Flooring Underlayment Guide: Materials, Applications, and Installation

New Floor Installation
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Not floor covering, not subfloor--flooring underlayment occupies a middle position in the anatomy of the entire flooring structure  What is this hard-to-define but often crucial part of your flooring?

Flooring Underlayment, Defined

Flooring is composed of many different layers.  What you see on the surface--laminate, wood, vinyl, for example--is not everything.  More layers below this top layer (called a floor covering or sometimes a finish floor) support that top layer.

Flooring underlayment is a thin material that rests between the floor covering at top and the subfloor at the bottom.  Because underlayment is a catch-all term, it can take different forms--soft like foam or felt, hard like cement board, fiberboard, or plywood.  Its purpose is to provide a smooth place for the floor covering to rest; to deaden footfall; to dampen some sound; and, in some cases, to act as a moisture barrier.

Here is one typical composition of flooring layers:

  • Top Layer - Floor Covering: The finish floor, such as hardwood, laminate, or tile.  This is the layer that you see and walk on.
  • Second Layer Down - Underlayment: Plywood, fiberboard, or cementboard, and is usually either 1/4" or 1/2".
  • Third Layer Down - Subfloor: Subfloor will already be in place (unless this home is under construction). The subfloor is typically made of OSB and ranges from 19/32" to 1 1/8" thick.
  • Fourth Layer Down - Joists: The wooden members, often microlam, which support all of the previously-mentioned floorings.

    Reasons For Flooring Underlayment

    1. Smooths Surface:  Underlayment provides a smoother and more predictable surface for the floor covering than the subfloor.
    2. Adhesion:  In the case of cement board, this underlayment provides a better place for ceramic tile to stick (and other types of flooring requiring mortar or adhesive) than just the subfloor.
    1. Lateral Stability:  As a residual effect, hard underlayment gives the entire floor better stability by running boards perpendicular to the subfloor and by simply adding more material to the entire structure.

    Is Underlayment Always Needed?

    Flooring underlayment can be a confusing thing. Sometimes it's needed, sometimes it's not. And sometimes the subfloor is really the same thing as the underlayment. So, let's boil it down to a few ideas you can count on.

    If you are remodeling, you will typically be laying down underlayment before installing the finish floor. Often, the existing finish floor or subfloor are in such bad condition that installing underlayment is necessary.

    If you are building a new house, the subfloor itself may be adequate for installation of the finish floor--eliminating the need for underlayment. If the finish floor is hardwood or engineered wood, all you need is a layer of rosin paper and you are good to go. But for all other types of applications, read on:

    Types of Flooring Underlayment

    • Plywood: Sheets of 4' x 8' A/C grade plywood cut to size.  A/C refers to plywood that has one relatively smooth face and one relatively rough side (for the bottom).
    • Underlayment Panels: Underlayment panels are interlocking and come in 2' x 2' tiles.  Fairly expensive, they do make installation go much faster and are great as a moisture barrier.
    • Foam and Cork: Soft "underlayment" materials of cork, felt, or foam that come in rolls and are "floating," rather than attached.
    • Cement Board: Cement board like Wonderboard or fiber-cement board like Durock used only for mineral-based floor coverings (tile or stone), not for wood flooring.
    • OSB: Orient-strand board may be used, but plywood is a preferred underlayment.

    What's the Best Flooring Underlayment?

    It depends on your situation, but 1/4" to 1/2" AC grade plywood tends to be the best flooring underlayment for many dry applications (hardwood, laminate, and engineered wood). The A-graded side is smooth enough even for the thinnest vinyl flooring.

    For mortared flooring, such as stone and porcelain or ceramic tile, a cement board will work best, as it is smooth, easy to cut, and mold-resistant.

    Can You Use Foam Underlayment Instead?

    Plywood underlayment is such a hassle--hauling the materials to your home plus the labor of cutting the sheets to size. Can't you just roll out foam or cork underlayment instead?

    Because these do not provide the same kind of structure as wood or cement board, we consider these to be a different type of underlayment altogether. In addition, these are floating underlayments, which means that they are not attached to the subfloor.

    Foam and cork "underlayment" do two things. First, they provide slightly--but only the very slightest--buffer between the finish floor and imperfections on the subfloor (or existing finish floor, if you choose to install on top of that). Foam and cork underlayments will account for imperfections like barely-protruding screw heads and knot-holes of no more than 1/8".

    A rule of thumb with foam or cork underlayment: Could you reasonably install the flooring without the foam/cork underlayment and get roughly the same results? In other words, look at this type of underlayment as a bonus rather than a cure for any subfloor issues.

    Do You Need Shiplapped or T&G Underlayment?

    No. You can use plywood underlayment with square edges. But if tongue-and-groove or shiplap plywood is available for about the same price, it's nice to have.

    Special Considerations

    Keep your underlayment as thin as possible. Thicker underlayments will begin to pose problems. Room ceilings begin to "lower" as you install thicker underlayment surfaces.

    Are you installing underlayment to provide structural stability to your floor? While underlayment can help with this to some degree, that is not the purpose of underlayment. Underlayment is there to help out with the flooring above, not the subfloor or joists below. If you have subfloor or joist problems, address those issues before dealing with underlayment.