Flooring Underlayment: Which Type Goes Under Which Floor

Underlayment is the layer nobody sees and the one most likely to be chosen wrong. The right one depends on three things: the floor going on top, the subfloor underneath, and what is under that subfloor — a heated room, a vented crawlspace, or a concrete slab sitting on soil. Hardwood nailed over a plywood subfloor wants a vapor retarder in one situation and nothing at all in another. A floating laminate floor needs a pad and, over concrete, a 6-mil plastic sheet. Vinyl plank with a pad already attached usually needs nothing added. Carpet needs a cushion thin and firm enough for the carpet. Tile needs a membrane, not a pad. This guide goes floor by floor, using the installation standards each industry publishes.

For the house-side checklist, see our guide to preparing your home for a flooring installation; this article covers what goes between the subfloor and the floor.

First, What Underlayment Is Supposed to Do

Underlayment does up to four jobs: controlling moisture vapor, cushioning and supporting a floating floor, reducing sound, and isolating a rigid floor from movement below it. No single product does all four, and some floors need none of them; the question is which jobs this floor, on this subfloor, actually needs.

Moisture is the job that matters most and the one with the clearest vocabulary. The National Wood Flooring Association (NWFA) defines a material's perm rating as the standard measure of its water vapor permeability: the higher the number, the more readily vapor passes through. Its Installation Guidelines, following the International Residential Code, sort vapor retarders into three classes. Class I is 0.1 perm or less — a vapor barrier, such as 6-mil polyethylene. Class II is more than 0.1 and up to 1.0 perm, which the NWFA calls semi-impermeable. Class III is above 1.0 perm. Those classes are the language to use when you ask an installer what is going under your floor.

Nail-Down Hardwood Over a Wood Subfloor

This is the classic hardwood installation, and the answer turns on what is below the subfloor. The NWFA's guidance is specific:

Over a conditioned space — a heated and cooled basement, or a living room below a second-floor bedroom — no vapor retarder is necessary, unless the flooring manufacturer says otherwise.

Over an unconditioned space, such as a vented crawlspace, a Class II vapor retarder may be used over the wood subfloor to slow moisture-laden air moving up into the flooring. The NWFA classes open pier-and-beam crawlspaces as unconditioned, so a home over a vented crawlspace falls in that group.

Where a Class I or Class II vapor retarder has already been installed on the underside of the joists, no vapor retarder should go over the subfloor.

The traditional sheet under a nail-down floor is 15-pound asphalt felt or asphalt-saturated kraft paper; the NWFA notes these are normally just shy of 1/64 inch thick, so they do not change floor height in any meaningful way. Whatever sheet is used, ask for its perm rating and its class rather than its name.

The most important line in that chapter is a warning. The NWFA says never use a vapor retarder over a wood subfloor to remedy a known moisture condition, and never install a wood floor over one. Paper is not a fix for a wet crawlspace. If the crawlspace has standing water, a damp smell, or a missing ground cover, that gets solved first — the pattern on our moisture under flooring page is what happens when it is not.

Two other NWFA rules surprise homeowners. Never install a wood floor over any type of carpet or carpet pad, and never install a new wood floor over an existing floating floor.

Floating Engineered Wood and Laminate

A floating floor locks to itself and rests on the underlayment rather than being fastened down, so the pad is part of the structure. NALFA, the laminate industry's association, puts it plainly: underlayment is crucial for laminate flooring. It supports the locking system, provides moisture protection, helps level minor subfloor imperfections, and adds thermal and sound benefits. Our laminate flooring and engineered wood pages cover the floors themselves; the nail-down vs. glue-down vs. floating comparison covers when floating is the right method.

