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HOUSE WORKS: Don’t make these three basement finishing mistakes


A beautiful finished basement is a great home feature, but enduring success depends on avoiding specific common mistakes that promote mould and harmful indoor air quality. - Canstock Photo
A beautiful finished basement is a great home feature, but enduring success depends on avoiding specific common mistakes that promote mould and harmful indoor air quality. - Canstock Photo

Finishing your basement is the most economical way to add comfortable living space to your home

Winter is the season when Canadians most cherish the idea of a finished basement. Sure, it’s great to have one any time, but as the winds swirl and snow blows outside, there’s something especially attractive about snuggling into a comfy couch or easy chair in your warm, bright, inviting subterranean space.

The thing is, most homeowners and some professional builders don’t realize how easy it is to finish a basement incorrectly. Even if things look great on the surface, common design mistakes can encourage mould, mildew and hidden corruption behind the scenes in your basement — none of which can be fixed easily after the fact. This is why the best way to finish a basement is to do it differently than the way it’s always been done.

The primary challenge when it comes to finishing a basement is maintaining good indoor air quality throughout your home. Comfort, sufficient light and a pleasant floor plan are all important, but they don’t matter much if your basement is polluting your indoor air with mould spores. The unique danger with basements is that moisture in the form of liquid water leaks, moisture vapour and condensation will enter wall cavities and floors, triggering the growth of mould and mildew on organic materials. A well finished basement is always dry, but also has some capability to handle unexpected moisture.

Basement Finishing Mistake #1: Having Too Much Hope

Even the wettest basements in the world look dry at some times of the year. And even the smallest amount of leaked water will have disastrous effects if it happens behind a wood frame stud wall or into the pile of broadloom. All this is why it makes sense to hold your basement finishing enthusiasm in check and give yourself a chance to prove for sure that your basement really is dry. It’ll cost tens of thousands of dollars to finish your basement in even the simplest way, and this is a significant risk if you go ahead without monitoring that your basement is truly dry over at least one full year before finishing.

Basement Finishing Mistake #2: Installing Carpet on Concrete

Even with the addition of thick underlay, wall-to-wall carpet on concrete basement floors is bad for two reasons. First, if you have any kind of a leak — no matter how small — it’s going to soak the carpet and kickstart mould. And second, carpet can encourage droplets of condensation to develop along the concrete floor during warm, humid summertime weather. Both these problems can be solved with the right kind of subfloor underneath whatever finished floor you choose. Every building supply store sells basement subfloor tiles these days. They create a slightly elevated space underneath to separate finishing floor from concrete and create a small drainage channel underneath in the event of a small water leak.

Basement Finishing Mistake #3: Inappropriate Rim Joist Insulation

Rim joists are the areas where the floor frame of the first floor of your house meets the outside walls in the ceiling of your basement. Almost every home I examine has mould growth on the fibre-based insulation behind vapour barrier plastic installed in rim joist areas. As common and code-compliant an approach as this is, it simply doesn’t work. The code definitely needs to be upgraded. There’s simply no way you can install plastic to seal out the warm, moist indoor air from infiltrating the fibre insulation, cooling within it during winter, and forming mould-promoting liquid water droplets inside. At least a three-inch depth of closed cell spray foam is the only way to seal and insulate rim joist areas properly.

This basement allows no organic matter to touch masonry walls or floors. When this strategy is applied to a truly dry basement, the result is long-lasting freshness and freedom from mould. - Stephen Hutchings
This basement allows no organic matter to touch masonry walls or floors. When this strategy is applied to a truly dry basement, the result is long-lasting freshness and freedom from mould. - Stephen Hutchings

Finishing your basement is the most economical way to add comfortable living space to your home. You’ve already got a roof and exterior walls, you just need to divide up the basement and make it nice. Just be sure to do it so mould doesn’t get a chance.

Steve Maxwell is a syndicated home improvement and woodworking columnist who has shared his DIY tips, how-to videos and product reviews since 1988.

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