Concentration of CO2 in the Atmosphere

A Question of Windows

The outer envelope upgrade was well underway during the summer, with taped Kohltech windows and Huber Zip System sheathing. All photos courtesy of H. Sloane Mayor.

Barb and Greg Whitchurch

Windows. Light. Views. A contrasting relief to the inside of our homes. Also, an added expense, a challenge to keeping the inside temperatures under control, a need for cleaning. But here, let’s take a look at the efficiency aspects of window choices.

In general, the more efficient a window, the better its longevity, ease of maintenance, energy savings, and personal comfort. Today’s best windows are triple- or quadruple-pane, triple-gasketed (no felt padding), Passive House-rated, have U-values well below 0.20 (R-6 or better) (ours are U-0.11 or R-9); and air infiltration well below 0.30 (ours are 0.03). These attributes help keep a home warm in winter and cool in summer.

Inside envelope of Intello variable permeable membrane with Tescon Vana acrylic tape for air and moisture control. The two right Kohltech windows are both operable.

Up here in the North, where we want our south-facing windows to bring in solar heat in the winter, the Solar Heat Gain Coefficient (SHGC) should be above 0.45. (Our windows are 0.62.) By contrast on the east and west-facing windows, we probably don’t want the summer heat to come into the house, and there is no appreciable sun there in the winter, so a lower SHGC is a good idea. Regardless of SHGC, south- and west-facing windows should have shading for the summer such as leafy trees, vines, awnings, and so on.

In the building industry nowadays, the terms “energy efficient” and “high performance” are tossed around quite cavalierly. Advertisers are hoping that people will jump to conclusions and not ask the hard questions. Any window supplier who hesitates to provide the above-mentioned specifications should be avoided.

Simply claiming that a window is “top of the line,” “new and improved,” or “ultra-high efficiency” doesn’t say enough. Likewise, claiming that a window has double-thick glass, one inch or better thermal pane or triple-pane, is low emittance (low E), with argon gas, or has foam-filled frames, is not sufficient. A friend of ours recently got a quote for some windows from a very well-known company that advertises heavily. But when he took the trouble to look a little further, he found that he could get Passive House-rated windows with better specifications for less money.

H. Sloane Mayor (, one of the principals at MAKE Architects in Hanover, NH, renovated her older home in Hanover with a deep energy retrofit (DER). Among the many decisions she faced was that of choosing window replacements. Previously, in her design of the 2018 U.S. Green Building Council (USGBC) NH Chapter Building of the Year, a LEED Silver DER private residence for a client, she had specified Passive House-certified Kohltech windows, which had performed incredibly well and had greatly pleased the owners ( So, she chose Kohltech for her own home.

Both of these projects were contracted by O’Hara and Gercke (, and the windows were acquired through Loewen Window Center in White River Junction, VT ( Other Kohltech placements include Summer Park Residences in Hanover, NH ( and the Bente Building at the AVA Gallery and Art Center in Lebanon, NH (

Siding and the new porch and deck added onto the back of the house.

A little deeper dive: although most people understand the importance of insulation, did you know that most older homes lose more of their heat through air leaks than through lack of proper insulation? In Sloane Mayor’s DER, the Kohltech windows support the overall insulating and air-tightness targets, allowing the home to switch over to heat pumps for heating and cooling. Note that the window installers must be trained in modern techniques to assure that they don’t compromise airtightness with old-school installation shortcuts — here, membranes and acrylic tapes come to the rescue.

As architects know, upfront cost increases, if any, are quickly offset by energy savings alone — even without considering the health, comfort, safety, environmental benefits, and the increase in the home’s value. Mayor’s design, material, and appliance choices have created a highly efficient, all-electric home that uses heat pumps (also providing hot water) instead of a furnace or boiler, and will cost very little to run and maintain. In such an all-electric home, on- or off-site solar PV can be leveraged to provide all of its energy cheaply and sustainably.

Our own 2001 home was “high performance” when it was built; in 2014 we built a Passive House (PH) addition. The two are connected with an enclosed breezeway. Last fall, we replaced all 110 square feet of Andersen thermal pane low-E, argon-filled, circa 2000 windows in our kitchen with PH-certified Klearwall windows. The difference has been striking. It’s quieter; we’re using far less firewood (we heat only with woodstoves); we can shut down our stoves and don’t have to reload during the night; the house is noticeably warmer when we get up in the morning. No question about it!

Barb and Greg are board members of and have their Passive House in Middlesex, VT,

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