Installing and Using Carpet over your concrete basement floor
All carpets are manufactured with synthetic fibers; nylon, polyester or polypropylene, and are immune from mold and mildew.
To support mold growth, certain specific conditions must exist: humidity consistently above 60%, temperatures between 50 and 90°F, continued darkness, a pH of 3 to 8, and a food source. As a form of thermal plastic, carpet cannot, in and of itself, be a food source for supporting the growth of mold and mildew. Things like dirt and food that get trapped in the carpet fibers can become a food source, but the carpet alone cannot.
There are other factors that promote mold and mildew, such as, if a building has a humidity reading that stays above 60%, there’s a lot more to worry about than the possibility of mold or mildew in the carpet. Under such moist conditions, other surfaces, like drywall, wood, and natural fiber materials will be prime targets for mold and mildew to grow on. Mold spores are everywhere and they are present in every building interior and unless moisture is controlled mold will grow.
In terms of indoor temperature, temperatures on the floor where carpet is installed are normally too cool and dry to pose a threat. The pH of carpet installed over concrete is predictably not at a level conducive to mold growth.
Carpet installed in a basement directly over concrete can be laid using either the direct glue-down method or a stretch-in over cushion. Virtually all adhesives used in the industry today do not support mold growth. There are also anti-mold cushions available that actually prevent the growth of mold, mildew and bacteria. Any carpet placed on these kinds of cushion is protected from potential threats that originate from beneath the carpet. Of course, moisture that comes up from beneath the slab does pose a problem, but it’s not related to the carpet.
Sources of moisture that would perpetuate the growth of mold or mildew in a residential basement include: leaks, “sweating” or condensation off cement block or poured walls, over-wetting from cleaning by an uninitiated operator or do-it-yourselfer, an unbalanced humidifier or HVAC system, or moisture in the substrate. The most likely sources of moisture coming up from the floor are either moisture in the concrete slab itself or moisture vapor emissions coming up from beneath the slab.
Moisture in a concrete slab comes from two sources: water of convenience, which is the term for the water necessary to mix the cement to a workable consistency, or free water, which is essentially water that doesn’t combine with the dry cement, but instead roams free inside the slab and constantly tries to escape. Water vapor that comes up from beneath a concrete slab usually comes from failing to install a low-permeance vapor retarder directly below the slab. Not using a barrier to stop the moisture sets the stage for problems.
Moisture related flooring problems are common today for a number of reasons. One of the biggest causes is rushing the job the concrete isn’t given sufficient time to dry to an acceptable level for installing floor covering. Often, the vapor barrier beneath the slab is missing, inadequate, or positioned incorrectly. Moisture protection should be placed directly below the concrete, and it should be from10 to 15 mil thick so it won’t tear. One good product is Stego Wrap vapor barrier, but there are several others available. Anything less can be compromised too easily.
A properly working and balanced HVAC and air filtration system, plus an owner knowledgeable in their efficient operation are of prime importance in preventing conditions that propagate the growth of mold and mildew in the living space. Maintaining carpet by vacuuming frequently with a high-efficiency vacuum that may also have a HEPA filter will keep carpet free of dust and dirt and free of substances that can infiltrate carpet and become food sources for mold and mildew. Cleanliness of all surfaces, not just carpet, is key to preventing mold and mildew growth in any space.
Synthetic carpet is a good choice in a basement’s concrete floor.
There is no reason for carpet not to be used below grade, or above grade, in residential or commercial applications. In fact, with the right cushion, carpet is actually the best flooring material to use as it adds comfort to the space, provides thermal insulation, and helps thwart sound and noise. Carpet is also safer for young children to play on, as a fall on carpet is much less threatening than a fall on hard surface flooring. Carpet breathes, which means that small amounts of moisture vapor under the carpet will have a chance to dissipate into the air, instead of being trapped under a non-permeable material like sheet vinyl flooring, for example. Simply put, there is no logical, rational, reasonable, or viable reason not to use carpet in a reasonably dry basement area.
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