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Community and Q&A

Unvented Roof Assembly Vapour Retarder

dric | Posted in General Questions on


Residing in Zone-5, I’m planning on incorporating 2″ of Roxul as the Exterior Continuous Insulation with an 3.5″ in the stud bay.  The roof assembly design will only permit the cavity to dry inwards due to the 2-ply roof membrane.

I understand the need to install a Class-2 Vapor Retarder, but unsure as to what qualifies as a Class-2 along with its location within the assembly.  I’ve also noticed the majority of the vapor retards I’ve come across have an adhesive backing.

To permit an effective air barrier/vapor retarder, would I install a plywood sheathing (say 1/4″) and install the Vapor Retarder directly over top.  Based on this layout the Vapor Retarder would be directly touching the Roxul Continuous Insulation.

Your feedback/confirmation would be appreciated.


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  1. Jon_R | | #1

    Best if you supply a drawing. Also review IRC R806.5 and roof total R value requirements.

    Membrain, Intello or wallboard with the right paint are Class II vapor retarders.

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