Underlayment is the thin layer rolled out between the subfloor and a floating floor. It cushions the floor underfoot, dampens sound, smooths very minor surface imperfections, and — over concrete — can help slow moisture migration when it includes a vapor-rated layer. What it is not: a fix for a subfloor that is genuinely unlevel (that needs a self-leveling compound or proper flattening) or wet (that needs testing and a real vapor barrier). Underlayment manages comfort, sound, and minor surface texture on top of a subfloor that already meets the manufacturer's flatness and moisture requirements — it does not replace correcting either problem.
On this page
- What underlayment actually does
- The types: EVA, IXPE, cork, felt, rubber & combination
- Which floors actually need it
- Moisture over concrete: why the barrier still matters
- Acoustics: IIC, STC, Delta IIC & IBC minimums
- Thickness: why more is not better
- Radiant heat: go thin, go low-R
- What to look for
- Myths & mistakes
- FAQ
What underlayment actually does
Underlayment does five real jobs, and it is worth being precise about the boundaries of each one:
- Comfort and cushion. A thin resilient layer softens the feel of a rigid floating floor underfoot compared with planks sitting directly on a hard subfloor.
- Sound — both impact and airborne. A quality underlayment reduces impact noise (footsteps, dropped objects) and, to a lesser degree, airborne noise (voices, music) passing through the floor-ceiling assembly. See acoustics below for how this is actually measured.
- Minor smoothing — not leveling. Underlayment can hide very small surface texture and minor irregularities in an otherwise flat subfloor. It cannot correct a subfloor that is genuinely out of level or has real high/low spots; those need to be corrected first (see our subfloor & moisture guide). Installing over an unlevel subfloor and expecting underlayment to compensate is a common, avoidable mistake.
- Moisture and vapor protection over concrete. Some underlayment includes a laminated film layer that acts as a vapor barrier or retarder, slowing moisture migrating up through a slab. Plain foam or cork without that film layer does not do this job — see moisture over concrete below.
- Thermal. Underlayment has some thermal resistance (R-value), which matters both for comfort over a cold slab and, in the opposite direction, for how well heat from a radiant system passes through — see radiant heat below.
The types: EVA, IXPE, cork, felt, rubber & combination
The common underlayment types trade off density, moisture resistance, and acoustic performance differently:
| Type | Density / feel | Moisture resistance | Acoustics | Notes |
|---|---|---|---|---|
| EVA foam | Soft, lower density | Basic — can compress and lose performance over time | Basic to moderate | Economical; suits lighter-duty or budget floating installations |
| IXPE foam | Denser, closed-cell, holds its thickness (low compression set) | Better than EVA — uniform closed-cell structure | Better than EVA, even at thin gauges | Common attached or separate pad under rigid-core (SPC/WPC) vinyl |
| Cork | Firm, natural material | Some moisture sensitivity — usually paired with a vapor barrier over concrete | Good, natural sound damping | Renewable; resists mould and compression well |
| Felt / fibre | Firm, dense | Varies by product — check the spec sheet | Good; often used to quiet a “hollow” sound over wood subfloors | Common under floating floors in condos and multi-family buildings |
| Rubber | Dense, heavy | Good, when rated for the application | Generally the strongest acoustic performer of the common types | Heavier and pricier; common in commercial and multifamily installs |
| Combination (foam/cork + film) | Varies with the base material | Adds a laminated vapor-barrier film layer | Depends on the base foam or cork | Marketed for direct use over concrete without a separate poly sheet |
The headline comparison: IXPE (cross-linked polyethylene) is denser and more uniform than plain EVA foam, giving it better long-term compression recovery, moisture resistance, and acoustics — which is why it's a common factory-attached pad beneath rigid-core vinyl, while EVA stays the more economical, lighter-duty choice. Cork trades some moisture sensitivity for natural sound damping. Rubber is generally the strongest acoustic performer of the group, at the cost of extra weight and price; combination products add a laminated vapor-barrier film to a foam or cork base.
