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Community and Q&A

Impact of Air Gap on R-Value

B_Hungerford | Posted in Energy Efficiency and Durability on

I have a multifamly building in Climate zone 5.  The building plans call for R20 rigid foam board above the roof deck and R30 fiberglass batt in the truss cord below.   Local code requires R49 roof/attic.   The insulation values added meet this requirement.  However the insulator installed the fiberglass batts to the bottom of the truss cord leaving a 6″ air gap between the batt and the bottom of the roof deck.  My thought is that in this configuration this is only a R-30 roof.  is that correct?

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  1. Expert Member
    Michael Maines | | #1

    Did they insulate the full height of the trusses at the perimeter? If so, it complies with the IRC and probably other codes, but it's best practice to keep the insulation layers as close together as possible to avoid thermal short circuits and potential moisture accumulation.

    If they did not insulate the full height at the perimeter, then I agree that you essentially have an R-30 roof.

  2. B_Hungerford | | #2

    Thanks Michael for the reply. I had totally forgotten about that method. Looking at the project agan yesterday that was a great option.

  3. charlie_sullivan | | #3

    I'll at that air sealing of all six sides of that cavity, and the corners between those sides, becomes more important when there is air space, and when that air space is bigger.

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