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

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.

Asked by Jason Macdonald
Posted Sep 16, 2012 5:21 PM ET
Edited Sep 16, 2012 5:21 PM ET

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5 Answers

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1.

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

Answered by Armando Cobo
Posted Sep 16, 2012 9:27 PM ET
Edited Sep 16, 2012 9:28 PM ET.

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?

Answered by Jason Macdonald
Posted Sep 16, 2012 9:35 PM ET

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....

Answered by aj builder, Upstate NY Zone 6a
Posted Sep 17, 2012 5:01 AM ET

4.
Answered by Armando Cobo
Posted Sep 17, 2012 10:59 AM ET

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?

Answered by aj builder, Upstate NY Zone 6a
Posted Sep 17, 2012 1:01 PM ET
Edited Sep 17, 2012 1:04 PM ET.

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