Which floors actually need it
Floating floors — most LVP/LVT, laminate, and floating engineered hardwood — are the installations that genuinely need underlayment, because nothing else sits between the plank and the subfloor. The one exception: planks with a factory-attached pad already bonded to the backing. Adding a second, separate underlayment under an attached-pad product doubles up the cushioning, which can let the click-lock joints flex more than the manufacturer designed for — don't double up, and check the product's instructions before buying a separate pad.
Glue-down flooring bonds directly to the subfloor across its full surface with adhesive, so it does not use a separate acoustic underlayment at all — the adhesive is the connection. Nail-down solid hardwood commonly goes down over building paper or 15 lb felt, but that layer serves a different purpose: it acts as a slip sheet that lets the boards move slightly without creaking, and offers light protection against subfloor moisture — it is not there for cushioning or sound damping the way floating-floor underlayment is, and it should not be confused with it. See our full floating vs. glue-down vs. nail-down guide for how the three attachment methods compare.
Moisture over concrete: why the barrier still matters
A concrete slab stays in slow moisture exchange with the ground beneath it, often for years. Installing a moisture-sensitive floor over concrete without addressing that generally calls for a vapor barrier or retarder — either a film-backed underlayment built for the purpose, or a separate poly sheet beneath a plain pad. Ordinary foam or cork underlayment, on its own, is not a vapor barrier; check the product specifically states a vapor-barrier or retarder rating before relying on it over a slab. This is also not a substitute for testing the slab itself — see our subfloor & moisture guide for how relative-humidity and calcium-chloride concrete testing actually works.
Acoustics: IIC, STC, Delta IIC & IBC minimums
In condos, apartments, and other multi-family buildings, acoustic performance is usually the reason underlayment is specified at all — often by the HOA or building code, not by choice. Three ratings describe it:
- IIC (Impact Insulation Class) — how well a floor-ceiling assembly blocks impact sound such as footsteps and dropped objects, tested under ASTM E492.
- STC (Sound Transmission Class) — how well the assembly blocks airborne sound such as voices, television, and music, tested under ASTM E90.
- Delta IIC — how many impact-sound points a specific underlayment adds on its own over a reference concrete slab, tested under ASTM E2179. It is the most apples-to-apples way to compare underlayment products directly, since it isolates the pad's own contribution from the rest of the assembly.
The International Building Code sets a lab-tested minimum of IIC 50 and STC 50 for sound isolation between dwelling units in multi-family construction. Many HOAs and building managers require a higher rating than the code minimum, so check the building's actual specification — not just the code floor — before buying underlayment for a condo or apartment installation.
Thickness: why more is not better
Most floating-floor underlayment runs roughly 1–3 mm thick. Going thicker, or choosing a softer pad than the manufacturer specifies, does not reliably mean a quieter or more comfortable floor — it can instead let the planks flex more at the click-lock joints than the joint design tolerates, which stresses the locking mechanism and can lead to separation, gapping, or a voided warranty. Follow the flooring manufacturer's stated maximum underlayment thickness, not a general instinct that more cushioning is always an improvement.
Radiant heat: go thin, go low-R
Over hydronic or electric radiant heat, the goal flips: you want underlayment with a low thermal resistance (a low R-value) so heat passes through to the room efficiently, rather than being trapped between the heating element and the floor surface. A thick or highly insulating underlayment slows that heat transfer and can make the system feel sluggish to respond. Use a thin, low-R product specifically rated radiant-heat compatible, and confirm compatibility with both the flooring and the heating-system manufacturer before installing.
What to look for
- Match to the floor type. Confirm whether your planks already have an attached pad before buying a separate underlayment.
- Vapor barrier if over concrete. Confirm the product states a vapor-barrier or retarder rating if it will sit on a slab — not just "moisture-resistant" marketing language.
- Acoustic rating if it's a condo or apartment. Check the building's actual required IIC/STC/Delta IIC, which may exceed the IBC minimum.
- Thickness per the manufacturer. Stay within the flooring manufacturer's stated maximum, even if a thicker option is available for sale.
- Radiant compatibility if applicable. Confirm a low-R, radiant-rated product if the room has in-floor heating.
Myths & mistakes
- Myth: "Thicker or softer underlayment is always quieter and better." Past the manufacturer's specified range, extra thickness or softness stresses the locking joints instead of improving sound or comfort, and can void the warranty.