Over concrete, the rule is consistent across both industries. NALFA's subfloor guide says the slab should be fully cured, at least 90 days old, and that you should always use a 6-mil vapor barrier and underlayment padding. The NWFA's floating-floor chapter says that over concrete, when the pad does not include a vapor retarder, a separate Class I retarder underneath it is necessary. Some pads combine the two layers; the data sheet says which. The NWFA also lists 6-mil polyethylene, closed-cell foam pads, and peel-and-stick membranes as Class I sheet-good retarders, and says to install a Class I retarder when slab tests come back above its limits, including relative humidity readings greater than 80%.

Softer is not better under a floating floor. The NWFA says underlayment beneath a floated wood floor should have a published compression resistance and density that meets the flooring manufacturer's minimums. A thick, squishy pad lets the locking joints flex with every step, which is how joints wear and open. NALFA adds that there is no single underlayment solution for every laminate installation, and points to its LF-03 standard for laminate underlayment performance.

Luxury Vinyl Plank: Attached Pads and Doubling Up

Much rigid-core luxury vinyl plank now arrives with a pad on its back. The Resilient Floor Covering Institute (RFCI) defines rigid-core flooring as modular flooring with a rigid polymeric core, a decorative top surface, and an optional soft underlayment pre-attached, and notes that a pre-attached cork or other acoustical underlayment is often applied for warmth and sound absorption.

If the plank has an attached pad, do not add a second one unless the manufacturer's instructions allow it. NALFA says additional underlayment should only go beneath attached-pad flooring when specifically permitted by the manufacturer, and the NWFA gives the same rule for engineered wood with an attached underlayment. Stacking pads creates the soft, bouncy base that strains click joints; the result is the gapping described on our vinyl plank separating page.

What may still be needed over concrete is a moisture layer. RFCI's moisture guide notes that, based on slab moisture levels, some manufacturers allow a one-part moisture membrane or a polyfilm underlayment under some floating resilient products, and recommends verifying that option with the manufacturer first. Each manufacturer sets its own maximum slab relative humidity. RFCI is also clear that manufacturer requirements for flatness, expansion space, moisture, and underlayment govern the installation. Our guide to choosing luxury vinyl plank covers the rest of the product decision, and LVP vs. laminate sets the two floating floors side by side.

Carpet Cushion

Carpet is the one floor where the underlayment is part of how the floor feels, and the industry standard is firmer and thinner than most people expect. The Carpet and Rug Institute's CRI 105 standard for residential installation requires that the cushion conform to the carpet manufacturer's recommendations for the specific product, and says residential cushion thickness should not exceed 7/16 inch. It also calls for cushion seams at right angles to carpet seams, or offset at least 6 inches.

CRI's consumer guidance adds a density rule of thumb: for most residential cut-pile carpet, cushion no more than 7/16 inch thick and no less than 1/4 inch, with 6 pounds per cubic foot density; for berber or low-profile carpet, no more than 3/8 inch thick with 8 pounds density. Hallways, stairs, and family rooms need thinner, firmer cushion than bedrooms. CRI warns that the wrong cushion can cause wrinkling, buckling, seam separation, and breakdown of the carpet itself — the thick, plush pad that feels best in the showroom is often the wrong one. CRI recommends always using cushion except for direct glue-down carpet. See our carpet installation page.

Tile: Membranes, Not Pads

Tile does not go over a cushion. It needs a rigid base, and the layer between the substrate and the tile is a membrane bonded into the assembly. The Tile Council of North America (TCNA) maintains the ANSI standards that define them:

Crack isolation membranes, under ANSI A118.12, sit between a concrete slab and the tile. TCNA explains that the membrane's internal make-up keeps movement in the concrete from transferring directly to the tile; it stretches where needed. Over a slab with shrinkage cracks, this is the layer that keeps a crack in the concrete from becoming a crack through the tile.

Waterproof membranes, under ANSI A118.10, are load-bearing, bonded membranes for thin-set tile and stone, used in showers and wet areas.

Sound reduction membranes have their own standard, ANSI A118.13, for bonded sound reduction under thin-set tile.

Uncoupling membranes are widely sold, but TCNA lists an ANSI uncoupling standard, A118.20, as proposed and still under development. Until it is published, an uncoupling claim is the manufacturer's own, so ask which ANSI standard a product is tested to.

