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Q decking, insulation, and vapor barrier

r_wiley | Posted in GBA Pro Help on

We’re building a house and the plans call for a screened in porch over a concrete slab. The design calls for full foundation wall heights with no basement access—the void is filled with stone and a slab to be poured over. The client would like to make the space beneath the screened porch and accessible basement area, so the engineer spec’d a 3” Q deck with #3 bars in the valley. The client believes we should put 2” of rigid over the Q deck and beneath the slab, along with a vapor barrier. The engineer thinks they should spray foam or foam board underneath. We are unable to find conclusive details for this quandary. What do you guys think?

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Replies

  1. stamant | | #1

    need to look at the thermal breaks between the conditioned spaces and the composite slab. likely want at least 4" of continuous insulation in the assembly and an ignition barrier. all of a sudden the 3" slab assembly gets thick in a hurry.

  2. Malcolm_Taylor | | #2

    r_wiley,

    Screened porches are one of those not-in, not-out spaces, and what you choose as an floor assembly depends on how wet it will get in there. If you never anticipate moisture beyond that from foot traffic, then an insulated slab is probably fine. If it is going to see water it needs to be waterproofed. My own preference would be to use wood frame construction and a membrane you can use a variety of floor finishes on.

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