Hello,
Assuming we are using Option 2 - Prescriptive compliance Path, the roof skylight of our project would need to abide with the U-value and SHGC listed on Table 5.5-3 (zone 3) of ASHRAE standard 90.1 -2007 SI (i.e. international version)
In our case, ASHRAE requires the U-value to be 3.92 and the Solar Heat Gain Coefficient (SHGC) to be 0.19.
It seems to me that this SHGC coefficient is extremely low and quasi- impossible to find on a market without paying a fortune.
Does the USGBC expect us really to install a roof skylight with a SHGC of 0.19? Isn’t there some kind of error? Perhaps there is a conversion to be made?
I would appreciate some help on this.
Many thanks,
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
April 16, 2010 - 4:19 am
SHGC 0.19? It's not an error. I've had this problem before. Insidently, have you managed to find a glass type with a U-value 6.81? They also, as far as I can tell, don't exist. Insidently, have you noticed that TABLE A.8.1B 's lowest listed SHGC for skylights is 0.27? To me this basically declares (and maybe rightly so) that you shouldn't install skylights in these climes or if you do the technology must be (theroretically) efficient enough.
The simulation method is not bound to these restrictions.
Christopher Schaffner
CEO & FounderThe Green Engineer
LEEDuser Expert
963 thumbs up
April 17, 2010 - 4:06 pm
As Jean says, the standard is trying to discourage large skylights in these climate zones. For smaller skylights up to 0.39 is allowed.
Also remember that SHGC includes any external shading installed.