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Mould Resistant Basements.

May 04, 2021

Five building strategies that combat musty basements and bad indoor air.

By Steve Maxwell

There’s a finished basement I know that’s both typical and dangerous. It’s in a small town recreational building near where I live in rural Canada, and this basement hosts a number of different groups and functions year-round. Other than the very slight musty smell of the place, I didn’t think much about indoor air quality in this basement until something happened to my son.

Within minutes of walking downstairs after several weeks of hot and humid summer weather, his eyes began running, he became congested, his throat itched and so did his ears. No amount of kleenex could keep up. I’d always been fine in this place, as I was that day, but as word got around in the crowd about my son’s sudden allergy symptoms, half a dozen people piped up.

“That always happens when I come down here.”

“My throat tightens every time I walk down those stairs.”

“Coming into this basement always makes my hay fever worse.”

This basement has always been visibly dry, it’s bright and it looks good. The only outward sign that something is wrong is that faint musty odour. The thing is, basement mustiness is always just the tip of the iceberg. It invariably means there’s some kind of hidden mould growth somewhere – enough to send signals to your nose – and it’s happening in many finished Canadian residential basements because of the way they’re built. The crazy thing is, basements are still being finished today in ways that virtually guarantee hidden mould growth and poor indoor air quality over time. By finishing basements following the five mould-busting strategies you’ll get here you’ll minimize the chance of mould growth and boost indoor air quality.

Basement Mould-Busting Strategy #1

Tolerate No Moisture

No matter how well you follow the other four strategies, mould will always win in the presence of sufficient moisture. That’s why you must never finish a basement space that’s anything less than 100 per cent dry, 100 per cent of the time. This is obvious to you, but not necessarily to your clients. In fact, it’s not just liquid moisture that will ruin a basement and air quality, but invisible moisture vapour, too. You can have a basement that has looked dry for decades, yet you can still have a moisture problem. How do you know? Polyethylene vapour barrier plastic is your best detection tool.

Cut 24” x 24” pieces of polyethylene vapour barrier, then tape them to the walls and concrete floor in several places. Leave them for a few days, and look for moisture building up on the inside face. What you see under the plastic is what will happen inside any walls you put up. If you do see droplets of moisture (and it’s not unusual), show your client, then remove the plastic, paint the bare masonry with a waterproof paint such as Drylok or Xypex, then test again. Only when a basement passes all moisture tests can it be safely finished.

Basement Mould-Busting Strategy #2

Eliminate Organics

When it comes to basements (and many other areas of life), designing things with multiple layers of safety is always the best idea. If your basement really was 100 per cent dry, 100 per cent of the time, then there’d be no danger using wood or paper-based drywall for finishing. But what if 100 per cent dry actually turns out to be only 95 per cent of the time? A little bit of moisture can feed a lot of moulds, but it has a harder time growing without organic matter to feed on. That’s why it makes sense to keep wood and paper building materials out of the basement, or at least well away from potentially moist masonry surfaces. This is where the still-common practice of using wooden wall studs in basements is especially foolish. Better to use metal studs or no studs at all. A growing number of foam-based, stud-free building materials for basements go a long way to eliminating organics in hidden places.

Basement Mould-Busting Strategy #3

Use Impervious Insulation

Moisture can invade a basement from two directions. It can travel in through masonry basement walls and floors in the form of liquid or vapour, but it can also come from humid outdoor air coming inside the basement. If this air is allowed to make it’s way into wall cavities, it can cause droplets of water to appear out of nowhere within the wall itself. Hidden mould is then guaranteed. What does this have to do with basement insulation? Quite a bit. All insulation products can be roughly divided into two groups: insulation that air can move through such as batts and loose fill; and insulation that air can’t move through such as rigid, high permeability foam. The best basement insulation doesn’t allow air to move through it at all, nor does it absorb moisture in the event of a liquid leak. This is why stud-free, non-organic basement wall systems have a big advantage over studs.

Basement Mould-Busting Strategy #4

Never Install Carpet Directly on Concrete

None of your clients like to walk on a hard concrete basement floor, and that’s why installing carpet and underlay are such popular basement strategies. Trouble is, they can also be very powerful mould breeding grounds, especially during hot, humid summer weather. If warm, outside air is allowed to make it’s way into the basement, it will settle on the floor and sneak into the carpet and underlay. Since the concrete floor underneath is always cool, it will cool down the air to the point where it can’t hold all the water it used to at higher temperatures. The result is tiny droplets of condensed water forming within the pile of the carpet and the underlay. Mould loves to grow in conditions like these, and in time the carpet may smell like a wet dog.

There’s nothing wrong with installing carpet and underlay on a basement floor, but only do it on top of a raised subfloor. This eliminates any chance that warm, humid air reaches the cool concrete. A non-organic interface between the subfloor and concrete floor is essential.

Basement Mould-Busting Strategy #5

Ventilate Only When Conditions are Right

If your client believes that ventilation is the best cure for basement dampness and moisture problems, you’ve got some education to do. Ventilation can actually be a source of damaging levels of basement moisture during some seasons. When it’s warm and humid outside, outdoor air transports moisture inside. As this outdoor air cools in the basement, it can’t hold as much moisture as it did. It’s not unusual for outdoor air at, say, 75 per cent relative humidity to skyrocket to 90 per cent or even 100 per cent relative humidity when it cools in a basement. Go ahead and open basement windows when outdoor temperatures are cooler than basement temperatures. That’s a good idea and will lower basement humidity levels in most cases. Just never open basement windows when it’s warmer outside than it is inside. The only way to lower basement humidity levels under these conditions is with a dehumidifier or air conditioner running behind closed windows.

If you look at newly-finished basements these days, most violate some or all the mould-busting strategies you’ve learned about here. So don’t let popularity guide you in the details of how you finish basements. What’s commonly done today will be considered completely inadequate tomorrow. After all, people get tired of grabbing a box of kleenex every time they go downstairs.

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