If ever your basement was going to smell musty, the height of hot summer weather is when it’s most likely to happen.
The thing is, musty basements are more than just annoying. They’re also an indication of poor indoor air quality and mould growth happening somewhere downstairs. The most likely cause is a seasonal moisture dynamic that’s easy to misunderstand, and it’s this misunderstanding that gets a lot of homeowners into trouble during warm weather.
Almost all mould requires an elevated level of moisture to grow. Maintain dry conditions in your home and mould/mustiness will not be an issue.
The thing is, when it comes to basements, liquid water leaks are only one source of damaging moisture. Just because your basement seems dry to the eye does not mean it actually is. Airborne moisture is the cause of a lot of musty odours, and to understand how this happens you need to understand something called relative humidity.
Holding moisture
All air has the ability to hold invisible water vapour, and this ability varies with temperature. The higher the temperature, the more water that air can hold. When warm, moist summer air is allowed to enter a cool basement, that air loses some of its ability to hold moisture as it cools down.
If the relative humidity of the outdoor air is high enough to begin with, and the temperature of your basement cool enough, the relative humidity of the infiltrating air can reach 100 per cent downstairs.
This is not because the total quantity of moisture in the air has increased. It has stayed the same. Rather, the relative ability of the air to hold moisture has decreased because the air is now cooler.
The resulting condensation probably won’t cause visible moisture to appear on surfaces, but it can still raise moisture levels within walls and especially deep inside basement carpets. Whenever it’s warmer outside than it is in your basement, the potential for cooling and basement condensation exists.
And though there is an effective solution, it took me years to figure out because it seems opposite to what you’d think would work.
The main thing to understand is that summer air is the source of moisture that’s triggering mould growth and mustiness in your basement, so you need to keep the air out. While musty smells might make you want to open basement windows, that is exactly the wrong thing to do when it’s hot and humid outside.
Basement windows should be opened only when it’s cooler outside than it is in your basement. Whenever it’s warmer outside than downstairs, keep the windows closed and use either a dehumidifier or air conditioner to bring basement humidity down below at least 70 per cent. In my experience this is the moisture threshold that triggers mustiness.
Aiming for a basement humidity level of 60 to 70 per cent is a good compromise between dry conditions and economical energy use.
Getting a humidity meter is the first step to managing your basement so that it’s free of mustiness all year. This is essential because it’s the only way you can know what’s going on down there humidity-wise.
Removing moisture
You’ll also need a dehumidifier or some kind of air conditioner. Both will remove moisture from the air, although an air conditioner delivers more drying action for a given amount of electricity consumed.
If you do opt for a dehumidifier, get one that’s rated to remove at least 70 pints of water per day. Many models are smaller than this and don’t deliver the drying power needed.
Are you planning to finish your basement? Carpet and underlay applied directly to concrete floors is common but also much more likely to trigger condensation than any other part of your basement. Always install basement carpeting on some kind of insulated, vapour-proof subfloor. Check an earlier column for a video lesson on basement mustiness.
