Basement Subfloors – All you need to know

June 7th, 2011

Basement Subfloor

When choosing flooring options for basement rooms, carpet surfaces as one of the most popular choices, especially for basement family rooms, home theaters and guest rooms. Carpet adds comfort and warmth to basement rooms, can be easily installed and, if compared with some other popular flooring choices, it can be less expensive as well.

However, unlike any other area in the house, basement floors are prone to moisture. The slab, which is made of porous concrete, sits straight on top of dirt. Wet dirt.
Some of the ground moisture will always be infiltrating the slab, and during the rainy season the problem can get much worse.

That makes it very unsafe to place padding and carpet straight against the slab, because they will be soaking all that moisture and there is a huge potential for mold growth.
This is the reason why so many carpeted basements smell musty after a while, even though there are no visible leaks.

Contractors typically deal with it in a number of ways. Some are temporary solutions that work for a limited period of time; some are just plain disastrous choices from the start.

Waterproofing PaintsWaterproofing Paints
Popular and relatively inexpensive, they are mostly a temporary solution. They can’t keep the water from infiltrating the slab. All they do is keep the water from evaporating into the basement, for a while. Give it enough time and the water trapped under the sealant will build up and the resulting pressure will cause the layer of sealant to crack and peel off. Just look at the manufacturer’s warranty in these products which is of only. In the best case scenario, that is how long it will take for you to have to strip the floors and replace the carpet. In our experience, it often happens much sooner than that.

Plastic over the Slab
This approach is often recommended by seasoned contractors in Forums and answer sites. Just like the sealants, a poly sheet lining the slab does nothing to prevent the water from seeping through the slab. It only traps the water underneath the plastic, preventing it from evaporating into the basement, which is  not a good idea.
According to the U.S. Department of Energy, basement floors and walls should be allowed to “breathe” so that the moisture wicking through the slab has a way to dry to the interior of the basement.
When you place a poly sheet over the slab, you cause the moisture to build up underneath and, without a chance to dry, there is no saying what is going to grow underneath.

Insulation Foam Board Over the Slab
It might seem like a good idea, after all it provides both a thermal break and a vapor barrier. The problem is that, just as it happens with the plastic sheet, when placed straight over the slab, the flat board doesn’t allow the air to circulate and moisture can build up underneath.

Moldy Plywood SubfloorWood Subfloors (plywood, OSB, pre-treated)
When trying to create a surface to attach the padding and carpet, many contractors will use in the basement, the same approach used in above grade areas: they will create a wooden subfloor. That is the worst possible choice for a basement subfloor.
Plywood, OSB or pre-treated wood, are still wood. They soak moisture, and being organic, they will eventually grow mold and rot. It is not a matter of if, but when.
Even if you do, as many will suggest, place the wood on top of a plastic vapor barrier.
Consider that basement water comes from more sources than just the ground. Think plumbing and water heater tank leaks, backed up drains, running faucets. If you happen to lack a battery operated backup sump pump, pump failure is a possibility as well.
A wooden subfloor will absorb all the water and, according to the U.S. Environmental Protection Agency, when not dry in less than 48 hours will develop mold.

Basement Subfloor Products

Because they were specifically engineered to work in basements, they are always the best choice. They all work under the same principle: They have or accommodate a flat surface, which is elevated from the floor by a system of channels or pegs.
This allows the air to circulate underneath, drying out the slab, all the while, keeping it from evaporating into the basement.
However, not all basement subfloors are created equal, and while some work fairly well, some have the potential to create problems. Here’s a quick look at them.

OSB Lined Subfloor TileOSB Lined Basement Subfloors
As stated before, because these subfloor options, commonly sold in hardware stores are lined with wood, while they are somewhat able to address the moisture that comes from the slab and protect the carpet, they will not withstand a water accident or a basement flood. Some also have a design flaw: the OSB overlaps the plastic mat and that allows the moisture evaporated from the slab to come in contact with the wood around the edges, creating mold problems.


Drainage Mats (Dimpled Subfloor Rolls)
Drainage mats work well to raise the flat surface off the floor and allow the slab to dry underneath. However, many contractors put a layer of plywood on top, which causes the same problems seen in OSB lined subfloor products.


Thermal Dry MattThermalDry Basement Subfloor Tiles
Thermal Dry tiles follow the same principles of hardware store brand subfloors and drainage mats: it creates a space between the raised, flat surface and the slab, to allow air to circulate underneath. With one main difference: it is 100% inorganic and waterproof. It has no OSB Layer and doesn’t need to be lined with plywood. Padding and the carpet can be laid directly over the sturdy, flat plastic surface. It acts as a vapor barrier and provides some Thermal protection as well. It installs easier than Drainage Mats, through and interlocking system, and individual tiles can be removed or replaced without disturbing the surrounding ones.
ThermalDry is mold resistant and is built to withstand  basement harshest moisture conditions, including a basement flood!

ThermalDry Basement Subfloor Tiles are only available through the Basement Systems and Total Basement Finishing Dealership Network. Contact us for a dealer near you!

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