What to Put Under Hardwood Flooring: Choosing the Right Underlayment

The layer you cannot see does more for your hardwood floor than most homeowners realize. What to put under hardwood flooring comes down to choosing the right underlayment material for your installation method, your subfloor, and the moisture and noise conditions of the room. The wrong choice can leave a floor squeaky, cold, or vulnerable to moisture damage, while the right one quiets footsteps, blocks vapor, and helps the floor perform for decades. This guide focuses on the underlayment material choices themselves, comparing felt, rosin paper, foam, cork, and combination products so you can match the layer to your floor.

Why the Underlayment Material Matters

Underlayment is not filler. It does specific jobs that change with the situation: it can act as a moisture or vapor retarder, reduce sound transmission, add a slight cushion, and provide a slip layer that lets nailed boards move without squeaking against the subfloor. Crucially, the right material depends on how the floor is installed and what sits below it. A nail-down solid floor over plywood has different needs than an engineered floating floor over a concrete slab. Choosing the material to match those conditions is the whole game.

Felt Paper and Rosin Paper for Nail-Down Floors

For traditional nail-down solid hardwood over a plywood or wood subfloor, a paper underlayment is the classic, proven choice.

• Asphalt-saturated felt: Often 15-pound felt, this provides a modest vapor retarder and a slip sheet that helps prevent squeaks as the floor moves seasonally. It is the long-standing standard under nailed solid wood.
• Rosin paper: A lighter red paper that mainly acts as a slip sheet to reduce squeaks. It offers little moisture protection, so it suits dry above-grade installations where vapor is not a concern.

Both roll out fast and cost little. Felt is the safer pick when you want some vapor resistance along with the slip and squeak benefits.

Foam Underlayment for Floating Floors

When you install engineered hardwood as a floating floor, foam underlayment becomes relevant. Standard polyethylene or IXPE foam provides cushioning, a degree of sound dampening, and a smoother feel underfoot. Foam is inexpensive and easy to roll out, but on its own it does not block moisture, so over concrete you need either a foam with an integrated vapor barrier or a separate poly sheet. Note that foam is intended for floating installations; it is not appropriate under nail-down solid wood, where the fasteners and the compressible foam do not mix well.

Cork Underlayment for Sound and Comfort

Cork is the premium choice when noise control and comfort top your list. A cork underlayment, often 3mm to 6mm thick, excels at deadening impact sound, which makes it popular in upstairs rooms, condos, and apartments where footstep noise travels to the space below. It also adds a touch of warmth and cushion and resists mold naturally. Cork does not provide vapor protection by itself, so over concrete it should be paired with a moisture barrier. It works under floating engineered floors and, in some glue-down systems, as a sound layer beneath the wood.

Combination and Vapor-Barrier Underlayments

Many modern products bundle several functions into one roll, which simplifies the job and reduces the number of layers.

• Foam with attached vapor barrier: A foam cushion bonded to a poly film, ideal for floating floors over concrete where you need both cushioning and moisture protection in a single product.
• Cork with integrated barrier: Combines cork’s sound control with a moisture layer for slab installations.
• Premium acoustic underlayments: Engineered specifically to hit sound-transmission ratings required in some condo buildings.

Combination products cost more per square foot but can be the most reliable choice when a room demands both moisture defense and quiet.

Over Plywood Versus Over a Concrete Slab

The subfloor below changes the underlayment decision more than anything else. Over a plywood or OSB subfloor in an above-grade room, moisture is usually less of a threat, so felt or rosin paper under nail-down wood, or basic foam under a floating floor, often suffices. Over a concrete slab, moisture is the constant enemy, because concrete wicks vapor even when it looks dry. There you must include a vapor barrier, whether a standalone 6-mil poly sheet or an underlayment with the barrier built in. Skipping moisture protection over concrete is the single most common cause of cupping and finish failure.

Thickness, and Why More Is Not Always Better

It is tempting to assume a thicker underlayment is automatically better, but with hardwood that logic can backfire. Under nail-down solid wood, paper underlayments are deliberately thin because the fasteners must grip the subfloor firmly; a thick, compressible layer would let the floor move and squeak. Under floating engineered floors, foam and cork commonly run 2mm to 6mm, and going too thick can create a spongy feel and stress the locking joints between planks. The right thickness is the one your flooring manufacturer specifies for that product, not the thickest roll on the shelf. When sound control is the goal, choose a material engineered for acoustics rather than simply piling on thickness.

Common Underlayment Mistakes

A few recurring errors undo otherwise good installations. The most serious is skipping a vapor barrier over concrete, which invites cupping and finish failure. Another is stacking two underlayments to “add cushion,” which over-softens a floating floor and can void the warranty. Using foam under nail-down solid wood is a mismatch that leads to movement and noise. Finally, ignoring the manufacturer’s stated requirements is a quiet but costly mistake, since many warranties hinge on using an approved underlayment. Read the installation instructions for your specific floor before you buy the layer beneath it.

Matching Underlayment to Noise and Moisture Needs

Think about the room before you buy. An upstairs bedroom or a condo over a neighbor calls for cork or a rated acoustic underlayment to control footstep noise. A basement-adjacent slab or any below-grade space demands robust vapor protection, pushing you toward a barrier-equipped product. A dry, ground-floor room over wood can get by with simpler, cheaper options. Always confirm your hardwood manufacturer’s requirements too, since some warranties specify an underlayment type and ignoring it can void coverage.

The Bottom Line

Deciding what to put under hardwood flooring means matching the underlayment material to your installation method, subfloor, and room conditions. Use felt or rosin paper under nail-down solid wood over plywood, foam under floating engineered floors, and cork or acoustic products where noise control matters. Whenever the floor goes over concrete, build in a vapor barrier. Choose the layer that fits the situation and check your warranty’s requirements, and the hidden layer will quietly protect your investment for the life of the floor.