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Vented metal roof and underlayment

smgarch | Posted in General Questions on

Hi Martin,

We’re planning on installing a metal standing seam metal roof over an existing vented cathedral ceiling. In zone 5, the existing conditions to remain are gyp. board ceiling and fiberglass batt insulation between the rafters.

After removing the existing cedar roof and skip sheathing the contractor installed plywood and an impermeable (less than .02) high temperature self-adhered membrane over the whole roof.

Will the impermeable membrane create a situation where the plywood can’t dry out?

Please confirm we should still vent the assembly at the soffit and ridge.

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    If this is a cathedral ceiling insulated with fiberglass batts between the rafters, it must have a ventilation channel between the top of the insulation and the underside of the roof sheathing.

    This ventilation channel must be connected to soffit vents at the eaves and a ridge vent at the peak.

    The need for a ventilation channels is not just a recommendation by experts -- it's also a code requirement when using an air-permeable insulation like fiberglass.

    Assuming that your roof has an adequate ventilation channel under the sheathing, the use of vapor-impermeable roofing underlayment is permitted.

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