Heat gain comparison between a non white roof and a cool roof

[immsk]

Hi Guys,

I am trying to do some calculations to figure out the decrease in heat gain through the roof due to the installation of a new water proofing membrane (Lexcan TPO 1.1mm). I have the SRI for the new membrane but I am at a loss on how to proceed from here. Any pointers would be helpful.

 

[IRstuff]

One choice is to calculate the amount of absorbed solar load vs. a conventional roof. However, there doesn't appear to be anything in the Lexcan literature that tells you where it transitions from 75% reflectance to 92% emissivity. Based on a 5600K blackbody profile for direct solar load, there's somewhere between 56% to 70% of the solar load applicable to the reflective regime and the remainder can be assumed to be applicable to the emissive regime.

TTFN
faq731-376

 

[Zesti]

Maybe you can do a "guesstimate" by guessing that the surface temperature for the light-coloured material might be around 40 degrees C whereas a dark roofingmaterial might get up to 60 degrees C.

Using these temperatures for a transmission heatgain calculation should give you some idea of the difference the roofing material makes.

Depending on the insulation of the roof the difference might not be that much because there are also walls/windows and internal heatloads that add up to the total heat load and these are unchanged by the roofingmaterial.

So while the heat gain through the roof might be influenced quite significantly, it might not be a significant difference for the total heatload.

Anyway, I always feel it is usefull to try and approximate the proportions of the effects involved before trying to calculate something to several decimal places, because it might not be worth the trouble.

 

[DRWeig]

Don't forget -- a roof gets dirty. Your 0.75 reflectivity will be less than 0.50 in a few years.

Good on ya,

Goober Dave

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[immsk]

thanks guys