Styofoam on outside of slab

Originally Posted By: rchoreyii
This post was automatically imported from our archived forum.

Does anyone know why some slabs have syrofoam on the outside? Is it to keep out pests or water? It rises all the way to the bottom of the sill plate or it appears to anyway.

Ron Chorey

LAS CRUCES Home Inspections

Originally Posted By: rpalac
This post was automatically imported from our archived forum.

in the northern region we use stryfoam rigin close cell insulation on the outside of the wall for ground insulation and moisture block. It is also used on inside perimeter of the slab for theral value not moisture.

There are several sites that you can see this depicted or

as written:

Basement Walls
Building scientists report that the best way to have a dry basement is to insulate the outside of exterior walls with a rigid fiberglass type "mat." Under the mat is a damp-proofing coating over the entire foundation, from footing to just below the finished grade. A carefully designed perimeter drainage system consisting of washed gravel, perforated plastic pipe, and filter fabric is also strongly recommended for locations with poor soil drainage. A waterproof paint over the room side of the foundation wall is also often recommended. Some foam insulations are impregnated with boric acid to discourage termite infestation. However, the borate chemical often slowly leaches out of most materials when exposed to ground water. Adding insulation to the interior of the foundation is often more cost effective for an existing building.

Insulating Slab-On-Grade Foundations
Slab-on-grade foundations are often insulated over the exterior of the footing/ slab edge, or between the interior of the footing and slab. Often the bottom of the slab is insulated from the earth to some extent as well. Each approach has its advantages and disadvantages. On the exterior of the footing/slab edge, insulation reduces heat loss from both the foundation and the slab. Sometimes a foamboard insulation is extended outward beyond the foundation for several feet. This offers more protection to the footing from freezing. It also allows the builder to dig a shallower footing without the risking of damage due to frost-heaving. All exposed parts of the insulation must be covered with metal, cement, or other type of membrane to protect it from damage. When installed on the interior of the footing/slab, it must be vertical between the footing and slab. This protects the insulation from insects and damage better than an exterior application while it isolates the slab from the colder footing. Insulating under an existing slab is usually impractical.

Bob P.

Originally Posted By: roconnor
This post was automatically imported from our archived forum.

In the NE it would usually be a FPSF … Frost Protected Shallow Foundation (25$ word … LOL).

The theory is that the foundations do not need to go full depth because heat from the building (retained by the insulation) will keep the soil below the footing from freezing and pushing the foundation up (e.g. "Frost heave" ... not a good thing).

Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong