Flooring Underlayment Guide: Types, Thickness, and Use

Q: What underlayment goes under tile?

Not soft pad. Tile needs a cement backer board, a uncoupling membrane such as a dimpled polyethylene sheet set in thinset, or a bonded waterproofing membrane, depending on the substrate and whether the area is wet. These products manage movement and moisture, not comfort. Foam underlayment under tile guarantees cracked grout and loose tiles because the tile assembly needs a rigid, continuous bond to the substrate.

What Underlayment Does

Underlayment is a thin engineered layer between the subfloor and the finished floor. It does four jobs, and no single product does all four equally well — which is why the aisle has so many options and why a recommendation without context is useless.

Moisture control. A film layer keeps slab vapor out of fiberboard cores, adhesives, and wood. This matters far more below grade than upstairs.
Sound reduction. It dampens both airborne noise and the hollow click of footsteps. A floor with the right pad sounds solid; the same floor without it sounds like a drum.
Texture leveling. Foam smooths minor subfloor texture so panel seams and small ridges do not telegraph through thin vinyl.
Cushion and thermal break. A slightly softer feel underfoot and a small insulating effect over a cold slab.

What underlayment cannot do is fix a bad subfloor. It evens texture, not structure. Dips deeper than about 3/16 inch over 10 feet need patching or panel work with or without pad, and a floor that deflects underfoot needs framing work rather than a thicker cushion. The acclimation and subfloor prep guide covers that side of the job.

Flooring underlayment guide — layer stack showing flooring over underlayment, vapor barrier, and concrete slab

Types Compared

Type	Thickness	Best under	Vapor barrier	Cost per sq ft	Notes
Standard foam	2–3 mm	Laminate, LVP over wood	No	$0.20–$0.40	Cheapest; add poly over slabs
Foam with bonded film	2–3 mm	Laminate, LVP over concrete	Yes	$0.35–$0.70	The default for slabs; tape the seams
Cork	3–6 mm	Engineered wood, laminate	No	$0.70–$1.60	Best natural sound dampening
Rubber or cork-rubber	2–6 mm	Acoustic-critical rooms	Varies	$0.80–$2.00	Condos and upstairs rooms; heavy
Felt or fiber	3–7 mm	Engineered and solid panels	No	$0.60–$1.40	Also acts as a leveling course
Uncoupling membrane	3 mm dimpled	Tile over concrete or wood	With membrane	$1.50–$3.50	Prevents crack telegraphing; not a cushion
Cement backer board	1/4–1/2 in	Tile on floors and walls	No	$1.00–$2.50	Adds height and stiffness

Prices are typical 2026 U.S. material costs and vary with roll size and brand. The important column is the "best under" one: a premium cork pad under vinyl plank is money spent on the wrong property, and a 2 mm foam under engineered hardwood is not enough cushion for the product's warranty.

Matching It to the Floor

Flooring	Over wood subfloor	Over concrete slab	Never use
Laminate (no attached pad)	2–3 mm foam	Film-backed foam	Thick rubber or cork beyond spec
Laminate (attached pad)	Nothing additional	6-mil poly if pad is not rated as barrier	A second foam layer
Rigid-core vinyl plank	2 mm foam or none if attached	Poly plus approved pad	Soft, thick cushion
Engineered hardwood (floating)	3 mm foam or cork	Poly plus approved pad	Bare slab without barrier
Nail-down hardwood	Rosin paper or felt, not foam	Not suitable for nail-down	Foam of any thickness
Ceramic or porcelain tile	Backer board or membrane	Membrane or backer board	Any soft pad
Carpet	Carpet pad, not underlayment	Carpet pad over tested slab	Rigid foam panels

The same laminate that takes basic foam over plywood needs a film-backed pad over a basement slab. The flooring is identical; the building physics are not. When a product's instructions and a retailer's advice disagree, the instructions win, because the warranty is written against them.

Moisture and Vapor Barriers

Moisture is the quiet failure mode of floating floors. Wood-plastic and fiberboard cores swell when they absorb slab moisture, and the first symptom is a row of peaked joints months after the receipt has been filed away. Concrete releases vapor continuously, and a wood subfloor over a crawlspace carries its own seasonal moisture.

Over any concrete slab: use a bonded vapor-barrier underlayment, or lay 6-mil polyethylene with seams overlapped 6 to 8 inches and taped, then the pad. Below grade, do not skip it.
Over a wood subfloor: standard foam is fine when the crawlspace is vented and dry. Check dishwasher and refrigerator supply lines — the two most common hidden leaks — before closing in a kitchen.
Test rather than guess: a calcium chloride kit or an in-slab humidity probe converts "probably fine" into a number you can act on. The acceptance limits in the product instructions are the ones that count.
Do not double up on film: a vapor barrier on both sides of a wood subfloor can trap moisture in the panel. One barrier, on the warm side of the assembly, is the rule.

If the subfloor is concrete, treat the vapor barrier as mandatory. If it is plywood or OSB over a conditioned crawlspace, it is usually optional — but the pad itself still has to match the flooring manufacturer's specification, or the warranty is void regardless of how well the barrier is installed.

