In the building which I’m modelling there are plastic skylights which are practically opaque. In the baseline shall I model the skylights according to tables 5.5 of ASHRAE 90.1, which requires SHGC values? Is comparing an opaque surface with a transparent surface sensible? Unfortunately the U-factor of those skylights is not available. Shall I model the skylights both in the baseline model and in the proposed building model with U-factor according to tables 5.5 of ASHRAE 90.1?
Best Regards
Francesco
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 14, 2012 - 5:06 pm
If any light comes through I would think they would be considered skylights and should be modeled as such. Comparing opaque with transparent makes no sense, so are they "practically" opaque or opaque? If fully opaque then I would treat it like a spandrel panel - in this case as part of the roof.
If you model as a skylight and do not know the U-value use the information in ASHRAE 90.1-2007 Tables A8.1A and B.
Francesco Passerini
engineer90 thumbs up
May 15, 2012 - 11:57 am
Thank you, Marcus.
I’m trying to get more information about those skylights. A question about tab. A8.1A (I’m using the SI version): in par. A9.4.1 “Air Films” for exterior surfaces R-value = 0.03 and for interior horizontal surfaces, heat flow up R-value = 0.11. Therefore for skylights the total air film resistance is 0.14, whose multiplicative inverse is 7.1W/(m^2*K). In tab. A8.1A some values are higher than 7.1W/(m^2*K). How is it possible? Does that table consider also air infiltration?
Best Regards
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 15, 2012 - 1:50 pm
Not sure about your first issue.
I do not think the table accounts for air infiltration.