The basement can account for 20 to 30 percent of the total heat loss of a home. This is because of the large, uninsulated surface area both above and below grade level. By adding insulation and sealing air leaks with caulking, weather stripping and an effective air vapour retarder, heat loss can be reduced to create a more comfortable home.


The most critical consideration when insulating a basement is moisture, both internally and externally generated. Moisture can ruin insulation and rot wood. Uncorrected moisture problems can also cause serious structural damage.

The first step is to ensure that the surface drainage is adequate. Eavestroughs, drains and pavement should be properly sloped to take the water away from the house. Basements often have high humidity, caused by dampness in the foundation walls. Dehumidifiers can decrease the humidity level if the problem is minor. Once the moisture problem has been eliminated, the basement can be insulated either from the inside or the outside. In most cases, insulating from the outside is best, though often it is necessary to insulate from the inside for economical and practical reasons. Sometimes a combination of both is required.


An R-value is simply a numerical representation of thermal resistance. The higher the number, the greater the resistance to heat transfer. RSI values are the metric equivalent of R-values. Different types of insulation have different R-values per inch of thickness. Even the same type of insulation can have a different R-value depending on its form. Glass fiber insulation has a higher R-value in batt form than in the loose fill form.

There are many different types of insulation on the market today. The following chart identifies types and R-values of the most common.

Material R-Value/Inch Common Form
Glass Fiber 2.0 – 4.2 batt, loose fill, rigid
Mineral Wool 3.0 – 3.2 batt, loose fill
Cellulose Fiber 3.4 – 3.6 loose fill
Vermiculite 2.3 loose fill
Plastic Board 3.7 – 6.0 rigid board
insulating basement
insulating basement


Insulating from the inside may require a wood frame and batt insulation. Another option is to use rigid board insulation, with prefabricated metal channels of wood framing to hold the insulation, followed by a layer of fire resistant material, mechanically fastened to the wall. Normally a vapour barrier is applied to the inside face of the basement walls up to grade level, and an air vapour barrier is installed on the warm side of the insulation. Insulation from the inside should not be attempted if there is any indication of a moisture problem. Obstructions such as plumbing and wiring, stairs, etc. can make the work more difficult and the insulation less effective. The advantages of insulating from the inside are that it can be done at any time of the year and in sections. It is often a cheaper and easier way to insulate the full wall and to achieve high insulating values.

In new houses, insulation on cement walls in some northern climates need only be applied from the subfloor and may terminate 8″ above the finished concrete floor. This prevents moisture problems and allows for better air circulation.


This involves excavating around the foundation, damp-proofing and installing rigid insulation. Flashing must be attached to keep the water out from behind the insulation, and a protective covering must be installed on the exposed sections of insulation. Moisture problems can readily be addressed and repaired. Rubble or brick foundations with moisture problems must be insulated from the outside. Foundation repairs, damp-proofing and the installation of a drainage system can be done at the same time. Because the mass of the foundation is within the insulated portion of the house, it will even out temperature fluctuations. The outside walls tend to be more continuous and easier to insulate once the soil is removed.

Increasing insulation levels in a home should be considered an improvement rather than a repair. The optimum amount of insulation depends on local climate, fuel costs, and the cost involved. To insulate a basement from the inside costs approx. $1.00 and up per sq. ft. From the outside, with rigid board insulation, insulation costs approx. $1.00-$2.00 per sq. ft.