The Dirt on Perimeter Foundation Insulation

Insulating below grade with under slab and perimeter insulation is a foundational part of an energy efficient building. When insulating under the slab and around the perimeter of the foundation, not only is R-value an important factor, but the compressive resistance of the insulation selected is just as important.

Plasti-Fab has a variety of insulation products to choose from for insulating an exterior foundation: PlastiSpan HD insulation, DuroSpan insulation, and higher compressive insulation boasting a psi of 25, 30, 40, and even 60 psi.

Following standard building practices, begin with the perimeter of the foundation. Regardless of whether a block or poured foundation has been chosen, the perimeter is insulated in the same method. The use of exterior foundation insulation reduces the likelihood of frost-heave under footings.

Insulating the exterior surface of the foundation wall provides the advantage of a fully insulated warm wall, which reduces the likelihood of degradation due to freeze thaw and of condensation forming on the inner face of the wall.

When a footing has to be placed above the frost line, external insulation can be adapted by placing it horizontally out from the foundation. This has the effect of raising the frost line to the level of the footing so there is no heaving of the foundation. Exterior foundation insulation isolates the foundation wall from outdoor temperature fluctuations, and provides a drainage plane which directs surface moisture to the drainage tile and protects foundation damp-proofing during back-filling.

With its outstanding resistance to moisture absorption, PlastiSpan insulation provides dependable long-term thermal performance when installed on the exterior face of the foundation wall.

What about drainage? In most basement construction a drain is installed around the foundation at the footing level. The use of external insulation does not decrease the requirement for drainage. If there is concern that the drainage system may freeze, or that frost may get under the foundation, a horizontal insulation board, minimum 600 mm (24"") wide, sloped away from the foundation, should be placed above the drain and the foundation.