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Roof Insulation Assemblies

Michael Roland | Posted in Green Building Techniques on

GBA blogs are filled with discussions of wall assemblies (insulation/vapor & air barriers), but I haven’t found much on roof assemblies, particularly cathedral types. Assuming rafter construction, Is it best to build them like walls (rigid insulation outside, breathe to inside) or is it best to seal the rafters on the inside with taped rigid foam and let the assembly breath to the outside? Do we need to avoid rigid on inside and outside as with walls? With timber frames and SIP roofs, can shingles or metal roofing be placed right on the SIP without creating a cold roof via an air space? Zone 5A.

Roland

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  1. GBA Editor
    Martin Holladay | | #1

    Roland,
    Q. "Assuming rafter construction, Is it best to build them like walls (rigid insulation outside, breathe to inside) or is it best to seal the rafters on the inside with taped rigid foam and let the assembly breathe to the outside?"

    A. Either way can work, but remember, many types of roofing will not allow the top of a roof assembly to "breathe." While cedar shingles, slate, and concrete tile are all vapor permeable, other types of roofing, including standing-seam metal roofing and membrane roofing, are vapor-impermeable.

    Q. "Do we need to avoid rigid on inside and outside as with walls?"

    A. It's generally best to avoid a "foam sandwich" with plywood or OSB between two layers of foam.

    Q. "With timber frames and SIP roofs, can shingles or metal roofing be placed right on the SIP without creating a cold roof via an air space?"

    A. Yes, the code recognizes unventilated roofs (although roofing doesn't go directly over the SIPs -- you still need underlayment like asphalt felt.)

    If you are planning to build a house with a sloping insulated roof, you may want to read this article: Creating a Conditioned Attic.

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