Cold Basements and Crawl Spaces

by Tim Gould

Second to the attic, the basement and crawl space are the next most important areas to seal and insulate. This is due to a few factors. During cool or cold weather, air pressure difference between the inside and the outside is greatest at the top and bottom of the house. Once the heat is turned on in your home, heated air is trying to force its way out at the top while cold air is being drawn in at the bottom.

While air leaks in basements and crawl spaces are generally smaller than in attics, they are still larger than the leaks in exterior walls. Additionally, sealing air leaks in the basement and crawl space areas prevent potential contaminants such as radon, mold spores, water vapor, and other subsoil "nasties" from entering the living area. It may also prevent pipes freezing from cold air that typically enters at the sills.

When evaluating how to seal and insulate the basement or crawl space areas, one must first decide whether it's best to include these areas in the conditioned space or separate them with a thermal boundary and treat them as unoccupied space. When considering sealing and insulating the floor (which is the ceiling of the basement or crawl space and therefore isolates those areas under the floor from the rest of the house) versus the foundation walls (which means including those areas in the conditioned living area), there are many factors involved, such as ground dampness, presence of pipes and ducts, foundation type, ease of access, future occupancy plans, cost, sill condition, and more.

In general, our New England climate favors sealing and insulating the floor between the house and the basement or crawl space, and treating them as unoccupied space. The cold ground causes significant heat loss through the floor (which again means the basement or crawl space ceiling). Remember from a previous article, it is a myth that heat rises-it actually moves toward cold, so warm air from your home will be sucked down through the floor and out the cold basement or crawl space if you don't insulate the floor. Also, waste heat from the boiler or furnace mostly goes down to the cold ground through concrete or stone foundation walls.

In summary, basement or crawl spaces are the second best areas (after the attic) to start when looking to conserve energy. In a future article, I'll discuss how to seal and insulate the basement or crawl space where both potential energy savings and practicality come into play. The many factors listed above need to be carefully considered and will influence the decision on a particular approach to sealing and insulating. Doing it yourself versus hiring a contractor will, of course, help to determine the approach, as well.

© 2008 Tim Gould. All rights reserved.