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SHGC .61 ufactor .47  RSS feed

 
Gary Hughes
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I'm trying to figure out how many windows or sq. ft. of windows to put on my south facing wall. I'm thinking of building a cob house 40'x24' with an earthen floor. I found a guy who can order SHGC .61 and a ufactor of .47 So if my building is roughly 1,000sq. ft. is it correct to put about 12% of that or 120sq.ft. of windows on the south side? I don't want to overheat in summer and I do want to get as much thermal mass heated up in winter, so how do I strike that balance? I do plan on overhangs and possibly exterior shutters or other means of summer blockage of windows if that would help? And at the same time I'm worried about heat loss in winter at night so I'm thinking of thermal blankets or something on the inside at night during winters. I really am having a hard time figuring out a balance that seems realistic. I don't want to put too much or not enough of the right or wrong glazing etc. I'm in Oklahoma NE. Thanks Gary
 
Gary Hughes
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The other thing I'm not understanding is I see passive solar houses and they are almost all windows or very little wall between windows on the south side, and yet if I'm understanding the 10-12% "rule" correctly then my south facing wall would have a lot of wall between windows which just seems counter intuitive.
 
Brian Knight
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Location: Asheville NC
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There are no hard rules, only generalized recommendations. The 10-12% is a good one to shoot for on the majority of projects. It can be adjusted up or down depending on the details. The solid wall area is important for structural strength. "Shear" strength is the main engineering characteristic that walls with a lot of windows need to pay attention to. Its possible to use a higher engineered solution like steel or simpson strong panels to make up for the lack of wall area but the most affordable way is to leave some south wall areas un-glazed and increase the standard framing connections for the smaller areas of solid wall. In my article I point to the corners and stairwells as being the most attractive locations for solid wall area.

Iam going to take the high road and advise you to check your window selection in RESFEN, a software program used for selecting windows. NE OK is in building climate zone 3 very close to 4. The 2012 IECC calls for prescription minimum Uvalues of .35 in zones 3 and 4. The code also says max SHGC of .35 (.4 for zone 4). The performance path allows high shgc windows with higher Uvalues but .47 is pretty low performance. SHGC of .61 is really good (for whole window) but doubt its a good balance. It would probably be better to have a lower SHGC if you can improve the Uvalues by a third which is much closer to typical passive solar windows.
 
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