Two cautions from TCNA. Membranes do not let you skip movement joints; there must always be soft joints in the tilework. And felt paper is not an anti-fracture membrane: TCNA says it generally does not provide suitable bond strength, does not hold up to moisture, and can promote fungal growth. No membrane fixes a floor that bends, either. TCNA gives L/360 as the traditional minimum floor rigidity, measured before the tile underlayment goes in. A springy floor shows up as cracked tile regardless of what is under it.

Sound Control: Reading IIC and STC

Sound ratings matter most in condos, townhomes, and any room over a bedroom. The NWFA explains the two ratings. Impact Insulation Class (IIC) measures impact noise through a floor-ceiling assembly — footsteps, dropped objects, furniture moving — and is tested under ASTM E492. Sound Transmission Class (STC) measures airborne sound such as voices and television, tested under ASTM E90. For both, a higher number means more sound attenuation.

The catch is that both ratings describe the whole assembly — the structure, the ceiling below, the floor, and the pad — not the pad alone. The NWFA notes that no single component can fulfill building requirements on its own. ASTM E2179, which measures how much a floor covering improves impact sound over a concrete slab, is the source of the "Delta IIC" figure some products publish, and its own scope warns that the same covering over different floor assemblies will not always give the same IIC rating. The NWFA recommends acoustical underlayments carry a published Delta IIC so products can be compared.

In multi-family buildings, the NWFA notes the model building codes call for a minimum 50 IIC and 50 STC (45 if field tested), and that many homeowners associations write stricter minimums into their CC&Rs, which supersede the code. If you own a condo, get the CC&R requirement in writing before choosing a floor; it can dictate the underlayment, and sometimes the floor itself.

Radiant Heat: Mind the R-Value

Every underlayment is also insulation, and over radiant heat that works against you. The NWFA says the R-values of underlayment pads vary and may compromise radiant heat efficiency, and that the pad's R-value should be accounted for when the heating system is designed. It also says not to use asphalt- or bitumen-type vapor retarders over radiant heat, which rules out the traditional felt paper, and that subfloor surface temperature should not exceed 80°F at the time of installation.

Carpet is no exception. CRI 105 says radiant floors require special consideration in choosing carpet and cushion and that the heated surface should not exceed 85°F; CRI's FAQ recommends a flat rubber or synthetic cushion no thicker than 3/8 inch. The general rule: over radiant heat, use the thinnest underlayment the floor's manufacturer allows, and give its R-value to whoever designed the heating system.

A Quick Reference by Floor

Nail-down hardwood over a crawlspace: a Class II vapor retarder, unless one is already on the underside of the joists. Over a conditioned room: usually nothing.

Floating engineered wood or laminate over a wood subfloor: a pad meeting the manufacturer's compression and density minimums. Over concrete: add a 6-mil polyethylene or other Class I layer unless the pad includes one.

Vinyl plank with an attached pad: no second pad. Over concrete, only the moisture layer the manufacturer allows.

Carpet: a firm cushion within the carpet maker's specification, no thicker than 7/16 inch, and thinner for berber.

Tile: no pad — a crack-isolation, waterproofing, or sound membrane matched to its ANSI standard, over a floor stiff enough for tile.

For a planning number on the floor itself, our cost calculator uses installed ranges such as $5–10 per sq ft for luxury vinyl plank, $4–8 for laminate, $8–15 for engineered wood, and $10–20 for hardwood or tile.

Renaissance Floors is a licensed C-15 flooring contractor (CSLB #1060673) based in Roseville, installing hardwood, engineered, laminate, vinyl, carpet, and tile across Greater Sacramento and Northern California. Our floor installation work starts with what is under the floor — the crawlspace, the slab moisture reading, the joist stiffness — before anyone chooses a pad. Call (916) 749-0272 or contact us and we will tell you what your subfloor actually needs.