Pre-Attached Pad: Skip or Stack?

Many vinyl and laminate products now ship with the pad bonded to the back of the plank. Stacking a second layer underneath feels like extra comfort and causes three specific failures: the floor becomes springy, the click joints flex past their design limit and chip, and a taller-than-tested stack voids the warranty.

Attached pad over wood: install directly and buy nothing extra.
Attached pad over concrete: check the specification sheet. Many attached pads are not rated as vapor barriers, so 6-mil poly still goes underneath.
Attached pad plus an acoustic requirement: some products approve a specific acoustic underlayment over a bare subfloor instead. Use the listed product, never generic foam.

When comparing prices, normalize for the pad: a floor with an attached pad at $3.80 per square foot against a bare floor at $3.40 plus a $0.45 pad is a closer race than it first looks — and the attached-pad version has fewer things to get wrong on site.

Sound Ratings and Apartment Rules

Two ratings describe floor sound, and they measure different things. Impact insulation class (IIC) covers footsteps and dropped objects — the noise your downstairs neighbour hears. Sound transmission class (STC) covers airborne noise such as voices and television. Underlayment mainly improves IIC.

Assembly	Typical IIC	Where it appears
Bare slab, hard floor	25–35	Rarely permitted above another dwelling
Laminate over 2 mm foam	45–55	Single-family; marginal in apartments
Laminate over 3 mm acoustic foam	55–60	Many condo rules accepted range
Cork or rubber 5–6 mm	60–70	Multi-family; often specified by HOA
Purpose-built acoustic membrane system	65–75+	High-rise and hotel construction

Those numbers describe tested assemblies, and a rating only applies to the exact flooring, pad, and slab combination that was tested. In a condominium, get the required IIC from the HOA or building department before buying, and keep the manufacturer's test report — an inspector may ask for it. In a single-family house, sound is a comfort question rather than a compliance one, and cork or rubber is the practical upgrade over foam.

Underlayment for Tile

Tile does not use soft underlayment. It needs a rigid, continuous bond to a substrate that will not move, and the layer between tile and subfloor is a membrane or a board.

Product	What it solves	Where it goes	Height added
Cement backer board	Provides a bondable, dimensionally stable surface	Floors and walls over wood framing	1/4 in floors, 1/2 in walls
Uncoupling membrane	Absorbs lateral movement so cracks do not reach the tile	Floors over concrete or wood	About 3 mm
Bonded waterproofing membrane	Stops water reaching the substrate	Showers, wet rooms, bathroom floors	Under 1 mm per coat
Foam tile backer panel	Lightweight board with waterproof core	Walls and floors, often showers	1/4 to 1/2 in

Deflection is the other half of the tile question. Ceramic needs L/360 and natural stone L/720, meaning the floor may deflect no more than 1/360 or 1/720 of the span under load. A membrane does not fix a bouncy floor; it manages movement in a floor that is already stiff enough. The Tile Calculator handles quantities once the assembly is settled.

Installation Sequence

Clean the substrate. Vacuum, then vacuum again. Grit under foam grinds holes in it and becomes audible under a floating floor.
Lay the vapor barrier first if one is required, running it up the wall by an inch or two and taping every seam. Overlap at least 6 inches.
Roll out the pad across the room rather than along it, so seams run perpendicular to the flooring direction and are less likely to line up with plank joints.
Butt the seams, do not overlap them. Overlapping creates a ridge that shows through thin vinyl and makes the floor rock.
Tape the seams with the manufacturer's tape, and tape the pad to the wall at the perimeter so it does not slide while the floor goes down.
Trim at walls and around obstructions with a sharp knife, leaving the expansion gap free. Pad should never fill the gap between the floor and the wall.
Install the flooring promptly. Pad left exposed to foot traffic tears and compresses before the planks ever arrive.

How Much to Buy

Underlayment has almost no cut waste: it trims with a utility knife and offcuts tape back into service on the next row. Buy the measured floor area plus about 5 percent for seam overlaps and torn edges.

Measure the floor area of every space the pad will cover, including closets and alcoves. The square footage guide covers irregular rooms.
Add stair treads and risers separately if the flooring runs up a staircase; each tread is roughly its width by its depth plus the nosing.
Multiply by 1.05 and round up to whole rolls. Rolls commonly cover 100 square feet.
Buy tape and spacers at the same time. One roll of seam tape covers roughly four to five rolls of pad.

If the pad comes in 100-square-foot rolls, three rolls cover 300 square feet, which handles a 240-square-foot room with 5 percent waste plus a small hall. Buying the fourth roll for a ten-square-foot shortfall is the usual mistake; measuring the hall with the room avoids it.

Worked Example: 240 Square Feet

A 240-square-foot living room in laminate with no attached pad, over a concrete slab in a below-grade room. The flooring needs a vapor barrier and a cushion.

Vapor barrier: 6-mil poly, 260 square feet at $0.15 per square foot = $39, plus one roll of sheathing tape at $12.
Pad: 3 mm acoustic foam, 252 square feet (240 × 1.05), at $0.55 per square foot = $139.
Total underlayment cost: $190, or about $0.79 per square foot of finished floor.
Compare: film-backed foam at $0.65 per square foot would replace both items for about $164 and save a layer of work.

