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Stephen's Column
The Road Less Travelled
Robert Frost, the famous American poet, wrote “Two roads diverged in a wood, and I – I took the one less travelled by, and that has made all the difference.” These words reflect both the uniqueness of individuals and the universality of the human experience. They can be extended to many circumstances. They can even be extended to Thermotech's approach to window energy efficiency.
Ask nearly any North American, or even the European, window sales person about energy efficiency and you'll get the same answer. They'll start talking about insulating value (either U-value or its inverse, the R-value). They'll talk as if a window is no different than a wall. They promote the idea that the lowest U-value produces the lowest energy bill.
In doing so, they are getting the science 100% incorrect, (see Stephen's Column – All Low E is not created Equal).
Besides from being wrong, equating a window's insulating value with its energy performance is brutal marketing. Even a 2x6 wall normally insulates about 600% better than a typical Energy Star (US northern zone) window. It follows then, that an energy efficient home would minimize the number and size of windows. Not exactly clever marketing for window makers.
Marketing aside, the simple fact is that windows are a unique element of the building envelope. Unlike walls, roofs and foundations, windows can be a source of heat gain. For buildings that need to be heated, this can be a very good thing.
Unlike most other window makers, Thermotech recognizes this fact. Although we offer a multitude of glazing options, we are biased towards high solar gain glazings. When combined with our slim insulated fiberglass frames these glazings produce super energy efficient windows. Windows that not only insulate well, but also capture solar gains.
Compared to a typical Energy Star (US Northern zone) window, our best triple glazed windows insulate about 50% better, and capture about 1/3rd more solar gain. This means that Thermotech's windows are dramatically more energy efficient than a typical Energy Star window.
Consider a 600mm x1200mm (about 24" x 48") casement window that faces south in Ottawa ON. If that window was a typical Energy Star casement, it will gain 28 kWh over the heating season. If it was Thermotech's best casement, it will gain 118 kWh over the heating season. That's a 400% improvement.
Surprisingly to most, that same south facing window will not cause extensive overheating in the summer. The sun is just too high in the sky. Consider a south facing window at 40 degrees north, in mid-June. It receives about half the solar gain it does in February. And this diminished solar gain is only 40% more than the solar gain of a north facing window.
Usually overheating comes not from south facing, but from east or west facing windows. On the longest day of the year, they can experience 115% more solar gain than a south facing window.
It is true that summertime solar heat gain can be controlled with low solar heat gain glass. However, doing so means the substantially more valuable wintertime solar gains for south facing windows are largely wasted.
It's usually more effective to control excessive solar gains with exterior shading. Exterior measures such as deciduous trees, vines, overhangs and awnings can all provide cooling season shading; without sacrificing cherished heating season gains. Interior window treatments such as blinds, shades and curtains are never as effective as exterior shading. Even white interior blinds only reduce solar gains by 20%.
Usually overheating comes not from south facing, but from east or west facing windows. On the longest day of the year, they can experience 115% more solar gain than a south facing window.
Clearly then there is more to window energy efficiency than insulating value alone. Thermotech has taken this less travelled road; a road that recognizes the unique opportunity presented by windows. This is the opportunity not only to reduce heat loss, but also maximize usable solar gains.
- Frame In? Frame Out?
- All Low e’s Are Not Created Equal
- Double Hung Inherently Flawed
- The Road Less Travelled
- The Affordable Energy Efficient House
- Green Priorities - Skin Deep Not Deep Enough
- Embodied Energy - As Important As Low Energy Design?
- Energy-Star's Solar Eclipse Ending?
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