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Community and Q&A

Insulated Slab

David Epley | Posted in GBA Pro Help on

I have a few questions about an insulated slab detail found in ‘strategies and details” section ( What is the ideal substrate underneath the rigid insulation board and where, if necessary, should a vapor barrier be placed? In the past I have constructed in this order… slab, vapor barrier, 4″ of gravel (passive radon vent integrated if EPA zone 1 or 2).

Thanks for the help!


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  1. GBA Editor
    Martin Holladay | | #1

    The GBA detail library includes several approaches to building a slab foundation, including:

    1. Monolithic slab

    2. Slab on Grade with Frost Wall

    3. Frost-Protected Monolithic Slab

    In cold climates, it's best to include a continuous layer of horizontal rigid foam under the entire slab. This insulation is placed on a compacted substrate of at least 4 inches of crushed stone. A layer of polyethylene or a similar vapor barrier material is placed on top of the insulation.

    The polyethylene vapor barrier should always be directly under the concrete slab, with no intervening layer of "blotter sand." If you're unaware of the problems associated with blotter sand, read this article: Concrete Floor Problems.

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