Basement Subfloors and Indoor Air Quality: Home-Improvement Health Implications

Reader Contribution by Steve Maxwell
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Nobody wants a moldy finished basement, but that’s exactly what too many homeowners end up with. Even after spending thousands of dollars, problems regularly emerge. The reason why? Moisture and the microbe growth it triggers. Too many basements are still being finished as if they were above-ground spaces, and the consequences are not limited to just the finished basement themselves. Poor basement air quality always spreads to the rest of your house, and that’s why a healthful finished basement and indoor air quality go hand-in-hand. What too many people don’t realize is that a healthy basement begins with a healthy basement subfloor.

While it’s entirely possible to install a warm, comfortable finished basement floor that doesn’t encourage mold growth, you need to get intentional about moisture control. Basements are essentially just fancy holes in the ground, and moisture in all its forms loves to flow into holes in the ground.

Two Kinds of Moisture

The first thing to understand is that liquid water and water vapour are two different things. Even if you don’t actually see liquid water in your basement, you could still have a moisture and indoor air quality problem. Here’s how .

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In the interests of making basement floors warmer and more comfortable under foot, it’s not unusual to install carpet and underlay directly on the concrete floor without the use of a basement subfloor. And while this seems like a good idea on the surface, there’s a very real danger of condensation developing within the carpet pile itself. And the warmer and more humid it is in the summer where you live, the more of a threat invisible carpet condensation and subsequent basement mold becomes. Solving the basement floor condensation problem is one reason why basement subfloor tiles are such a good idea.

How Basement Subfloor Tiles Work

Subfloor tiles came on the scene in the late 1990s and they make it easy to create healthful, comfortable finished floors in basements. One of the most commonly available tiles are made by a company called DRICORE. Made of nominal 7/16-inch-thick by 2-foot by 2-foot pieces of oriented strand board (OSB) with tongue and groove edges, tiles interlock with their neighbours as they go down. The underside of the most common type of basement subfloor tiles has a layer of strong plastic with little legs extending down underneath.

This design does several good things. First, the plastic keeps the OSB up and separate from any liquid moisture that might come up through the concrete floor or walls. This little space won’t help if 6 inches of water floods into your basement, but will make leaks up to 1/2-inch deep a non-issue. Although you should never finish a basement that hasn’t proven itself completely dry over a number of years, the air space underneath subfloor tiles allows small amounts of unexpected moisture to make it to the floor drain or simply seep away through pores of the concrete. This air space is key and the reason why basement subfloor tiles discourage the growth of mold even though the OSB on top is not mold-proof.

As useful as it is to have a basement subfloor assembly that can handle minor leaks in stride, this is not the most important moisture-related advantage offered by basement subfloor tiles.

Another way subfloor tiles prevent mold and mildew is by preventing warm, moist indoor air from penetrating the carpet and underlay from the top down, then cooling against the concrete and losing some of its moisture as condensation. The dimpled plastic on the underside of the subfloor tiles acts as a two-way vapour barrier. Small amounts of liquid water are kept down where it belongs, but also moisture laden air never gets the chance to come in contact with the concrete, eliminating  one leading cause of poor basement air quality – condensation within carpet and underlay.

Currently, OSB-type basement subfloor tiles come in two versions. The most common type has those raised plastic feet I was telling you about. There’s also a version of DRICORE with extruded polystyrene foam underneath instead of the plastic. This provides more insulation value at slightly higher cost, with slightly lower capacity to deal with leaked moisture.

Leave Basement Floor Uncoated

So, if liquid and condensed moisture are the leading causes of basement mold, mildew and poor indoor air quality, doesn’t it make sense to apply a waterproof coating to all basement floors before finishing, just in case? If your observations show that no liquid water is coming through the bare concrete of your basement floor, there’s still a reason you might NOT want to coat that floor with a waterproofing compound “just in case”.

If your basement ever does leak a bit of liquid water in the future and it’s sitting underneath some of those subfloor tiles, bare concrete could be your friend. Despite the fact that your concrete floor is at least 4” thick, water that doesn’t make it to the main drain of your house may still be able to seep away through pores of the concrete. In fact, more leaked water could disappear by soaking through an unpainted concrete floor than ever does make it to the main drain. Sometimes too much diligence ends up back-firing.

If you’re planning to finish your basement, check out my online basement finishing video course here. It’s called HOW TO FINISH YOUR BASEMENT, it’s currently free, it’s the most detailed course of its kind in the world, and it has saved many people from making poor basement finishing choices that affect the healthfulness of their home. This course is useful for people who want to finish their basement themselves or to be informed as they hire and manage a contractor to do the job. HOW TO FINISH YOUR BASEMENT remains free until October 31, 2020, so visit and bookmark while you can.

Steve Maxwell is a DIY expert and longtime contributor to MOTHER EARTH NEWS. He and his family homestead on Manitoulin Island, Canada, cultivating a little patch of  farmland surrounded by a sea of forest. Connect with Steve at, and read all of his MOTHER EARTH NEWS posts here.

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