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Window shading in a cool climate - is it needed?

I'm designing a small up/down duplex in a mountain climate with cold winters and cool summers. Do I really need or want window shading overhangs with a relatively high south glazing ratio (13%)?

Today is a fine June day with weather that is not at all out of the ordinary for the area. Today's high is about 10C. The indoor temperature is 15C. A little extra solar heat gain would be quite desirable now, not to mention in March, April, May, September and October. Since solar shading is the same in mid-August as mid-April, I'm pretty hesitant to block the solar gain that I want in April in order to stay a little cooler in August. HOT2000 is definitely indicating that I will need more heat if I block a good portion of the solar heat gain in March and April.

I'm assuming that I can control overheating by just opening a window or in extreme cases opening windows and running bathroom and kitchen exhausts, plus the HRV in exhaust mode. It is probably only hot enough that I would normally consider closing windows during the day for one or two weeks per year. Humidity is low, so not a concern.

The details:

Up/down duplex, 800sqft interior per unit
13% unshaded S windows with high SHGC
4% unshaded E&W windows with low SHGC
250 sq ft of 2.75" Gypcrete per unit as thermal mass, plus some doubled drywall
R44 walls, R80 ceiling, R40 floor over unheated basement
50N in SE British Columbia, 4400 HDD (Celsius)
Can provide HOT2000 modelling details if it helps

My plan is to use a small, high roof overhang on the S side for the top floor providing minimal shading and no shading on the bottom floor. If those 1-2 weeks of hot weather per year are just unbearable, I'd consider adding shading in the future. Most guidelines would indicate I'm headed in a bad direction, but I'm not so sure they apply to my specific climate. I know that builders on the nearby Canadian prairie use significant shading in a similar heating climate, but their summers are much hotter than mine.

I have read with interest A Contrarian View of Passive Solar Design, but I'm designing with far less solar mass and without active heat transfer techniques (and I'm leary of any techniques running air through slabs). I'm also not a big fan of adjustable awnings.

Does anyone know of other examples of this working? Am I crazy to consider this?

Asked by Lars SG
Posted Tue, 06/17/2014 - 15:55

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Lars,
Some types of software, notably PHPP, are supposed to be sophisticated enough to provide an answer to your question. I have my doubts about such software, and I feel perfectly comfortable advising you based on my experience.

Based on my experience, I wouldn't proceed with your plan. I would reduce the area of the south-facing glazing, or I would provide better shading for the windows, or both.

Your glazing ratio is quite high -- on the upper end of what passive solar designers have used historically. The usual advice for such a house would be to provide lots of thermal mass located somewhere in the house where the sun will shine directly on the mass, and to provide roof overhangs that are wide enough to completely shade the glazing on June 20.

You don't want to do that. If you go ahead with your plan, I advise you to install a powerful air conditioner.

Opening windows or turning on fans to cool an overheated passive solar house only works if someone stays home all day to make the needed adjustments. What happens in March if you have too much south-facing glass? The homeowners come home from their jobs at 5:00 p.m. and discover that it's 80 degrees F inside.

Of course, there are many factors to consider. I don't know if you are planning to install high-solar-gain glazing (the type of glazing recommended by passive solar designers for south-facing windows) or the more commonly available low-solar-gain glazing.

And climate matters -- so seek local advice from experienced designers in British Columbia.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 04:30

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