Canadian Contractor

Basement humidity-busting tactics to beat summertime mustiness


September 1, 2020
By Steve Maxwell

Build Topics

As a contractor, no doubt you’ve been asked to solve tricky problems over the years. Now and then you probably even get blamed for problems you didn’t even create. Summertime basement humidity and mustiness is a common case in point. Whenever outdoor temperatures rise higher than the temperature inside a basement, the problem of warm-weather condensation can occur. This triggers the kind of basement mustiness and mold growth that everyone hates. Problems like these can be your fault if you finish a basement without understanding key moisture issues, especially as they relate to basement floors. And I’m not talking primarily about leaked water here, either. The real culprit behind mold-promoting basement moisture is water vapour and invisible condensation. This is where it’s important to understand what’s behind basement moisture dynamics and how to make sure you’re not responsible for finishing basements in a way that turns musty in a few years.

Strategy#1: Always install an impervious subfloor

What would happen if you built a house, insulated the walls with fibre-type batts, but neglected to install a vapour barrier? Condensation would occur within the wall cavities during cold weather, that’s what. The same dynamic occurs with basement floors when they don’t include a vapour-impermeable subfloor on top of the concrete. If warm, summertime air is allowed to make its way through, say, carpet and underlay, eventually that warm, humid air will begin to cool as it makes its way through the carpet and close to the floor. And if this air gets cool enough, the air can easily drop to the dew point. The resulting water droplets forming within the carpet or underneath a hard-surface floor is the last thing you want happening on your watch. The solution? Install vapour-impermeable subfloor panels

before any type of finished basement flooring goes down. The most widely available option is Canadian-made Dricore subfloor panels. Besides making feet feel warmer in winter, these panels prevent humid air from ever getting to the cool concrete condensation zone. The plastic layer on the bottom of the panels is key to the vapour barrier action. These panels also make floors feel warmer in winter and more pleasant to walk on.

Strategy#2: Educate your clients about basement window management

Water vapour and condensation is the biggest hidden cause of mold, mildew and mustiness in finished basements. And we all know that ventilation is the cure for mold, right? Not always. Understand that whenever the temperature outside a house is higher than the temperature inside the basement, the possibility of condensation in the basement exists. And the greater this temperature difference from inside to outside, the greater the condensation potential. So, how does this shake down into actual homeowner management of the basement you’ve just finished? Here’s a simple rule of thumb that does not fail: never open basement windows when it’s warmer outside than it is in the basement. Why? Because the warm, moist outdoor air itself can be the source of moisture. Explain to your basement clients that they need to keep a humidity meter in the basement and take active steps to lower humidity if it gets any higher than 65 percent during the summer. How? There are only two reliable ways: a dehumidifier (decent but not great), or air conditioning (uses less power than a dehumidifier for a given amount of moisture elimination, plus it cools the space). Opening basement windows when it’s hot outside simply causes basement humidity to rise. Try it yourself and see. You can finish a basement properly, but if your client often opens windows when it’s hot and humid, chances are moisture problems will appear anyway. It won’t be your fault, so you need to explain things sooner rather than later.

Photo credit: Steve Maxwell
A heat recovery ventilator (HRV) is a permanently installed household air exchange appliance. It trades stale indoor air for fresh outdoor air during winter while keeping most of the heat energy in your home. An HRV is the single most effective way to combat window condensation and it also boosts indoor air quality.

Strategy#3: Mechanical ventilation

You’ve probably noticed how often people are concerned about indoor air quality in their homes these days. The crazy thing is, most of the health-conscious clients who talk about this are barking up the wrong tree. They think they can enjoy excellent indoor air quality by avoiding materials that include chemical contaminants. A good start, yes, but never enough on its own. Even if you finished a basement using nothing more than organic tofu, without a molecule of urea formaldehyde anywhere, indoor air quality would still be less than ideal. People breathe, viruses build up, and there’s always off-gassing of something somewhere in the house. This is where one of two kinds of mechanical ventilators can help. You need to choose the right one for the situation and instruct your clients to use it properly.

A heat recovery ventilator exchanges outdoor air for indoor air while retaining a large portion of the energy invested in heating or cooling that indoor air. HRVs are ideal for use during the heating season because they extract moisture from the outgoing air stream while also preventing that moisture from combining with the incoming air. An energy recovery ventilator reclaims the energy invested in conditioning that indoor air, but it also preserves whatever moisture profile exists indoors. This means ERVs are ideal for use during summer months because it keeps humidity outdoors better. HRVs, by contrast, are superior for wintertime use because you want moisture to leave the house, not stay inside it.


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