- Myth: "Underlayment waterproofs the floor." Only a product specifically rated as a vapor barrier or retarder addresses moisture from below — plain foam or cork does not, and none of it substitutes for testing the subfloor.
- Myth: "Every floor needs underlayment." Planks with a pre-attached pad, glue-down flooring, and nail-down flooring either don't use a separate underlayment or use a different-purpose layer (a felt slip sheet) entirely.
- Myth: "All foam underlayment performs about the same." EVA and IXPE are both foam, but they differ meaningfully in density, moisture resistance, compression recovery, and acoustic performance — the type matters, not just the thickness.
- Myth: "Meeting the IBC's IIC/STC 50 minimum is always enough." Many HOAs and building managers set requirements above the code floor — check the building's actual spec, not just the code minimum.
Frequently asked questions
Does every floating floor need a separate underlayment?
Not if the planks already have a pre-attached pad bonded to the backing — adding a second underlayment on top doubles up the cushioning, which can let the click-lock joints flex and can void the manufacturer's warranty. Check the product's installation instructions for whether a pad is already attached before buying a separate underlayment.
Is thicker or softer underlayment always quieter and better?
No. Most floating-floor manufacturers cap underlayment thickness at roughly 1–3 mm; going softer or thicker than that specification lets planks flex more at the locking joints, which stresses the joint and can cause separation, gapping, or a voided warranty. Match the product to the manufacturer's stated maximum, not to "more cushion is always better."
What's the real difference between IXPE and EVA underlayment?
They're both foam, but IXPE (cross-linked polyethylene) is denser and has a more closed, uniform cell structure, which gives it better moisture resistance, better long-term compression recovery, and better acoustic performance than standard EVA (ethylene-vinyl acetate) foam. EVA is softer, less dense, and more prone to compressing (and losing performance) over time, but it's also the more economical option — a reasonable trade-off for lighter-duty or budget installations.
Do glue-down or nail-down floors need underlayment too?
Not the same kind. Glue-down flooring bonds directly to the subfloor with adhesive, so it doesn't use a separate cushioning underlayment. Nail-down solid hardwood commonly goes down over building paper or 15 lb felt, but that layer's job is acting as a slip sheet and light moisture retarder to reduce squeaks — it isn't there for cushioning or sound damping the way floating-floor underlayment is.
Does underlayment stop moisture coming up through a concrete slab?
Only if it's actually rated as a vapor barrier or retarder — plain foam or cork on its own is not a moisture barrier. Over concrete, look for underlayment that specifically states a vapor barrier or retarder rating (often a film layer laminated to the foam), and confirm the slab has been tested for moisture first; see our subfloor and moisture guide.
What underlayment should I use over radiant floor heating?
A thin underlayment with a low thermal resistance (low R-value), so the heat isn't trapped between the heating element and the room. A thick or highly insulating underlayment slows the heat's path upward and can make a radiant system respond more sluggishly. Always confirm the specific product is rated radiant-heat compatible before installing it over an in-floor heating system.
Sources
This guide describes standard test methods and code minimums as defined by the bodies below; we are not affiliated with any of them. Product-specific figures (thickness maximums, R-values, vapor ratings) vary by manufacturer — always check the specific product's installation instructions.
- ASTM International — ASTM E492 (laboratory impact sound transmission / IIC), ASTM E90 (laboratory airborne sound transmission loss / STC), and ASTM E2179 (effectiveness of floor coverings in reducing impact sound transmission through concrete floors / Delta IIC).
- International Code Council (ICC) — International Building Code (IBC) sound-isolation provisions, which set the lab-tested IIC 50 / STC 50 minimum between dwelling units.
Keep going: confirm your slab is ready in subfloor & moisture; compare attachment methods in floating vs. glue-down vs. nail-down; or see how underlayment fits luxury vinyl, laminate, and engineered hardwood. Not sure where to start? Use the how-to-choose framework.
Educational information — not professional advice. Check the manufacturer's installation instructions and a qualified professional before you buy or install. Last verified: 23 July 2026. See our Disclaimer & Terms.