Roughly three percent of a mid-range flooring job, and it is the layer that decides whether the slab's moisture reaches the laminate core. On a below-grade installation, the combined film-and-pad product is usually the better buy because it removes one installation step and one opportunity for a taped seam to be missed.

Frequently Asked Questions

Do I need underlayment for laminate flooring? ▼

Most laminate requires a pad unless the planks already carry an attached one. A 2 to 3 mm foam pad over a wood subfloor is the minimum, and a film-backed foam is the normal choice over concrete because the bonded film acts as a vapor barrier. Check the installation sheet for the approved thickness range: too thick a pad lets the locking joints flex, and too thin a pad does nothing. If the laminate has an attached pad, buy nothing extra.

Can I put two layers of underlayment down? ▼

Generally no. Doubling the pad makes the floor feel soft, which sounds good and behaves badly: the click joints flex past their design limit, the locking tabs chip, and the floor can develop a hinge line where two pad seams cross. Manufacturers specify a single layer of an approved product. The exception is a separate vapor barrier, which is a film rather than a cushion and goes under the pad, not in addition to it.

What is the best underlayment for sound? ▼

For impact noise between floors, a cork, rubber, or cork-rubber blend in the 3 to 6 mm range outperforms cheap foam, and acoustic membranes sold for condominiums carry published impact insulation class ratings. Look for a tested assembly rather than a thickness claim: a product's IIC number only means something when it is tested with the same flooring and slab as your installation. In multi-family buildings, the required rating is often set by the HOA or building code, so get the number before buying.

Is underlayment the same as a vapor barrier? ▼

No. Underlayment is a cushioning layer: foam, cork, rubber, or felt. A vapor barrier is a thin film, usually polyethylene, that stops moisture vapor moving from a slab into the floor. Some products combine the two by bonding a film to the back of a foam pad, which is why the two ideas get confused. Over a concrete slab you need the vapor barrier function, whether it arrives attached to the pad or as a separate sheet underneath it.

What underlayment goes under tile? ▼

Not soft pad. Tile needs a cement backer board, an uncoupling membrane such as a dimpled polyethylene sheet set in thinset, or a bonded waterproofing membrane, depending on the substrate and whether the area is wet. These products manage movement and moisture, not comfort. Foam underlayment under tile guarantees cracked grout and loose tiles because the tile assembly needs a rigid, continuous bond to the substrate.

How much underlayment do I need? ▼

Measure the floor area, add closets, halls, and stair treads, then add about 5 percent for seams and torn edges. Rolls commonly cover 100 square feet, so a 240-square-foot room needs 252 square feet, or three rolls. Underlayment has almost no cut waste because offcuts can be taped back in on the next row, which is why 5 percent is enough where flooring itself needs 10 percent or more. Buy one roll of seam tape per four or five rolls of pad.

Does underlayment make a floor warmer? ▼

A little, and mostly by feel. Foam, cork, and felt reduce the rate at which heat leaves your foot, which is why a padded floor feels warmer than bare concrete even at the same temperature. The measured insulating value is small: a few millimetres of foam adds roughly R-0.5 to R-1. If warmth is the actual goal over a slab, an insulating subfloor panel or radiant heat does far more than a thicker pad, and a thick pad can interfere with radiant systems by blocking heat transfer.

Can I install underlayment over an existing floor? ▼

Only over a sound, flat, fully bonded hard surface such as vinyl or tile, and only if the new flooring manufacturer allows installation over that substrate. Cushion vinyl, carpet, and loose-lay sheet goods must come out. The old floor adds height that affects doors, transitions, and toe kicks, and any movement in the old layer telegraphs into the new one. In most residential jobs, removing the old floor and starting at the subfloor is faster than building on top of a surface you cannot inspect.

References

National Wood Flooring Association, Installation Guidelines: Underlayment and Moisture Control — pad selection and slab requirements, nwfa.org
North American Laminate Flooring Association, Installation Guidelines — approved underlayment types and thickness limits, nalfa.com
Tile Council of North America, TCNA Handbook for Ceramic, Glass, and Stone Tile Installation — membranes, backer boards, and deflection criteria, tcnatile.com
ASTM International, E492 Laboratory Measurement of Impact Sound Transmission and E989 Impact Insulation Class — how IIC ratings are determined, astm.org
APA – The Engineered Wood Association, Underlayment for Floor Coverings — panel grades and surface requirements, apawood.org
U.S. Environmental Protection Agency, Indoor Air Quality: Flooring and Adhesives — low-emission products and ventilation during installation, epa.gov

Disclaimer: This guide explains common residential underlayment products and installation practice for general planning. It is not an installation specification or a warranty statement. Approved underlayment types, thicknesses, and moisture limits vary by flooring manufacturer and product line, and multi-family sound requirements are set by local codes and building rules. Always follow the installation instructions supplied with your flooring and confirm vapor barrier and acoustic requirements before purchase.