There are two ways of insulating the footing in the slab-on-grade condition
Case 1: Over the footing or slab on the outer surface
The major advantage of this type of insulation is that it reduces heat loss from both the footing and the slab. Sometimes, a foamboard insulation may be extended several feet beyond the edge of the foundation to further protect the footing from freezing.
Case 2: In between the footing and slab on the inside
In this situation, the insulation, by being vertical at the joint between the footing and the slab, provides better isolation of the slab. The colder footing is isolated from the slab. Also, this insulation protects the foundation system against insects.
In both cases above, the slab also needs to be insulated from the earth, either by placing insulation on the bottom (in new construction) or on the top (in existing buildings).
Insulating the Slab
In new construction, the slab can be easily insulated from the earth below it by placing insulation below it. A typical cross-section of a slab-on-grade, from top to bottom, will be: concrete floor slab, 2-3 inches of sand, 1-2 inches of rigid insulation, 6 mil thick vapor barrier (usually of polyethylene plastic) which acts as a moisture retarder, 4 inches of gravel and under-slab drainage pipes to drain away any water leakage in the vicinity of the slab This method, however, is impractical for existing buildings which need to be retrofitted. Insulation may be applied on the top of the existing slab. A typical cross-section, from top to bottom, will be:
- Finish flooring
- Rosin paper
- Subflooring
- Rigid foam insulation laid inbetween treated furring strips (wooden or steel framing members, usually 1" x 3", used to provide an even nailing base) which are attached to the concrete slab. The furring strips can be avoided with the insulation being placed on the slab itself. This creates a floating floor assembly.
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