Insulating concrete foundations from inside the basement

Before installing insulation

Pressure from the surrounding soil, groundwater, underground gases—your home’s foundations are severely tested. Before you decide to insulate foundation walls, a few checks are necessary.

Inside

Since the walls to be insulated are concrete and located partially below grade, you must first of all make sure they are in good condition:

• Structurally, by checking for cracks, crumbling of the concrete, and so on; and
• In terms of resistance to infiltration by water and radon gas, by ensuring that no cracks or traces of water infiltration or efflorescence have formed.

If you notice any anomalies, you must take the required corrective actions.

Outside

Make sure the house foundations aren’t subjected to undue water pressure, by ensuring that:

• the ground slopes away from the house at a drop of 3/4 inch (2cm) per foot for a distance of 5 to 6 feet (1.5 to 1.8m) all the way around the perimeter, to ensure water flows away from the house; and
• downspouts also drain water 5 to 6 feet (1.5 to 1.8m) away from the house.

Foundation walls: minimum thermal resistance

The minimum thermal resistance values for new buildings in Quebec were recently increased. When renovating, if you can’t meet those targets (e.g., because of a lack of room or because accessing voids is impractical), you should try to come as close as possible to them, to lower your heating bill and increase comfort.

A few best practices

Surface to be insulated

Condensation tends to form on the concrete in foundation walls. Application of an initial uninterrupted layer of insulation (seamless insulation) over the entire concrete surface is therefore recommended. This delivers a two-fold benefit:

• Better insulation because thermal bridging, i.e., heat transferred to the foundation through the wall framing, is eliminated; and
• No potential for contact between the concrete and materials that can absorb water (e.g., wood, batt insulation) and thus be a source of mould.

When foundation walls are insulated, the end joist must also be insulated: this area is often neglected, and is a major source of air leakage.

Cinder-block or stone foundations shouldn’t be insulated for their full R value from inside unless a waterproof membrane is installed outside.

Choosing the proper materials

The most effective materials for insulating the entire surface of foundation walls include:

• extruded or expanded polystyrene boards; and
• sprayed polyurethane foam.

If this type of seamless insulation cannot be installed, there must be a vapour barrier (air barrier membrane) against the portion of the interior foundation wall that is below grade. Safety warning:

Do not use building tar paper indoors, as it is likely to emanate toxic fumes.

Sprayed polyurethane, because of its high adherence and expansion when applied, is the most effective material for insulating the top of the foundation wall (concrete) and the cavity formed by the end joist and the floor framing members (joists, beams, etc.).

A vapour barrier must prevent humidity in the air inside the house from infiltrating the wall and damaging the structure as well as the insulation. The most common method is sheet polyethylene placed between the insulation and the interior finish material.

Electrical and plumbing elements

Electrical boxes (wall boxes, switches, etc.) in insulated walls must be of the airtight type, or wrapped in sheet polyethylene that is in turn sealed to the vapour barrier covering the insulation.

To prevent freezing, water lines must be on the inside (warm side) of any foundation wall insulation.

Two standard, effective ways of insulating a foundation wall

Seamless insulation plus fibre insulation between wooden studs

• 1in. (25mm) polystyrene board across the entire concrete surface (R-5) or, if the concrete surface is irregular, ¾in. (2cm) of sprayed polyurethane foam directly on the (R-4.5);
• 2 x 4po (38 x 89mm) studs on 24in. (600mm) centres;
• 3½ in. (89mm) batt insulation (R-12);
• code-compliant vapour barrier and ½in. (13mm) gypsum wallboard.

Rigid-board insulation plus nailing strips

• 2 lap-jointed thicknesses of 1½in. (38mm) extruded polystyrene board (R-15); note that at this thickness, the material acts as a vapour barrier as long as the joints and ends are sealed;
• 1 x 3in. (19 x 64mm) nailing strips;
• ½in. (13mm) gypsum wallboard.

Main features of insulating materials

Several types of insulation are available on the market, each with its advantages and disadvantages.