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Community and Q&A

Capillary break necessary for above-grade slab?

John Cuddy | Posted in General Questions on

I’m planning to build in coastal Delaware (zone 4).  We’re in a flood zone, so we plan to use a 4 foot above-grade CMU stem wall and slab for the entry level of the house.  At 4 feet above grade, does my slab still require a capillary break (gravel layer), or can the slab insulation just sit on fill dirt?  Thanks in advance.

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  1. Andrew C | | #1

    The wicking ability of concrete is orders of magnitude higher than wood. Yes, you should have a crushed gravel layer and a vapor barrier touching the concrete. And ideally you'd have some rigid insulation (used) between the gravel and the vapor barrier. There are a number of articles here on GBA if you're searching, especially if you have a GBA membership. (If you are planning to build, the price of a GBA membership will pay for itself many times over.)
    See also pictures in articles like BSI-059 Slab Happy at the Building Science Corp site. They all show gravel and vapor barrier...and in zone 4 you need to insulate the perimeter of the slab.

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