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Is thermal mass an issue when reroofing with metal over shingles?

I live in Charleston, S.C. (zone 3) and while I understand the structural considerations of installing a metal roof with purlins over older asphalt shingled roof, how does the thermal properties of the roof come into play on hot summer days?

Can the roof continue to absorb the heat in spite of having reflective panels, and could that heat be lost to the air space created by purlins or would it continue to radiate into the attic space?

Thanks

Asked by Marlo Sutphin
Posted Mon, 01/20/2014 - 15:33
Edited Tue, 01/21/2014 - 07:07

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5 Answers

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1.
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Marlo,
If you install metal roofing on purlins over old asphalt shingles, then of course the asphalt shingles will get hot during the summer. But then again, they always did.

You shouldn't be depending on your roofing to keep hot summer weather out of your house. If you want a better thermal separation between your hot roofing and your living space, you need to install more insulation.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/20/2014 - 16:18

2.
Helpful? 0

Thanks Martin, I figured that the shingles would continue to be hot and they always have gotten real hot but I guess I was wondering if that negates the thermal advantages of having a metal roof . I am having a hard time deciding whether to tear off my shingles or to reroof over them and am trying to comprehend the efficiencies of both options. Thanks again.

Answered by Marlo Sutphin
Posted Mon, 01/20/2014 - 16:31

3.
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Marlo,
My advice is to depend on your insulation, not your roofing, to keep your house cool during the summer.

Choose metal roofing if you like, but if I were you, I wouldn't count on your roofing to keep your house cool.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/20/2014 - 16:52

4.
Helpful? 0

Shingles exposed to direct sunlight absorb a lot more heat via direct radiation, and only when it reaches a sufficiently high temperature that the combined re-radiation of heat back at the sky and localized convective cooling matches the absorption does it stop heating. When you put the shingles in the shade, they won't get nearly as hot.

Metal roofing heats up quicker to the re-radiation balance, but the amount of heat radiating toward the sky is higher than it radiates toward the shingles, since a clear sky typically has a cooler radiation temperature lower than or comparable to the temperature of your house. Bottom line, the shingles will run cooler as long as they aren't in direct contact with the metal roofing. But the color of the metal roofing (or more precisely, it's solar reflective index or "SRI") still matters, since that has a strong direct effect on the peak and average temperature of the metal. The heat transfer of the metal to the shingles is primarily radiated if there's a 1.5" gap, and the emissivity of the under side of the metal roofing makes a difference too.

The shingles aren't worth stripping unless you have three layers or something, and then it's more about the long term care of your roof decking than the thermal mass of the shingles.

Answered by Dana Dorsett
Posted Tue, 01/21/2014 - 13:12

5.
Helpful? 0

Thanks Dana! That's very helpful. While I certainly don't depend on my roof to keep the heat out, it helps to understand the implications of both options. As you stated,I plan to create an 1.5" air space between the metal and the shingles, and while I'm sure the wood purlins can create some thermal bridging and there is still radiated heat transfer, any heat mitigation would be helpful. Any further design tips for this situation are welcomed. Thanks

Answered by Marlo Sutphin
Posted Tue, 01/21/2014 - 16:57

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