[Greenbuilding] South-facing windows are net energy gains

[John Straube]

Most windows are energy holes, this is not a bad rap, it is a current reality in the same way that most cars dont not get 60 mpg, or go 0-60 in 3 seconds.  Some do, most dont.  Triple-glazed windows in common wood, vinyl or aluminum frames are actually not always that good. A few good fiberglass and vinyl products are out there, but it is actually quite difficult if you did down into what they actually produce and deliver.
A very few windows are, over the season, able to gain more energy than they lose regardless of which direction they face, which is what the ER rating describes.
The ER assumes that all windows have full access to sun and no shading by adjoining buildings, window reveals, or terrain and even no flyscreens.  Hence, you should look for an ER of around -5 to -10 to actually get a zero net energy window.
As stated, a south-facing window with an ER of +5, might perform as a true (including real life shading losses) of ER -4.  Still not a bad deal.  Just BTW I would guess 99+% of windows installed in cold climate North America have an ER of less than +5 and so are NOT net energy gainers.

Another serious caveat that I see in houses all the time is the major error in design caused by the reasoning that "If 1 m2 of window gains more heat over the season than it loses, then I will cover my entire south wall with such windows" (or cover 50% of wall or whatever).  On average over the season means that even super good ER+10 fixed windows loose 5 times or more times more heat than a super insulated wall when the sun is not shining brightly (which is 12 hours/day in northern climates Dec/Jan/Feb).  Adding large areas of south superwindows, especially those with postive ERs runs a real risk of overheating in the fall and spring on sunny days, and cold temperatures and demand larger heating systems on January nights.

The ER rating is based on a pretty mediocre house (built to code essentially) which means the house assumed has a heating load from October through April. As you make the house better, the heating season shortens (to say Nov through March) and a great proportion of the heating load is Dec through February.  The ER would change, in that it would become significantly more negative, in a super insulated house because there is less sun available during the times of the day and the year that you need it.

As a consequence, in climates as far south as Boston, adding windows with an ER of 0 (a tremendously good value for an operable window) or so, ignoring any real world shading, to a well insulated airtight house results in more energy consumption.  If it were truly not zero energy, I could add more window with no energy consequences.  If you try this modeling exercise in Edmonton, it is even more true (as the window R-value matters more in really cold places and solar heat gain is not available in cold weather in northern locations).

All of this is just to say, ER is not a very good indicator of the energy balance of a window in a well insulated house.  It is an indicator of a good window.  But U-value (R-value) is actually a more important indicator for most super insulated houses in northern cold climates and choosing a window based on ER can lead to sub optimal choices.

BTW. Students and I built a quad pane fixed window system with insulating frames for our super insulated solar decathalon entry in '09.  I estimated this window to have an ER of around +20 to 25.  With this level of performance (an R-value over about 8.something U<0.12) with low-iron high solar gain glass, we could add as much south glass as we wanted and it did not cause the house to use more or less energy over the season in the Toronto (HDD6800F) climate.  What it did require was exterior operable shades controlled by a computer to avoid melting the interior during periods of bright sun and modest outdoor temperatures.  Go to team-north dot com for photos.   This is merely an extreme built example of this thread of thought.

> Windows That Perform Better Than Walls
> 
> If you choose the right glazing, your windows can gather more energy than they lose
> 
> The common perception that windows are “energy holes” is a bad rap. Since today’s high-solar-gain triple-glazed windows gather more heat than they lose, good windows perform better than an insulated wall. After all, a wall can only lose energy, while windows can gain energy during the day to balance energy lost at night.

Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture
www.buildingscience.com

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