By Greg Whitchurch
Intus Windows will launch its “One Window, One Tree” campaign on Earth Day, Sunday, April 22. For every commercial window sold, they will sponsor the planting of a tree, to contribute toward offsetting the carbon footprint of the building.
Intus manufactures passive-house-quality triple-paned windows and doors and they are partnering with tree-planting organizations to plant the trees near the facility where the window was installed. More information is available at their website https://www.intuswindows.com/.
Although at this time Intus only supplies windows to commercial projects, let’s talk about how one might check into a window’s specifications for residential projects, too. The National Fenestration Rating Council (http://www.nfrc.org/) is a U.S.-based organization that tests and rates windows and doors. Until very recently, some top-line European windows have been considered the best of the best, as far as both highest performance and most reasonable cost are concerned – and with good reason. But there are American and Canadian competitors entering the market.
A window’s ability to keep heat in (or out) through its thermal resistance is reflected in its U-value (http://www.nfrc.org/energy-performance-label/). Here in the U.S. we typically speak of R-values – which is simply the inverse of the U-value (U=1/R or R = 1/U). So while a typical “high-end,” U.S.-made window might have U-values in the 0.30s or even 0.40s, actual high-performance windows will be less than 0.20. When we bought our Intus windows in 2013, we paid less for them than the best name-brand U.S. makers, and they provided more than twice the insulating protection – almost as much as a two-by-four fiberglass-insulated wall. The R-3 to R-4 we’re used to finding for windows becomes R-8 to R-11 when you move upscale.
Low-e (low-emissivity) windows have a coating inside the outer pane of glass and, in triple-pane windows, inside the middle pane of glass in order to reflect heat back into the house. The cavities between glass layers can also be filled with inert gas (usually argon or krypton) to reduce conduction of heat through the window. The spacers at glass edge matter a lot, too.
Window frames are relatively thin but must have enough “beef” in them to support the window safely for decades, and perhaps to support opening, closing, slamming, etc. The construction of the frame is, therefore, additionally complicated when one attempts to make them super-insulating. Multiple small air channels throughout the frame provide strength while insulating; some are foam-filled.
Operable windows (sliding, double-hung, casement, awning, hopper, etc.) also risk air leakage around the moving contact surfaces, along with increased conduction through the hardware and the extra framing materials. Most European windows eschew the sliding and double-hung operations because of the severe engineering challenges presented, the additional expenses, and the resulting loss in performance. They mostly opt for “tilt-and-turn,” where the window can tilt inwards from the top, or swing inwards like a door. Once one gets used to the change, the clear advantages regarding bad weather, bugs, breezes and cleaning can make one wonder why anyone would choose otherwise.
The glass itself can vary widely in its characteristics. Of course, tempered, tinted, mullioned windows are available. The window’s solar heat gain coefficient (SHGC) describes how well it’ll let the sun through to help heat your home. We used the U-value and SHGC specifications to determine what performance characteristics we wanted for each outside wall of our house. North: fewer windows, low U-values, SHGC didn’t matter. East: we have a hill there so SHGC was unimportant, but low U was. West: western sun in the winter is pretty sketchy so, again, low U guided us. South: high SHGC was very important, more important than the U-value.
My message is paying attention to just the two specifications briefly discussed above can guide you in the right direction. Low U-value for better insulating; high SHGC for solar gain. If you’re building or remodeling to the Passive House standard, all of these characteristics are automatically included in the modeling.
Barb and Greg Whitchurch are board members of Vermont Passive House and own a net-zero passive house with solar PV and hot water in Middlesex, Vermont. http://bit.ly/2nRCdGL (802)223-2416.