with a variation of specs that enable pros to finely tune and optimize window placement and performance depending on a wall elevation’s orientation and exposure to the sun.
Except very few builders do that; the vast majority choose the same performance values for every window in the house. And why shouldn’t they? In addition to cost and convenience issues, the latest rules to qualify for federal energy tax credits and the Energy Star for Homes program make no distinction—and for the tax credits, not even for different climate zones—among window values and options beyond a maximum U-factor and solar heat gain coefficient (SHGC). Even the two prevailing green building certification rating systems, LEED for Homes and the National Green Building Standard, defer to National Fenestration Rating Council or Energy Star criteria for window values as a prerequisite to certification (albeit with climate zone and glazing ratio exceptions).
Both rating systems do offer extra credit for windows with higher performance values and allow “any” SHGC value in northern climates, but make only scant mention of passive solar design as a hedge against heating energy demand.
Setting and promoting common denominators for U-value and SHGC has, in turn, prompted window manufacturers to focus on selling products that meet those standards and qualify for tax credits and other financial incentives, even though they have every commercially available glazing configuration at their disposal.
“They want to supply one product for all orientations, or for the whole country,” says Dariush Arasteh, deputy leader of the Windows & Daylighting Group at Lawrence Berkeley National Laboratory (LBL). “It’s the most efficient thing for them to do in terms of manufacturing and distribution.”
Suppliers don’t disagree. “It’s more economical to buy and sell and stock the most popular glass options than stocking a wider range of glass,” says Kevin Vilhauer, an engineer with Milgard Windows.
Which begs the question: Are there enough reasons to consider each elevation’s exposure when specifying windows to offset the convenience and cost incentives, from tax credits to volume discounts, of ordering across-the-board performance values for the whole house?