Insulation Of Foundations

Foundation walls enclosing heated space should be insulated full height.

When insulation is applied on the outer surface of a wall or a slab perimeter, it should be of a type not susceptible to water damage, such as expanded or extruded polystyrene or insulation capable of draining water, such as high density, rigid glass fibre insulation. In addition, the insulation should be protected above grade with 1/2 in. (12 mm) cement parging on wire lath applied to the exposed face and edge (Fig. 118).

Where insulation is applied to the inner surface of foundation walls, the below-grade portions of the insulation and any associated wood strapping must be protected by a moisture

Concrete wall insulated with batts — horizontal strapping.

Concrete wall insulated with batts — horizontal strapping.

Note: Wall strapping may also be installed vertically as in conventional stud walls. Horizontal strapping allows for easier installation of plumbing and wiring

R-20 (RSI 3.5) batt insulation between joists vapour barrier

R-12 (RSI 2.1) batt insulation vapour barrier

'/z" (12.7 mm) gypsum board moisture barrier (below grade only)

2" x 4" 38 x 89 mm) strapping at 24" (600 mm) on centre keep framing members away from the foundation wall by standing off strapping, or using rigid insulation as a spacer caulking

8" (200 mm) concrete wall

Note: Wall strapping may also be installed vertically as in conventional stud walls. Horizontal strapping allows for easier installation of plumbing and wiring

Effective Thermal Resistance R-12.0 (RSI 2.11)

barrier in the form of a minimum 2 mil (0.05 mm) polyethylene film or two coats of bitumen applied to the inner surface of the wall to grade height. If the insulation is a type which will not readily absorb water (for example, expanded polystyrene), no damp-proofing is required behind the insulation, but associated wood strapping should be protected by, for instance, partial wrapping in polyethylene. However, in order to allow any moisture which does enter the wall to escape, neither the inner nor the outer surface of the above-grade portion of the wall should be dampproofed (Fig.


Rigid board-type insulation should be bonded to the wall with cement grout or synthetic adhesive applied in bands forming a grid pattern. This pattern of bonding is recommended to limit warm moist air movement behind the insulation since this can cause condensation and ice build-up between the wall and the insulation. If a protein-based adhesive is used to bond the insulation to the wall, the adhesive should contain a preservative.

Due to its high potential for contributing to the rapid spread of fire, any plastic insulation applied to the inside of basement walls must not be exposed, but should be protected with an acceptable finish. Other types of insulation must also be covered to protect them from damage. Where fire protection covering is required, insulation should be held in place by mechanical fastening to framing members at least at the top and bottom of the insulation and around all openings.

Insulation is normally placed between the studs of preserved wood foundations. Preferably, the cavity should be filled to prevent air pockets and the possibility of convection loops being set up within the cavity.

Normal weight concrete, that is, concrete with a density of about 150 Ib./cu. ft. (2400 kg/m3) is normally used in basement construction. Lightweight concrete may be used to achieve higher thermal resistance but should have a 28-day compressive strength of at least 2000 psi (15 MPa).

In the following illustrations, the insulation is shown extending the full height of the basement wall. With hollow core concrete block walls, convection currents may occur if the wall is not insulated over its full height. The bottom edge of the insulation should be sealed by caulking and, in the case of batt-type insulation, by solid blocking.

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