Earth Slabs and Conditioned Crawl Spaces

When you talk about building a house, the crawl space may not be the most discussed feature. Generally, building codes require the bare earth of a crawl space to be covered with a 6 mil thick poly sheeting which acts as a vapor barrier. The goal of the vapor barrier is to keep moisture from moving out of the ground and into the crawl space thus preventing mold, mildew and condensation. Additionally, the under floor of the house must meet insulation values laid out in the residential energy code.

You can enhance the crawl space area by pouring a 3-4" concrete "Earth Slab" under the entire house. The main purpose of the Earth Slab is to allow us to create a conditioned crawl space. It also acts as a radon barrier which, in some locations, can be a significant concern. And of course, it keeps out insects and rodents. Because the slab is not structural the rules for it are not very stringent. They don’t require any reinforcement or great attention to the surface finish. In some locations, a vapor barrier under the concrete is required by code. Regardless of code, it’s a good idea to lay a vapor barrier down before pouring the slab. The plastic sheeting will reduce any wicking of moisture through the concrete.

In our homes, the rat slab is one component of our conditioned crawl space, which in turn is just one component of our energy compliance package. The major concern when designing a crawlspace is to keep the area dry while also minimizing infiltration of outside (unconditioned) air into the house. This requires some type of ventilation and insulation. Typically, the approach is to vent the foundation by placing rectangular vent channels in the foundation as it is poured and insulating the underside of the floor joists for energy efficiency. This approach contributes to the "Stack Effect" (Read more about the stack effect by clicking HERE and HERE)

We take a different approach and stop the stack effect by eliminating the static vent channels and by sealing and insulating the foundation. (using ICF block - which allows us to eliminate the under floor insulation) Then we’re adding a crawlspace ventilation system that pulls in a small amount of conditioned air from the HVAC system. That air conditions the crawl space and is passively exhausted to the outside.

Stopping the stack effect is important because it allows us to control the fresh air that is introduced, and filter it before you breath it. It improves indoor air quality, and according to Mike O'Brien from the Portland Office of Planning and Sustainability, it will add 100 years to the life of a home.

Mark Mecklem - Miranda Homes