Crawlspace ventilation and air circulation within them is very important. Without it, damage can occur from moisture.
In crawlspaces that are considered a “cold space”, a general rule of thumb is that a minimum of 1 sq. ft. of venting should be provided for each 500 sq. ft. of the crawlspace. At least two vents should be used to maximize airflow. The vents should be located as high as possible to catch naturally occurring breezes. Increased ventilation for crawlspaces with dirt floors is recommended. Insulation and vapour barriers in the crawlspace are also important e.g. ensure heating ducts and the underside the 1st floor are insulated; protect all water supply lines protected from freezing. Venting of unheated crawlspaces should be provided at all times.
Crawlspaces that are a “warm space” should be treated like a regular basement. That is, they should be heated and have appropriate air circulation; cold air returns to the heating system if forced air, insulated walls / floors and appropriate vapour barriers.
Read more about foundations & basements.