Hi guys –
Is there a simple formula to determine the optimum roof pitch and soffit width combination that yields the most winter solar gain, while minimizing solar gain in the summer months?
A friend reports that at his building lot, the June sun rises to a 63 degree zenith above the horizon. In January, it rises to a 13 degree zenith. Local latitude is 49.30.00 North.
The house would be sited and built with passive solar in mind, with the main roof plane facing directly to the solar south. It may later on be retrofitted with electricity generating panels.
URLs for reliable solar sites would also be welcome.
Thx.
Replies
"Is there a simple formula..." Unfortunately, the answer is probably "no". You might take a look at the following website, however:
http://www.eere.energy.gov/consumer/your_home/designing_remodeling/index.cfm/mytopic=10280
Pierre,
Good question. The answer will always be somewhat of a compromise, since there is a significant time lag between the day with the highest sun angle (the summer solstice) and the hottest period in summer. To put it another way: during the spring equinox (March 21), most people like the sun to stream into their house; during the autumnal equinox (September 21), sun may be less welcome.
Check out this spreadsheet from ourcoolhouse.com
http://www.builditsolar.com/NewsLetter/newsletter.htm
If you can't find the answer here it doesn't exist.
The site owner posts here occasionally, maybe he'll see your post and chime in.
Joe H
Thank you for your replies.
MartinHolloday makes some good points.
At your latitude, an angle of roughly 60 deg from horizontal would be a good bet. This will maximize solar gain on that surface with an emphasis on the winter. If you have very cloudy winters (you didn't say where you are), this angle may not be very good for PV systems vs. solar thermal (domestic hot water or space heat).
For summer shade, draw a 60 deg angle across the section of the south wall. Try to have enough overhang to shade most of the window at that angle. A little science, a little artistic license. Combined you end up w/ a good performer that looks reasonable. Draw about a 20-30 deg angle to check the height of the window to the underside of the eave. Don't let the hard numbers dictate it all, but don't ignore them, either.
If you are going for passive solar, direct gain type of strategy, protect south glass w/ the eaves. Avoid east and especially west glass that is not protected (eaves don't protect them much)(also see my comment about windows in another comment).
Mark Williams, HR, OR
Thank you - that makes sense to me.