Sealing your basement floor before installing carpet is a good idea.
Carpet is a common choice as a finish covering for the basement floor. It is a good choice overall because it softens the surface and increases the comfort level when you walk across the floor. Installing carpet also helps to insulate the interior of a basement by keeping the floor warm in cold weather. Applying some type of sealer on a slab floor before installing carpet will prevent condensation as well as protect the slab.
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Moisture and Concrete
Concrete is not impervious to moisture seeping or "wicking" through. It is a porous material that continually allows some seepage even after concrete has cured and set. Water vapor from beneath the slab can amount to as much as 3 to 5 pints or more per 1,000 square feet a day. A basement with some ventilation windows, doors and HVAC will dissipate this vapor enough so it won't be a significant problem. Ambient air temperatures and humidity will also cause a basement floor to "sweat" when the interior temperature drops below the dew point.
Several types of basement floor sealants are available, ranging in price and effectiveness. One of the best types of floor sealants is epoxy, a two-part chemical mixture that penetrates deeply into the concrete and is the most expensive and time-consuming floor sealant to use. Urethane and acrylic sealers are somewhat less expensive than epoxy and require less time to set before installing carpet. Densifiers are another type of floor sealant that prevent moisture from wicking to the surface of the floor, although a good vapor barrier installed under the slab may negate the need for this type.
Acrylic paint is one of the best types of paint to use before installing carpet on your basement floor. It acts as a sealant by penetrating the pores in the concrete and is easy to apply with a paint roller. Solvent-based or water-based acrylic paint is also widely available and relatively inexpensive, although it doesn't penetrate as deeply as epoxy and urethane floor sealants.
Installation Tip for Carpet
Once your basement floor has been sealed, you can install the carpet either with or without padding. Stick-on carpet tiles are one choice that doesn't require any padding. However, padding will only enhance the acoustic and insulation qualities that make installing carpet a good idea to begin with as well as provide an even softer surface to walk on. Talk with a professional carpet installation contractor, or do some research online before installing carpet on your basement floor.