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Custom polyiso roof-vent baffles + traditional unfaced fiberglass batts + poly vapor barrier (conditioned space) = Success?

lmackinn | Posted in Energy Efficiency and Durability on

If I build custom roof-vent baffles with polyiso board then fill the remaining cavity with traditional unfaced fiberglass batts, can/should I also install a Poly vapor barrier prior to dry-walling? If so what mil thickness would be recommended?

I am concerned that the PolyISO board would create a vapor barrier and then the poly roll would create another barrier and any leaks could cause moisture to become trapped in the batts and rot the rafters.

If you agree with that analysis then please address my second concern if you would. If no poly vapor barrier under the drywall and only the foil vapor barrier on the custom roof vent baffles, wouldn’t moist warm air from the interior conditioned space condense on the PolyISO board in the winter and then need to dry to the inside? Would I then need a moisture management system in the living space?

Finally, would it make sense to use unfaced PolyISO and just do the poly vapor barrier? I’m not even sure if they make unfaced PolyISO. Do they?

Thanks a lot,

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  1. GBA Editor
    Martin Holladay | | #1

    Yes, you can get polyiso with vapor-permeable facings if you want that type of foam. Check with a roofing supply house.

    However, most residential builders who use rigid foam to make site-built ventilation baffles end up choosing EPS, which is somewhat vapor permeable. For more information on this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.

    Building codes do not require the use of interior polyethylene or a vapor barrier; in some climates, however, a vapor retarder is required. This requirement can be satisfied with vapor-retarder paint.

    I don't recommend the use of interior polyethylene for this assembly. What you want is an interior air barrier, not an interior vapor retarder. For more information, see Do I Need a Vapor Retarder?

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