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Community and Q&A

Venting space between layers of roof sheathing

thomasfagan | Posted in Green Building Techniques on

Working on a remodel of a building with a flat roof. Currently the roof assembly from top down is TPO membrane, 1/2″ plywood, sloped overframing, 1/2″ plywood, 2ft deep flat trusses with vented airspace over batts, gwb ceiling. 

The goal remove the gwb and batts, install new closed cell spray foam on the bottom of the lower 1/2″ sheathing, paint foam with firefree88 for thermal barrier, then paint the bottom of the foam, trusses, ect. Will be a industrial exposed framing look. 

I am a bit concerned about removing the inward drying potential for the airspace between the layers of plywood. Is this a reasonable concern or is condensation in that area unlikely enough to not worry about it?

Climate zone 5b, Bend Oregon

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  1. Expert Member
    Akos | | #1

    In-between spaces are always trouble with unvented roofs. The big problem is that any moisture that makes it in there, has no way to get out. You always want the SPF to be adhered directly to the sheathing under the roof membrane.

    If it is not possible to remove the 2nd layer of CDX and spray directly under the roof deck, I would look at figuring out how to vent this space or just keep the vented roof as currently is. Besides exposed trusses, there are many other ways to get industrial loft look.

  2. Jon_R | | #2

    +1 on not creating cold air spaces that can't dry well. There is probably some interesting discussion to be had about the relative importance of the various reasons for this.

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