Underlayment for hard-surface flooring is a key component in many installations. Depending upon the type of finish flooring under which it is installed, it can quiet impact and ambient sound; improve thermal insulation; smooth out little floor imperfections; help inhibit noise from traveling into the room below; act as a vapor barrier and provide moisture protection that helps keep the finish flooring dry. But that wasn’t always so. Underlayment has come a long way since its introduction as a way of dealing with imperfections in subfloors.

Early resilient flooring (chiefly vinyl asbestos tile and linoleum) was so thick and rigid that imperfections in a plywood subfloor did not show through. As resilient flooring manufacturers reduced the thickness and stiffness of their products and began using patterns, imperfections in the plywood transferred through to the surface became more noticeable, evoking complaints from homeowners.

In the 1960s, flooring installers started using Masonite (hardboard) over a subfloor to reduce surface imperfections and improve the look of resilient flooring. However, hardboard is not an ideal underlayment material because it is dense and difficult to nail or staple into, and the nails or staples leave impressions on the surface. In addition, achieving good joints in hardboard is difficult.Another concern was, as the backings on resilient flooring became thinner, the resin in the adhesive sometimes transferred brown staining from the Masonite onto the face of the finish flooring.

Rough-cut plywood underlayment was also imperfect. While accepting nails, it did not have precision edges, compromising any installation over it. Knots or voids under the face could eventually cause a visible dent if a heavy weight compressed the finished floor in that location. In addition, wood can shrink and expand in response to temperature and humidity, which can damage the finished floor.

In the 1980s, a precision-cut, smooth-sanded, solid veneered plywood product imprinted with a nailing pattern was introduced as an underlayment option. Widely embraced by the flooring industry for use under resilient floors, this type of engineered plywood is still a very popular underlayment for installations where appearance of the finish floor is important. However, it does not have the added value that later forms of underlayment have included. 

Particleboard underlayment is smooth and hard and uniform in thickness, can be nailed or stapled to the floor, or bonded to it with adhesives. It is suitable for laminate, resilient, and seamless flooring likely to remain moisture free. But because it swells on the edges when wet, it is not a good choice under vinyl tiles, as any swelling could cause the installed tiles to separate from one another.

Like resilient flooring in its early days, today’s engineered wood flooring and laminate flooring products are thick and rigid, precluding cosmetic imperfections from telegraphing through. Therefore, underlayment for this type of flooring – as for hardwood flooring – addresses sound control, moisture control, thermal insulation, cushioning when walking and other issues.

Underlayment for Floating Floors

When floating floors came to the U.S. market in the early 1990s,corkboard and foam-like products attached directly to the flooring were commonly used to reduce sound transmission. For floors installed over a concrete substrate, 6-mil polyurethane was taped down to prevent moisture from migrating up.

Cork underlayment became popular in the 1970s as an underlayment that offered much desired sound control in high rises and other residences. A costly form of underlayment, it is an effective insulator of sound; the thicker the cork layer, the better the sound reduction. A separate moisture barrier is needed for installations where moisture will be present. It is used today under floating floors, engineered wood flooring, VCT, and ceramic tile, but does not improve walking comfort and won’t reduce the sound of footfall within the room.

Foam underlayment, typically about 1/8 inch thick and sold in rolls, can be used under wood or laminate atop plywood or concrete subfloor in areas where moisture not likely to be a factor. If moisture is a consideration, the foam should have a moisture barrier attached on one side. Foam, unless particularly dense or made of certain upgraded formulations (including foam rubber), does not address sound control between floors or improve the sound of walking on the floor.

One alternative underlayment for floating floors, first used in commercial applications, wasdense rigid fiber board, which required saw-cutting at the jobsite and precise gluing to accomplish tight, even joints. While rigid fiberboard absorbed the click-click of footfall and boosted R-ratings, it was less successful in maintaining consistent floor heights from room to room.

In the 1990s, manufacturers introduced stand-alone non-rigid rollout products, including fiber underlayments, that could be under the underlayment/engineered wood planks or under floating floor systems that did not have attached underlayment and offered a host of other advantages, to boot. Easier to work with than earlier underlayments, non-rigid roll-out fiber underlayment typically does not raise height concerns, has a high enough insulating value to help with boosting R-values (Keeping floors warmer in winter and cooler in summer) and provides much higher impact resistance.

Some rollout fiber products are made entirely or substantially from third-party certified recycled textiles, enabling the product to help earn LEED credits in projects aiming for LEED certification. In addition, some fiber underlayment includes a physical anti-microbial additive that inhibits mold growth.

Underlayment for Tile and Stone

Until the 1980s, clay and quarry tiles were set in thick mortar beds of sand and cement mixed with water at the job site. Because the tiles are inherently quite absorbent, the mud set well in the traditional time-tested formulation. By the early 1980s, many installations were using denser ceramic tiles that required slower setting time, which in turn necessitated use of latex additives in the field that accomplished just that.

By in the late 1980s and in the early 1990s, some companies brought to market pre-packaged thick bed mortars that included latex additives, eliminating “guesstimating” onsite.

About the same time, mix-and-pour underlayments that enable installers to easily achieve a desired bed height were introduced and gained converts not only for use under tile and stone but also for under vinyl, where leveling of the subfloor was also needed. (This kind of application can pour over cement or wood on wire lathe.) In today’s market, there are numerous self-leveling products, including those that are rapid setting, with cure times of four to six hours. 

Today, tile installers can also opt for fiber underlayments designed especially for use under ceramic or porcelain tile that can be used with latex modified thinset mortar both below and above the underlayment. The underlayment can help cushion the floor and absorb impact and airborne sound while also smoothing out little subfloor imperfections. When used with ceramic tile, some formulations can preclude transfer of concrete subfloor cracks telegraphing through to the tile or grout above. Some membrane systems feature self-adhesive backing, eliminating need for glue on both sides, saving installers time and effort.

Duane Reimer is the technical director of MP Global Products, a manufacturer of Made-In-America recycled fiber underlayments including QuietWalk® for laminate and floating wood flooring; VersaWalk™ universal underlayment for wood, laminate and LVT; Insulayment® for glue-down and nail-down hardwood or engineered wood flooring and more. Located in Norfolk, NE, he can be reached at 888-379-9695 or go to www.quietwalk.com.