Basements are often one of the most overlooked areas of a home when it comes to insulation as people feel that they do not use them much and that they do not need to keep them warm. Studies have shown in fact that they are one of the highest priority areas for upgrading insulation.
Adding fiberglass batt insulation to wood walls (or blowing in insulation into the wall cavity if there is little or nothing present, adding a vapor barrier to the warm side of any insulation or unsealed areas around doors and windows. The space between the floor joists where they meet the exterior walls also often need to be insulated as there is none present or it has been previously removed during a renovation etc. The underside of bay windows, cantilevers, dormers or any floor that extends beyond the line of the exterior basement wall will require 6-8 inches of insulation.
In terms of insulating concrete walls, it depends on what the basement is going to be used for before deciding how to approach them. At a bare minimum the building code requires that in any heated area, 2 feet below grade of exterior concrete walls must be insulated. Typically this is done with a foil faced or rigid insulation. However, if the area is going to be finished or drywalled at a later date, a homeowner may want to insulate the full height of the basement wall.
Basement walls are unique because they must handle significant moisture flows from both inside and outside the house. The preferred method, from a building science perspective, is to insulate the wall on the outside with rigid insulation suitable for below-grade installations, such as extruded polystyrene or rigid fibreglass.
The advantages are as follows:
Insulating the outside of the basement works well with damp proofing and foundation drainage. Rigid fibreglass or mineral wool acts as a drainage layer, keeping surface and ground water away from the foundation. The basement walls are kept at room temperature, protecting the structure, reducing the risk of interior condensation and increasing comfort.
The disadvantages are the disturbance of landscaping, the need to cover the insulation above grade, and the relatively high cost.
Interior insulation can be used. This can be done when finishing the basement by using batt insulation in the stud cavities or by installing extruded polystyrene and strapping on the face of the perimeter walls. If the basement won’t be finished, you can install rolls of polyethylene encapsulated fibreglass over the wall. The advantages of interior installation are cost and ease of construction. The disadvantages of interior installations are as follows: The basement walls are now at the temperature of the soil or the outside. Any moist air moving through the wall from the inside will condense on the wall.
Usually, there is a moisture barrier against the foundation wall and a vapour retarder on the room side of the insulation. As a result, the wall has poor drying potential. Never apply interior insulation to a basement with moisture problems. Fix the moisture entry problems before insulating (see CMHC’s publication A Guide to Fixing Your Damp Basement).