In laminate flooring installations, the underlayment is the thin, flexible sheet of resilient material that is installed on top of a subfloor in order to act as a base for the laminate planks or tiles you're installing. While the subfloor provides the foundational support for the room, the underlayment is what supports the flooring material itself, intended to enhance and protect the flooring. It has several functions: It slightly levels the flooring; it cushions the laminate planks; it reduces sound transmission; and it slightly improves the floor's R-value (its resistance to heat transmission).
Proper selection and installation of underlayment is important because it can affect how your laminate flooring feels and how it wears over time. The type of underlayment chosen should be based on the environment, the type of floor chosen, and your own personal needs for space. It is very important to follow all vendor-provided instructions about underlayment when installing laminate flooring, as improper installation or the use of the wrong underlayment can void the manufacturer's warranty. You should also be sure the underlayment (and the subfloor below it) is level and smooth to get an even finish on the laminate flooring.
Working With Concrete and Plywood Subfloors
Concrete subfloors: When installing over a concrete subfloor, the main concern is moisture. Concrete is a porous material that can allow water to seep up into it and through to your flooring. This can cause adhesives to loosen, flooring planks to warp, and mold and mildew to develop. With a concrete subfloor, you will need an underlayment that acts as a vapor barrier in order to prevent moisture from penetrating through to your laminate installation. The normal choice is a thin foam pad material made of polyethylene or polypropylene, which rolls out in sheets.
Plywood subfloors: A breathable foam-type underlayment is generally used when installing laminate flooring over plywood or OSB subfloors. In most cases, you will not want to use a vapor-barrier-type underlayment when installing laminates on a standard plywood subfloor, because wood is a natural material that needs to breathe. Installing a vapor barrier over this can cause moisture to become trapped within the material, leading to the growth of mold or warping in the material. The exception is when laminate flooring is being installed in a high moisture area, such as a bathroom or basement, where a moisture barrier underlayment is a good idea.
There are any number of underlayment products to choose from when choosing an underlayment for laminate flooring, and it helps to know the features to look for.
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Insulation and Noise Reduction Considerations
R-value: This refers to the heat conductivity of a given material. The higher the R-value of the underlayment, the less heat will be able to move through it, while the lower the R-value is, the less insulation the underlayment provides. A high R-value underlayment will make the floor feel warmer underfoot in the winter.
Noise reduction: Some types of underlayment can act as a barrier to the transmission of noise. This can be important in certain working environments where silence is necessary, or in bedrooms where peace and quiet are preferred. Laminates can be noisy floors, and a good underlayment will help deaden the sound transmission.
Types of Laminate Underlayment
A common mistake is to use carpet padding or another soft, thick underlayment, thinking that this will somehow increase the comfort of walking on a laminate floor. But this will serve only to make the floor flex notably in a manner that can loosen joints. The proper underlayments for laminate flooring are very thin foam layers that slightly cushion the flooring but don't allow noticeable movement underfoot. These products are applied over the subfloor by rolling them out and sealing the seams with adhesive strips, which are sometimes preattached to the underlayment.
Standard foam underlayment: This consists of a thin layer of foam material that acts as a cushion between the subfloor and the laminate flooring. This is the most common form of laminate underlayment, and it is often used in cases when installing laminate flooring over a plywood or OSB subfloor. However, this should not be used in high-moisture areas, as it does not provide any vapor barrier characteristics. Standard foam underlayment is sold by rolling it out over the subfloor
Combination foam underlayment: As the name suggests, this type of underlayment combines standard foam with a vapor barrier layer that will protect the installation from moisture. Slightly more expensive than standard foam, this underlayment is appropriate for both plywood/OSB and concrete subfloors and can be used in most environments. This is the type of underlayment that should be used in moist locations, such as basements and bathrooms.
Cork underlayment: This material is used in flooring installations where noise reduction is important. One of the most expensive underlayment choices, cork can sometimes add as much as 50 percent to the cost of laminate flooring installation. While it does not provide any more cushioning or comfort underneath a laminate installation, it can be used to level out uneven surfaces between rooms. It also has natural antimicrobial characteristics.
Cork is not moisture-proof, so when it is used as an underlayment in basements or bathrooms, it is usually laid over a sheet of plastic that serves as the principal moisture barrier.
Laminate Flooring With Attached Underlayment
Today you can find many laminate floors that have been manufactured with underlayment padding built into the underside of the material. With these materials, you do not need to purchase and install a separate foam padding underlayment. In most cases, these flooring materials are installed directly over the plywood or OSB subfloor. Some types are also suitable for wet locations without an additional vapor barrier seal.