Make Attic Conditioned Space

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Joined: Mon Nov 11, 2019 7:20 pm

Make Attic Conditioned Space

Post by Magilla » Mon Nov 11, 2019 7:57 pm

After a comprehensive energy audit, I've settled on DIY-insulating the roof deck with XPS rigid foam board (e.g., Owens-Corning Foamular). The rafters are 2x6 (24" OC), so I'd start by building a 2" vent chute from soffit to ridge by gluing 2" thick foam "furring strips" to each rafter side and then gluing a form board to the strips to create a vent channel. Subtracting the 2" space of the vent, would give me 3.5" of insulation (R-17.5) before coming flush with the bottom edge of the rafter. Then I'd go perpendicular to the rafters with another layer(s) of foam board to bring the total R-value to around 30, and taping all the seam joints. (I live in the Wash DC area).

The attic has blown insulation on the floor joists but plenty of unsealed penetrations to the living space below. There's an air handler up there that gets cooked every summer. Insulating the roof sheathing underside is intended to extend the air handler's life and mitigate thermal losses in the flex ducting.

Do I need a vapor barrier somewhere in this DIY project, such as a sheet of poly as the final layer over the foam board? What adhesive is recommended for foam-to-wood and for foam-to-foam? Other suggestions? Thanks.

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Re: Make Attic Conditioned Space

Post by Shannon » Mon Nov 11, 2019 8:59 pm

I think your idea should work. As for needing a vapour barrier or if the design will be aloud is up to your local inspection office . Also there could be an issue with the exposed foam on the final layer. It should be ok exposed usually in an non living area of the home but some places could have different rules. One change I would suggest for sure is using canned Sprayfoam to seal the first layer into place to stop air flow from coming around it into the insulated ceiling cavity.
Run the plan through your local office.

Btw many manufactures have adhesive that is ok with foam Lepages PL line has 2 I believe.
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