A properly insulated basement provides comfort and is a good way to save on your energy bill. Basements, however, have specific features that must be kept in mind when it comes to insulating the foundation from the inside—starting with the fact that a variable portion of the walls is below ground. In this Tips & Tricks instalment, we look at the problem from all sides so you’ll know what to expect when planning such a job.

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.


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.


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 (2 cm) per foot for a distance of 5 to 6 feet (1.5 to 1.8 m) 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 1.8 m) 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.

Portions of wall above and below grade Required thermal resistance factor
Full wall height below grade R-17
More than 50% of wall height above grade; i.e., exposed to outside air R-24.5 (R-29 in municipalities located farther north)
End joist, i.e., the wood member running around the floor structure on the ground floor and resting on top of the foundation wall R-24.5 (R-29 in municipalities located farther north)

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.

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 (e.g., sheet polyethylene or 15 lb felt paper) against the portion of the interior foundation wall that is below grade.

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 wall1

Seamless insulation plus fibre insulation between wooden studs

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

Rigid-board insulation plus nailing strips

  • 2 lap-jointed thicknesses of 1½ in. (38 mm) 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 3 in. (19 x 64 mm) nailing strips;
  • ½ in. (13 mm) gypsum wallboard.

Main features of insulating materials

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

Material Features
Rigid-board insulation
(expanded or extruded polystyrene)
  • High thermal resistance per unit of thickness (generally R-4 or 5 per inch)
  • Allows for uninterrupted layer of insulation
  • Lightweight and easy to cut
  • Difficult to fit and seal if surface is irregular
  • More expensive than batt-type insulation
  • Must be covered in a fireproof material (e.g., gypsum wallboard)
Sprayed polyurethane  
  • Performance superior to all other types of insulation (R-6 per inch)
  • Complete adherence even on the most irregular surfaces
  • Leaves no air spaces that could lead to mould growth
  • Fills the smallest gaps (foam expands on application)
  • The most expensive type of insulation
  • Must be covered in a fireproof material (e.g., gypsum wallboard)
Batt-type insulation
(fibreglass, cellulose or mineral-wool)
  • Lightweight, easy to cut and fit between stud framing
  • The least expensive type of insulation
  • Mineral wool is best for basements because of its water repellency
  • Lower thermal resistance (slightly less than R-3 per inch)
  • Must always be covered in a vapour barrier on the warm side of the insulated surface


    1Guide pratique Novoclimat 2005 de l’Agence de l’efficacité énergétique du Québec.