Designing dry, warm basement floors
Dry, warm, basement floors are designed to manage:
- Water at ground level — gutters and downspouts connected to splash blocks on soil sloped away from the building;
- Water at the footing — perimeter pipe drainage at the footing;
- Water in porous materials — a capillary break (for example, free-draining gravel or a nonporous sheet good like polyethylene) underneath the slab, between the concrete walls and the soil, and between the footing and the foundation walls.
- Floor surface temperature — subslab rigid insulation warms the basement slab and elevates its temperature.
The basement floor is dry, you think
Before you put any finished flooring down on what appears to be a dry floor, it’s a smart thing to determine just how dry it really is. Many basement concrete floors don’t have a capillary break or vapor barrier installed underneath them and evaporate what can be quite a bit of water off of their surface, water that is wicking from the soil up through the concrete.
There are two simple tests to determine slab moisture transmission (see “Concrete in Practice 28 – Concrete Slab Moisture”). One involves just taping down a sheet of plastic for at least 16 hours and seeing if the underside of the plastic is wet (ASTM D4263). If it’s dry, congratulations – you can put down any finished flooring product you want to. If it’s wet, you might want to consider the next test, the anhydrous calcium chloride test (ASTM F1869), which can tell you the RATE of moisture transmission. Knowing this rate means that you can check flooring manufacturer recommendations on moisture transmission rates and see what types of flooring can handle what your slab is doing.
Keep in mind that many types of flooring go down with water-soluble adhesives; that can be a real problem if your slab is drying a lot of water through its surface.
Carpet in basements
Many, but certainly not all, carpets and carpet cushion are vapor-permeable, so if your basement slab is transmitting moisture, it can continue to evaporate up through the pad and carpet. On the other hand, if the carpet or pad is on a cool slab and it is loaded with dirt, pet and human dander, it’s just about a perfect place to cultivate dust mites and even mildew.
The bottom line?
If your basement slab is un-insulated and/or damp or transmitting moisture (and most are):
- Avoid installed wall-to-wall carpeting. Choose a hard surface material that meets manufacturer recommendations for what you know to be how much moisture is transmitting up through your basement slab.
- Maintain gaps for air circulation. Keep absorbent materials up off the basement floor and keep gaps between your slab and all objects, such as furniture.
- Keep your basement floor clean. Damp mop or vacuum your basement floor on a regular basis, based on use and traffic. If you’re using area rugs, launder or clean these on a regular basis and ideally, give them regular exposure to direct sunlight by taking them outside.
- Manage relative humidity in the basement. Use an Energy Star-labeled dehumidifier to keep the interior relative humidity at or below 60%.
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