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If I insulate a surface with two sheets of rigid foam insulation with thickness x, or with one sheet of otherwise identical rigid foam insulation with thickness 2x, how should I think about the performance differences between those two situations? What about if there was an intentional air gap between the two?

I'm thinking of an application exposed to cold air, around 10 to 50 deg F (-12 to 10 deg C). I'm also assuming that the warm side of the insulation would be faced with aluminum foil. I'm especially interested in expanded polystyrene.

Thanks!

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2*1x or 1*2x would be the same, but 2*1x with an air gap would be better than 2x by itself. Same principal as double pane glass; trapped air slows the transfer of heat.

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    Thanks a lot! I will mark this as the answer tomorrow. Follow-up question: If I went with the 2 x 1X with an air gap, what are the most important or common mistakes that I might make that would screw up the whole plan?
    – capet
    Commented Sep 10, 2020 at 20:32
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    Excellent question. If the trapped air isn't fully sealed into the cavity you'll effectively negate the benefit of one panel. If air can flow freely the insulation value of the second sheet is lost, just like an insulated building with the windows open does you no good. It's for this reason that I rarely leave such gaps. It's difficult to replicate the performance strategy of a double-pane window when assembling components in the wild.
    – isherwood
    Commented Sep 10, 2020 at 20:48
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    If you find yourself with a gap, just fill it with some loose batting or more thin foam sheets. Isherwood is correct that without careful engineering, you probably will not realize the potential of that increased insulation thickness.
    – Puddles
    Commented Sep 10, 2020 at 21:25
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    Gaps can also lead to sheetrock sweating, overtime is will stain and weaken the wall.
    – mreff555
    Commented Sep 10, 2020 at 22:34

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