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2012 IECC - Slab-on-grade Zone 4B

I will have an ICF stem wall that will transition into the ICF house wall. The interior will be a slab-on-grade design. This is for a Zone 4B (desert) climate.

1 - Should the vapor barrier be on-top or below the rigid EPS?
2 - What thickness is good for a vapor barrier?
3 - Will 3" of rigid EPS suffice for a Zone 4B climate?
4 - Any links to some details showing the above condition?
5 - Should crushed & compacted rock be used below the EPS and concrete slab?

Asked by Peter L
Posted Tue, 04/08/2014 - 00:50

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Peter L.,
Q. "Should the vapor barrier be on top or below the rigid EPS?"

A. On top. To understand why, see Concrete Floor Problems.

Q. "What thickness is good for a vapor barrier?"

A. The usual minimum recommendation is to use 6 mil polyethylene. If you are worried that the polyethylene will be damaged during the concrete placement, you can choose a more rugged material if you want. Remember, though, that vapor barriers don't have to be perfect to work well. A few holes are fine.

Q. "Will 3 inches of rigid EPS suffice for a Zone 4B climate?"

A. Most experts would answer yes, but there is no single answer to this question. To know how thick your insulation should be, you first have to define your goals. Most Passivhaus buildings would include much thicker sub-slab insulation, but there is a vigorous debate over whether this is a cost-effective measure. To answer the question for your own project, you have to run the numbers. My guess is that any additional rigid foam beyond 3 inches would have a very long payback period in your climate.

Q. "Any links to some details showing the above condition?"

A. Sure. Here is a link to a detail (although the foam in the detail isn't as thick as what you are planning): Slab on Grade with Frost Wall with Rigid Foam.

Q. "Should crushed & compacted rock be used below the EPS and concrete slab?"

A. Yes. For residential jobs, the usual recommendation is for 4 inches of crushed stone. However, local practices vary widely, due in part to differences in soil characteristics. If you have any doubt on this issue, consult with a soils engineer.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/08/2014 - 05:44

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