GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Capillary break necessary for above-grade slab?

jcofvienna | 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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |