Essential Information About Flooring Underlayment: Key Considerations Before Making a Purchase

With more than twenty years of practical experience in renovating, repairing, and enhancing residences, Lee has been offering home improvement guidance for many years.

While many people perceive a home’s floor as merely the top layer of a visible covering, it actually consists of a system of multiple layers, each playing a vital role in the floor’s functionality and longevity. Among these layers, the underlayment is particularly important, even though it often goes unnoticed.

Flooring underlayment is positioned directly beneath the surface layer of the flooring, enhancing the stability, comfort, and sound insulation properties of the flooring material.

What is Flooring Underlayment?

Flooring underlayment consists of either rigid or flexible materials placed beneath the floor covering and above the subfloor. Its purpose is to level out any irregularities in the subfloor, enhance comfort underfoot, safeguard against moisture, and offer a degree of soundproofing. Common soft underlayment materials include foam, cork, and rubber, while rigid options typically consist of plywood and hardboard.

Tip

Underlayment and subfloor are often mistaken for one another since both serve as foundational layers for flooring. Consider the subfloor as the structural foundation, while the underlayment functions as a stabilizing layer.

The Role of Underlayment in Flooring Systems

In residential buildings, the typical flooring structure is made up of four distinct layers. Beginning from the uppermost layer and moving downwards, these layers include:

• Flooring refers to the final, visible surface of a floor, which can include materials like hardwood boards, carpets, ceramic tiles, or vinyl. It is the layer that is both seen and walked upon.
• Beneath the visible flooring lies a layer known as the underlayment, typically measuring around 1/4 to 1/2 inch in thickness. Its primary function is to create a smooth, even base for the flooring material. Various materials can be used for underlayment, selected based on the specific requirements of the flooring. Common options include plywood, hardboard, and cement board, although a thin layer of foam padding is often used as well.
• Subfloor: This component, made of OSB or plywood, is an essential part of the home’s framework and is installed prior to the underlayment and flooring. These OSB or plywood panels play a crucial role in the home’s structural integrity, offering support and stability to the flooring system when secured to the floor joists. Typically, the thickness of the OSB or plywood ranges from 19/32 inch to 1 1/8 inch.
• Joists are horizontal wooden supports that are positioned on foundation walls and beams, playing a crucial role in the structural integrity of the entire framing system. They are commonly constructed from 2-by-10 or 2-by-12 lumber, or from engineered microlam materials.

Advantages of Flooring Underlayment

Flooring underlayment refers to a lightweight layer positioned between the surface flooring and the underlying subfloor. This term encompasses a variety of materials, each tailored to complement the specific type of flooring it serves.

In contrast to the subfloor, which is integral to the structural framework of a house, the underlayment primarily functions to create a level and smooth base, facilitating the straightforward and visually appealing installation of the top flooring. Additionally, it can fulfill other roles, such as reducing the noise of footsteps, enhancing the comfort of the flooring underfoot, and, in certain instances, serving as a moisture barrier. The main functions of underlayment include:

• The underlayment creates a more even and reliable surface for the flooring material compared to the subfloor, which primarily serves a structural purpose.
• Enhances bonding: When it comes to cement board, this underlayment creates an ideal surface for ceramic tiles to adhere to. Tiles struggle to stick properly when placed directly on a subfloor, which tends to expand and contract.
• Enhances structural integrity: A durable underlayment can significantly boost the overall stability of the flooring. This is especially beneficial in older residences, where the subfloor is often made of boards instead of OSB or plywood panels.

Gentle Padding Materials

Foam and cork underlayments serve dual purposes. They create a minimal cushion between the upper floor covering and any irregularities in the subfloor beneath it (or the existing floor if new flooring is being laid directly over the old). These underlayments help to level out minor flaws, including slightly raised screw heads and tiny knot holes.

Flexible, soft underlayments lack the structural support offered by sheets of wood or cement board, which makes them classified as floating underlayments. Consequently, they remain unattached to the subfloor.

Depending on the circumstances, these materials are generally not recommended as substitutes for a solid underlayment placed directly on a subfloor. However, they can be an effective option when laying a new floor covering directly on an existing floor that remains in good condition.

A layer of foam or cork serves as an excellent underlayment when installing laminate flooring over an existing wooden or ceramic tile surface. Additionally, placing foam or cork over a solid underlayment can provide an efficient sound-dampening layer, reducing the hollow sound and sensation of laminate flooring when walked on.

Stiff Flooring Underlays

Numerous types of flooring are ideally placed on a solid underlayment that is laid over the subfloor. Common materials for rigid underlayment include plywood and hardboard like luan. While subfloor panels like DRIcore, cement board, and OSB may not traditionally fit the definition of underlayment, they are occasionally considered in this category.

• Plywood: Cut sheets of A/C grade plywood measuring 4 by 8 feet make for an excellent underlayment option. A/C grade indicates that one side of the plywood is relatively smooth while the other is rough, suitable for the underside. Depending on your needs, plywood with a thickness of 1/4 to 1/2 inch is often the ideal choice for underlayment in various dry applications, such as beneath hardwood, laminate, and engineered wood flooring. The A side is smooth enough to accommodate even the thinnest vinyl flooring. While shiplap or tongue-and-groove plywood sheets can be used for underlayment, standard straight-edge sheets are also a viable choice.
• Underlayment panels are designed to interlock and are available in tiles measuring 2 feet by 2 feet. One popular brand in this category is DRIcore. Although they can be quite pricey, they significantly expedite the installation process and serve effectively as a moisture barrier. These panels are particularly suitable for use beneath carpeting or laminate flooring laid over a concrete slab, as they elevate the flooring slightly above the concrete surface.
• Cement board, including products like Wonderboard and fiber-cement boards like Durock, is often utilized specifically for flooring installations that require mortar, including materials like stone, porcelain, and ceramic tiles. This type of board is characterized by its smooth surface, ease of cutting, and resistance to mold.
• OSB, or oriented strand board, can serve as an underlayment for certain types of flooring, but plywood is typically the favored choice for flooring that requires a solid underlayment.

When Underlayment Is Unneeded

In many cases, a type of flooring underlayment is necessary for various floor coverings. However, there are situations where underlayment is not needed, and it could potentially damage the flooring system.

Pre-Installed Laminate Underlayment

A clear instance where extra underlayment is unnecessary is when it is already integrated into the flooring material. Currently, many laminate flooring options feature underlayment that is pre-attached. In such cases, not only is additional underlayment superfluous, but adding another layer can actually compromise the stability of the flooring. Manufacturers of laminate flooring generally advise against the use of loose underlayment when the flooring already has pre-attached underlayment.

Recent Development

In newly constructed buildings with solid and even subfloors, it is possible to install carpeting directly on a carpet pad that is affixed to the subfloor, or to lay hardwood flooring on a basic layer of rosin paper placed over the subfloor.

Using Old Flooring as Underlayment

The existing floor covering may potentially function well as your underlayment. For instance, it’s quite typical for laminate flooring to be installed directly over existing sheet vinyl. Similarly, new luxury vinyl can frequently be placed over old vinyl without any issues.

Carpets are frequently installed directly on top of existing hard surfaces without any underlayment, aside from the carpet padding. Refer to the guidelines for your new flooring type to determine the appropriate underlayment to use.

Reconstructed Underlayment

In remodeling projects, it is common to require a thicker underlayment to address any shortcomings in the subfloor. If the subfloor is significantly damaged, it may be necessary to remove it entirely and install a new subfloor layer prior to adding the underlayment and the final flooring.

In a bathroom affected by water damage, it may be necessary to take out the existing plywood or OSB subfloor down to the joists. After that, a new subfloor should be installed, followed by a cement board underlayment, and topped off with a fresh layer of ceramic tile flooring.

Unique Factors to Take into Account

Minimize the thickness of your underlayment. Using a thicker underlayment can lead to complications, as it may cause height discrepancies between areas with varying flooring types. This can result in challenging transitions, especially if the floor height in one room is significantly elevated compared to the next. Additionally, in spaces with low ceilings, a thicker underlayment can further reduce the perceived height of the room.

Take into account how the thickness of the underlayment can influence the dimensions of doors and trim: adjustments may be necessary to maintain a level fit. In kitchen areas, a thicker flooring material can impact the clearance from the floor to the bottom of the countertop, potentially complicating the process of removing and replacing appliances.