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Should sub slab insulation either EPS or XPS be placed above or below the sub slab vapor barrier?

Jason Macdonald | Posted in GBA Pro Help on

We will be using ICF’s with an 8″ core, using a capillary break on top of the footing. Sub slab insulation will consist of 2 layers of 2″ foam board with offset seams. Sub Slab Vapor Barrier will be Vapor Block 10 type A.

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Replies

  1. Expert Member
    Armando Cobo | | #1

    Polyethylene vapor barrier is always in direct contact with concrete; right in between the concrete and the foam.

  2. Jason Macdonald | | #2

    If the vapor barrier is between the concrete and the foam, what protects the foam from insects and other degradation? Can an additional layer of poly be placed beneath the insulation for protection? Other ideas?

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Jason, Overthinking overdoing...

    Underslabs don't need multiple layers with offset seams or taping of seams.

    More is not better. The plastic is your vapor barrier for water and radon not insects or whatever degradation. Obtain a proper foundation plan. Here, BSC, your concrete contractor, your architect, your ICF supplier, your GC....

  4. Expert Member
  5. Aj Builder, Upstate NY Zone 6a | | #5

    Armando, in my area interior foam is basic foam. Termites avoided by placing foam inside or under.

    Curious to costs of products you list and toxicity and climate areas where needed. Thinking the price may be double or more?

    Southern homes on slabs impacted by termites to benefit?

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