What's Causing Air Leakage and Moisture in Your Basement
Last Updated: Apr 11, 2025One of the unusual places that humans live is below the ground. It is something we have done since living in caves and hollows. From that time, we have gained a better understanding of creating healthier and more sustainable living spaces. Unfortunately, the concept of living in caves has stuck with us through the millennia as some primordial sense of safety.
Basements, however, are far from that truth. Understanding the science of basements, in simple terms, allows us to make healthier decisions on how we use and live in them. This article talks about ways to make things better and even livable in your basement.
Table of Contents
- Types of Basements
- Air Leakage
- Managing Moisture
- Bottom Line
Types of Basements
This article assumes your basement is full height, finished (in my particular case), and is usable space in a probable heating climate. If a basement is four feet or shorter in your geographic location, full of dirt, open to the outside air, or other strange things, then that is called a crawlspace, a dirt crawlspace, or even a dirt basement. We are not specifically talking about those creatures of a different stripe in this article.
Air Leakage
Understanding the air movement into the basement clarifies that there is value in air sealing the basement from the outside to reduce the inflow of dirty, dusty, moldy, and moist air from the outside. The typical entry places are around old windows (these are usually the last place people spend money on) and between the floor joists and rim beams.
Windows
The windows' most significant issue is not the windows themselves but around the window, between the window and the wall. This is where the bulk of air leakage occurs. If the house is more than 20 years old, most likely, there is just a piece of fiberglass insulation stuffed between these two building components. Insulation can slow down the thermal movement between inside and out, but it does nothing for air movement. As a low-cost solution, I recommend sealing up the old windows themselves with tape—such as aluminum tape, creating an air barrier tape between the operable window and the frame—or properly caulking around the trim between the wall and window to make it airtight. Try to find a zero VOC (volatile organic compound) caulking to use. This is the quick fix version.
The ideal version is to remove the trim, and add some spray foam, both an insulator and an air barrier, and reinstall the trim. There are now some low VOC foams out there, but not many. If you can’t tolerate foam due to chemical concerns, then tape up between the window and the wall directly with the air barrier tape of choice, and put the trim back on. Of course, if you have gone to all that trouble of tearing everything out, consider replacing the window. Energy efficiency grants and rebates are available in many locations.
Stephen Collette
Stephen Collette is a Building Biologist, Building Science Consultant, LEED Accredited Professional, and a Heritage Professional. Stephen is the owner of Your Healthy House and lives in Lakefield, ON with his wife and 2 daughters.
