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Can 1.5″ and 2.0″ unfaced XPS be used as a continuous rafter baffle?

Andreas Seibert | Posted in GBA Pro Help on

I am considering using either 2.0 or 1.5 XPS as a continuous rafter baffle (from soffit to ridge vent) using a 2″ vent between the XPS and the the roof deck. The rafters are nominal 2x10s and the underside of the rafter will be insulated with Roxul and then covered with finished plywood with a high quality latex paint (Class II or preferably Class III vapour retarder). I have only seen 1″ XPS recommended by GBA. Can a thicker board be used or is there concern re: drying to the exterior given the relative impermeance of the thicker board?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can 1.5-inch and 2.0-inch unfaced XPS be used as a continuous rafter baffle?"

    A. Yes, in my opinion. For more information on this topic, see Site-Built Ventilation Baffles for Roofs.

    In that article, I wrote, "There really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
    Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
    The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
    Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting."

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