What if a roof slab which is covered with an assembly having excellent SRI(>78) & U V values as per Energy modeling requirement, is covered with an non-compliant material (say grey gravel), will not the underlying assembly protect the building from being heated-up owing to its excellent UV & SRI values? and should it not be that the non-complaint top layer be exempted?
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Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11474 thumbs up
January 16, 2018 - 8:40 am
Faisal, that won't work. Much (I'm afraid I can't quantify how much -- maybe another expert could) of the SRI performance comes from the material being able to reflect light and heat away from itself. It needs an airspace facing the sky to do this, and arguably direct exposure to the sky.
Michelle Teague
Architect, LEED ConsultantPolk Stanley Wilcox Architects
39 thumbs up
January 16, 2018 - 1:22 pm
Why would you cover the high SRI material with anything else? If the high SRI material is causing glare when viewed from above or adjacent space then covering with a green roof or photovoltaics could address the problem and contribute to other credits. The topmost material is the only one that counts for this credit because it is about changing the microclimate that dark roofs create. Best of luck on your project.
Faisal
LEED AP(BD+C)January 17, 2018 - 7:48 am
Thanks tristan....your are right....but say if we ponder over above methodology a little further...
(please correct me if I am wrong in below assumption)
Now we know that the SRI of a material is its ability to stay cool in the sun by reflecting solar radiation and emitting thermal radiation, in our case the underneath thermal insulating polystyrene board(with requisite U R values) will not experience any exposure to sun and hence its reflecting properties will not come into picture.
Now the top most layer( gravels) are exposed to sun, due to its poor reflecting properties(owing to its dark color) there'll be considerable amount of temperature gain and consequent cooling by radiation( emissivity), here the heated-up gravels though would be able to transfer its heat to the surrounding upward space(air) but won't be able to transfer heat through underlying polystyrene board( due to its requisite R & U values)...and hence there wont be any heat gain by underlying room.
But since the surrounding air at roof would get heated up(from radiating gravels) which then would be used as intake by FAHU, this will drive extra energy from FAHU.
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11474 thumbs up
January 17, 2018 - 8:50 am
Faisal, the polystyrene will slow conductive heat transfer from the gravel to the underlying room and building, but it won't stop it. If the gravel is hot, more heat will be transferred to the building, driving up cooling loads.
If you put a high-SRI surface on your roof and then cover it with a low-SRI surface, you will get low-SRI performance, which doesn't fit with LEED.