Most cold-climate builders know that building codes require slab foundations to be insulated. But the terminology used to describe insulated slabs is quite confusing. In this article, I’ll try to clarify the current muddle over the many types of slab foundations.
First, some definitions
There are at least four kinds of slab foundations:
- A thickened-edge slab, also known as a monolithic slab, integrates the footing and the slab in a way that allows all of the concrete to be placed at the same time. The perimeter footing of a thickened-edge slab isn’t very deep.
- A slab with frostwalls has perimeter walls resting on footings installed below the frost line—generally three to four feet below grade. This type of foundation requires concrete to be placed on three different days. The perimeter frostwall footings are usually placed first, followed by the perimeter frostwalls. The last foundation component to be placed is the slab. A slab with frostwalls can be insulated or uninsulated.
- A frost-protected shallow foundation (FPSF) is a monolithic slab that includes enough insulation (either vertical insulation at the slab perimeter, or horizontal wing insulation outside the footprint of the building, or both) to prevent the soil under the footings from freezing. This type of slab sometimes, but not always, has a continuous layer of horizontal insulation under the entire slab.
- An insulated raft slab resembles a frost-protected shallow foundation, with two main differences: an insulated raft slab usually has a uniform thickness rather than a thickened edge, and an insulated raft slab always includes a continuous layer of horizontal insulation under the entire slab.
Regardless of what type of slab you intend to install, remember that site preparation is essential. In most cases, site preparation consists of…