Will southwest facing windows get too hot in zone 5A?
We’re currently working on plans for our new house and our design and plot plan have southwest facing windows for the three kids’ bedrooms. I’m concerned about the late afternoon summer sun shining directly in the windows and overheating those bedrooms. There will be some shade on the two bedrooms furthest to the south, but the corner bedroom will not get any shade for a few hours.
I estimate the biggest problem in the early and late summer assuming I decided to put horizontal awnings above the windows to block the high sun.
Ive attached the plot plan showing the home orientation. The few trees closest to the house will need to come down since they are diseased chinese elms. The home is rotated this direction to allow a walkout on the northwest side.
Will the 2-3 hours of directly sunlight from 4-7pm really be a problem or am I worrying too much? Will I be able to resolve the issue with interior shading or low-e windows? Or are my only options rotating the house or installing awnings that cover the top two thirds of the windows?
Thanks in advance!
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Replies
Erik,
You don't want low-e windows; what you want are low-SHGC windows.
SHGC stands for "solar heat gain coefficient." For more information on choosing the right type of window glazing, see All About Glazing Options.
Excellent! Thank you. Will the low-SHGC windows alone solve the problem without making any other design sacrifices? With low-SHGC windows do I even need the horizontal awnings to block the higher mid-summer sun?
Erik,
Any window facing southwest will increase your cooling load on summer afternoons. That doesn't mean you won't be comfortable. If your air conditioning system is designed properly, you can keep the room at any temperature you want. The bigger the windows, and the higher the SHGC of the glass, the higher your air conditioning bill will be.
Designing a house is a balancing act. You have to decide what size windows you want. Some people like very big windows, and just write the electric company a big check every month.
All low SHGC windows are low-E. Low SHGC is a matter of the low-E coating type(s) and which window surface(s) the coating(s) is (are) on.
The amount of solar gain to the bedroom is a function of the site shading factors & orientation of the window, and the size of the window. There is no free lunch, to limit the cooling load means you limit the size and SHGC factor of the window, but you'll lose some of the desirable passive solar gain in winter.
Adding thermal mass to the room commensurate with the window sizing can mitigate most of the peak temperature issues for that 2-3 hour insolation period without losing the desirable wintertime solar gain performance in a zone 5A climate.
Whether any special treatment needed or not is questionable though. Operable exterior roll down shades could always be added if your worst fears become manifest, but it's probably not a big deal unless you have an unusually large window/floor area ratio in that room. It makes a difference whether you're talking about 10 square feet vs. 50 square feet of SW facing glass in a 150-250' bedroom. You don't need to live in the dark, but you probably don't need a really great "sunset view" either.
Dana,
You wrote, "All low SHGC windows are low-E." That's true.
But it's worth pointing out to GBA readers (perhaps not to you) that not all low-e windows have a low SHGC. You can buy low-e windows with a (relatively) high SHGC.
The corner bedroom does have a large amount of windows. About 35-40 sf west-sw and 30 to the north-nw. Looking at the plot plan, I could rotate the entire house counter clockwise to make the direct sun higher in the sky. Would it be worth it or do you think there wouldn't be much difference between a higher angle sun and a lower sun? I guess I am thinking a higher angle might naturally reflect more heat off the glass.
Erik,
If you haven't built the house yet, and you are worried about the area of west-facing glazing, why not just eliminate some of the windows or make them smaller?
Zone 5A is heating dominated, rather than cooling dominated. Use the sun to your full advantage for the heating season, as nature intended. Situate the long axis of your house so that it faces (solar) south and design windows on that side to be about 12% of so of the conditioned floor space, integrating thermal mass, as necessary. Use BeOpt, Sketchup or any number of programs to design overhangs. In general, see the energy balance, for yourself, using BeOpt or HEED, or using any of several, free, programs. Find the HIGHEST SHGC, double pane windows you can and put them in the south side. It's a little hard these days to find much over 0.5 SHGC, but that's because of government, energy star bs. But it's still a fact that in a heating dominated climate, at least with more sun than, say, Vermont gets, south facing windows are net energy gainers over the heating season, and that the cooling season energy penalty is relatively minor.
What happened to the "Green" movement? First, a few years ago, we re-learned what our ancestors knew inside and out--that you put the broad side of a house toward solar south, with windows for heat gain in a heating dominated climate. Then, some later Greenies forgot all over again, and started advocating low-e and keeping the sun out. I live in the north part of Zone 4A, used the tried and true techniques of passive solar on my house, and I feel real comfortable that the sun doesn't send me a bill in the winter. Even for me, in 4A, much less 5A, the small cooling penalty to pay is insignificant relative to the heating gains in winter. My heating season is pretty much January and February, now, and it used to be about November to March.
South is one thing, southwest is another. It takes some pretty long overhangs to really shade the summer sun adquately, and going as high as 12% for a window/floor ratio on a SW window could get pretty steamy inside in July. Simulating energy use is always worthwhile, if you have time to learn the tools.