Hardwood Floor Underlayment Guide: Felt, Foam, and Moisture

Underlayment is one of those flooring terms that gets used three different ways by three different people, which is why so many homeowners end up confused about whether they even need it. Some installers mean a moisture barrier. Some mean a foam pad. Some mean the plywood layer on top of the joists. All three are called underlayment at one point or another, and picking the wrong one for your project can cost you a floor.

Here is what underlayment actually does, which type your specific installation calls for, and where people get it wrong.

What You Will Find in This Article

What Hardwood Floor Underlayment Actually Is

Underlayment is the thin layer that sits between your subfloor and your finished hardwood. It is not structural. The subfloor carries the load, and underlayment does three narrower jobs:

  • Moisture control. It slows water vapor moving up from below into the back of your planks.
  • Sound dampening. It reduces the hollow click of foot traffic and, in multi-story buildings, cuts noise transmission to the space below.
  • Minor smoothing. It bridges tiny surface imperfections and helps reduce squeaks where boards meet subfloor.

Note what is not on that list. Underlayment does not level a subfloor. If your subfloor is out of flat by more than about 3/16 inch over 10 feet, no underlayment will fix that. You need a leveling compound or you need to sand the high spots down. Skipping that step and hoping a pad will absorb the difference is how floors end up with hollow spots and separated seams.

Do You Need Underlayment Under Hardwood

Almost always, yes, but the answer depends entirely on how the floor is being installed. Here is the short version:

Installation Method Underlayment Needed What to Use
Nail down or staple down over plywood Yes Asphalt-saturated felt or kraft paper vapor retarder
Glue down over concrete No separate layer Moisture-control adhesive that acts as the barrier
Float over any substrate Yes, always Foam or cork pad, with attached vapor film over concrete
Nail down over a slab with sleepers or plywood Yes, two layers Poly vapor retarder on the slab, felt on the plywood

The Main Types of Underlayment

Asphalt-Saturated Felt

The traditional choice for nail-down installations, sold as #15 or #30 felt. It is inexpensive, it works, and it has been the default for decades. Look for felt meeting ASTM D-4869. The #30 weight offers a bit more vapor resistance than #15. One caution: do not use asphalt products under square-edge flooring, since the asphalt can telegraph through at the seams.

Asphalt-Laminated Kraft Paper

A kraft paper product with an asphalt layer laminated inside. It handles more or less like felt, lies flatter, and tends to be cleaner to work with. It is a solid choice anywhere felt would be used.

Red Rosin Paper

Rosin paper still gets sold as flooring underlayment, and it should not be. It is a slip sheet. It reduces squeaks and keeps the planks from sitting directly on the subfloor, but it offers essentially no vapor resistance. If your only concern is noise on an upper floor with conditioned space below, it is acceptable. For anything over a crawlspace, basement, or unconditioned area, it is not enough.

Foam and Cork Pads

These are for floating floors only. They compress slightly underfoot, absorb sound, and come in thicknesses from 2mm to 6mm. Cork costs more and performs better acoustically. Foam is cheaper and more common. Both are available with an attached polyethylene film for use over concrete.

Important: never use a foam pad under a nail-down or glue-down floor. The compression that makes it useful for floating installations is exactly what causes fastener movement and adhesive failure in the other two methods.

Polyethylene Vapor Retarder

Six-mil poly sheeting, used directly on a concrete slab before a plywood subfloor goes down. It is a moisture barrier, not an underlayment in the acoustic sense, and it does not replace the felt layer that goes on top of the plywood.

Choosing by Installation Method

Nail Down and Staple Down

Roll out felt or kraft paper across the entire subfloor, overlapping seams by about 4 inches, then staple it in place. Run it up the wall slightly at the perimeter if you are in a humid climate. The planks then get fastened right through the paper into the subfloor.

Before any of this, test moisture. The National Wood Flooring Association's moisture testing procedure calls for a minimum of 20 readings per 1,000 square feet, averaged. The moisture content of the subfloor and the flooring should be within 4% of each other before a single board goes down. No underlayment on earth compensates for skipping this.

Glue Down

Modern urethane adhesives are formulated to serve as the moisture barrier themselves, which is why you do not put a separate sheet product underneath. Adding one would prevent the adhesive from bonding to the substrate, which defeats the entire installation.

Match the trowel to the adhesive. Adhesive manufacturers specify a trowel notch size for a reason, and using the wrong one changes your spread rate. Too little adhesive and you get hollow spots. Too much and it squeezes up between the boards.

Floating

The pad is mandatory here, since nothing mechanically attaches the floor to the substrate. Butt the pad edges together rather than overlapping, and tape the seams. If you are floating over concrete, use a pad with an integrated vapor film or lay 6-mil poly first.

Leave a 1/2 inch expansion gap around the entire perimeter and at every vertical obstruction. This applies to all three methods, but it matters most with floating floors, which need room to move as a single unit.

Installing Over Concrete

Concrete is where underlayment decisions get serious, because slabs release moisture for a long time and the consequences show up months after the job looks finished.

Test the slab first. It should be at least 30 days old before testing means anything. NWFA guidance sets clear thresholds: if a calcium chloride test reads above 3 pounds, or relative humidity testing reads above 75%, or a calcium carbide test reads above 2.5%, you need an impermeable vapor retarder with a perm rating below 0.15. Below those thresholds, a retarder is not strictly required, though it remains a good idea for on-grade and below-grade installations.

Solid hardwood is generally not recommended below grade at all, regardless of what you put under it. For basements, engineered construction is the appropriate product.

Sound Ratings for Condos and Second Floors

If you live in a condo or an HOA-governed building, check your bylaws before ordering anything. Many buildings require a minimum acoustic rating for hard-surface flooring, expressed as STC (sound transmission class, for airborne noise like voices) and IIC (impact insulation class, for footsteps and dropped objects).

A requirement of IIC 50 or higher is common, and some buildings specify higher. Underlayment products are tested and rated for these values, and the rating is measured as part of an assembly, meaning the pad plus the subfloor plus the ceiling below. Ask for documentation, and keep it. Building management may ask to see it.

Cork generally outperforms foam at the same thickness for impact sound, which is why it shows up so often in condo specifications despite the higher price.

Common Underlayment Mistakes

  • Using foam under a nail-down floor. The pad compresses, fasteners loosen, and the floor develops movement and squeaks within a year.
  • Doubling up pads to boost sound ratings. Two pads do not add their ratings together, and the extra compression can void your flooring warranty.
  • Treating rosin paper as a moisture barrier. It is a slip sheet. It was never designed to stop vapor.
  • Skipping moisture testing. Underlayment manages vapor. It does not fix a wet slab or a subfloor that has not dried out.
  • Overlapping foam pad seams. Butt them and tape them. Overlaps create ridges that telegraph through the finished floor.
  • Ignoring the flooring manufacturer's instructions. Manufacturer requirements override general guidelines, and installing against them is the fastest way to void a warranty.

Getting the Right Materials

We carry the adhesives, trowels, levelers, and blades that go with a proper installation. Browse our adhesives and levelers collection to match the right products to your method, and if you are still choosing material, our solid hardwood flooring pages list the approved installation methods for each product.

Because we buy direct from the same mills that supply the major brands, you get the same material without the retail markup. We have shipped more than 50,000 floors over 30-plus years, and every order comes with people who know how these installations actually go.

Not sure which underlayment your project calls for? Call our Austin, Texas team at 888-705-7555 and ask for Marcus, Curtis, or Jody. Tell them your subfloor type, whether you are on grade or below, and how you plan to install, and they will tell you exactly what you need. It is a five-minute conversation that saves a lot of expensive regret.

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