Renovation & Design

MIKE HOLMES: Finished basement needs proper insulation

Postmedia/Basements can be the perfect environment for mould and mildew to thrive. Insulating the walls and floor properly helps prevent future moisture issues.

One of the most popular renovation jobs is to redo a basement. Done properly, a finished basement can add a lot of livable space to your home. It usually becomes a place where the kids -- and sometimes adults (What guy doesn't dream of having a man cave?) -- go to play. A finished basement can add value to the house if you ever want to sell.

When finishing a basement, the most important thing to make sure of is that it's properly insulated. Otherwise, you're just going to create the perfect environment for mould and mildew. That's not the kind of thing you want your family exposed to. Breathing in mould spores can lead to serious health problems, such as respiratory issues.

Most contractors would probably say it's only necessary to insulate a metre below the outside grade. They might think because the temperature below the frost line stays pretty constant, you have to worry only about insulating the top half of the basement walls. Not only is that wrong, it's a sure way to create moisture problems.

Insulating your basement does two things: It keeps the basement warm in winter and cool and dry in summer. Remember, a lot of moisture can build up in the summer because of all the heat and humidity. Your basement insulation controls that for you.

Basements are mostly underground. The temperature of the basement floor stays fairly constant, but as you go up the walls toward the ceiling, the temperature rises. So the temperature near the ceiling of your basement is always higher than at floor level. This leads us to a problem. When a basement is finished but not properly insulated, warm air from inside the finished space can come into contact with the foundation walls, behind the insulation. The foundation walls are going to be cooler because of the temperature of the soil on the other side. Warm air holds moisture. So when the warm air comes into contact with the cold foundation walls, it will start to cool. That's when you get condensation.

Condensation in your finished basement can build up in insulation and the wood framing and even pool on the floor behind finished walls. From there, your basement can become a breeding ground for mould spores -- any parent's nightmare.

If your basement is insulated the same way as an above-grade wall -- with wood studs against the wall, batt insulation in between and vapour barrier over that -- you will have air movement, which can lead to condensation and mould problems.

Can you imagine trying to fix a mould and moisture issue after the basement is finished? There goes the $20,000 you already put in and add more for the cost of fixing and replacing everything. Unfortunately, too many people don't have to imagine a situation like that. They're living with it.

Ideally, you want to have rigid foam insulation in a basement against all the outside walls and the floor -- a minimum of five centimetres on the walls and 2.5 cm on the floor. All rigid foam insulation is mould and mildew resistant. It won't hold moisture, even if you had a flood in your basement. Another great thing about rigid foam is that it comes shiplapped, like tongue and groove, so each piece fits tightly against the next without gaps.

Make sure your contractor uses an adhesive that is rated for foam when they're gluing the foam to your basement walls and floor. If it isn't, it can ruin your insulation. Ask them what glue they're going to use and then do your homework. Also check if they're tuck-taping each seam and using spray foam to fill any gaps around the edges. That's why it's important to be around during a renovation and not just see the finished product at the end. Once the walls are up, you can't know what might be behind them.

Insulating the basement this way creates a thermal break between the air inside your basement and the air outside. It gets rid of any air movement behind the walls. It's like a beer cooler, which has rigid foam insulation, too.

After the rigid foam is on the floor and walls, it can be studded over. I would go with a treated lumber -- like PinkWood -- that's mould, moisture, termite and fire resistant. I'd also use mould-resistant drywall.

Catch Mike Holmes in his new series, Holmes Makes It Right Tuesdays at 9 p.m. on HGTV. For more information, visit For more information on home renovations, visit


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