The lowdown on basements, cellars and crawlspaces

By Peter Meridew


This discussion deals with homes that are “intentionally air-tight” and where the basement, cellar or crawlspace is part of the conditioned [heated] space – section 9.32. The next question is whether the basement is part of the occupied space, or not. Fairly obviously, a crawlspace will hopefully not be an occupied space, but a basement may have several, partitioned rooms, a washroom and potential occupants, thus requiring both supply and exhaust air quantity calculations.

The fresh air supply and exhaust ventilation quantities will be calculated based on occupants and floor area – depending on the regulation being applied, but CSA-F.326 will just require 20 cfm supply to each partitioned area when there are no occupants and with no specified maximum floor area.

With a properly-designed, central forced air heating system, the fresh air to a crawlspace will obviously be provided via this system, but the quantity will depend on the relative heat loss of this space as compared to the total home heat loss. This is a pet peeve of mine, but it seems to work out some of the time.

When basements are part of the main floor heating system, they are seldom warm enough because they experience year-long heat loss. The furnace fan must run continuously.

According to the BC Code, the forced air system supplying an eight-ft. high, unoccupied basement, or crawlspace only needs to have one outlet for each 425 ft2 of full-height floor area and proportionately larger floor areas for lower ceiling heights, so a four-ft. crawlspace is good for 862 ft2 floor area, and so on. If no return air opening is provided, which is acceptable, then at least one transfer grille must be installed to the floor above − 12 sq. in. for each 1,000 sq. ft. of floor area. Typically, a three-in. by 10-in. register (16 in.2 free area) will be OK, but remember to remove the damper blades.

With a non-forced air system (electric or hydronic − these would be my preference, because independent thermostat settings can be made for less-used spaces) for the crawlspace the ventilation system is still only required to supply the calculated air volume as long as that above-mentioned transfer grille is provided. One is minimum, but two would be better and located in a pantry, closet, or laundry room to avoid having the cooler air causing complaints. Ventilating these spaces requires a careful approach and knowledge of whole Code sub-sections.



Definitions of basements, cellars and crawlspaces can be quite tricky and should be checked against the local building code. Some are defined as 50 per cent above grade or not 50 per cent above grade, and so on. For the purposes here, we are not discussing basements that have outdoor air louvres for cross-ventilation, or basements with dirt or gravel floors. If this is your situation you can check the National Building Code, or your local Code section 9.18 for more natural ventilation details.

Some experts say that ventilating this type of a basement simply causes more problems. In either of these situations, the floor above must be made  air-tight from the basement and be very well insulated. Water and drain pipes will need special attention to avoid freezing. Similarly, ductwork must be well insulated and joints made air-tight.