Quick answer
Floor underlayment is a thin layer installed between the subfloor and the finished floor, such as foam, cork, rubber, felt, plastic sheeting or thin plywood panels, that cushions the floor, smooths small flaws, blocks moisture from below or reduces noise, depending on the type.
Some underlayments do one job and some combine two or three. The flooring maker’s instructions decide which kind is allowed, and many floating floors now come with a pad already attached.
Quick facts
The key facts about Floor Underlayment, with sources:
- Sits between
- The subfloor (structure) and the finish floor 1
- Over concrete (NWFA, as reported)
- At least 6-mil polyethylene or material rated 0.15 perm or less 1
- Cushion pad (typical maker rule)
- At least 1/8 in thick foam, rubber or similar 1
- Example plastic lap
- Seams overlapped 6 in and taped (one vinyl maker) 3
- Multifamily sound code
- IIC 50 lab / 45 field for floor-ceilings between units (California 2025 CBC 1206.3) 4
Key takeaways
- Underlayment is the in-between layer: above the subfloor, below the floor you see.
- Each type has a job: cushion, moisture control, sound control or a smooth base.
- Over concrete, a moisture barrier is the most common requirement.
- If your floor has an attached pad, adding another layer can void the warranty or make the floor unstable.
- Underlayment cannot fix a floor that is out of flat or bouncy; that is subfloor work.
Underlayment in plain English
A finished floor is a stack. At the bottom is the structure: joists and a subfloor of plywood or OSB, or a concrete slab. At the top is the floor you walk on. Underlayment is whatever thin layer goes between them.
The word covers very different products. Under a floating laminate floor it might be a roll of thin foam. Over a concrete basement slab it might be a sheet of plastic.
Under tile or sheet vinyl it might be thin plywood panels screwed down to make a smooth base. In multi-unit buildings it might be a rubber mat meant to stop footstep noise.
What they share is purpose. A Hardwood Floors magazine article on NWFA guidance groups pads for floating floors into three jobs: cushion, moisture barrier, and sound deadening, often combined in "2-in-1" or "3-in-1" products 1.
The flooring maker sets the rules. The Resilient Floor Covering Institute notes that rigid-core vinyl often comes with a pre-attached cork or other acoustic underlayment, and that the maker’s requirements for flatness, moisture and underlayment should govern the install 2.
How underlayment does its jobs
Each function works in a different way. Most products do one or two of these.
- Cushioning. A soft layer, often at least 1/8 inch of foam or rubber, takes up small bumps and gives a floating floor something to rest on 1.
- Blocking moisture. Concrete slabs release water vapor. A plastic film or closed-cell foam slows that vapor before it reaches a wood or vinyl floor 1.
- Reducing sound. Rubber, cork and dense foams absorb the impact of footsteps so less noise passes to rooms below or echoes in the room 2.
- Smoothing. Thin underlayment panels give glue-down or tile floors a smooth, stiff surface. One vinyl maker tells installers to add an extra layer of APA-rated underlayment if the subfloor bounces 3.
- Separating. A layer between the floor and the subfloor lets a floating floor move with temperature and humidity without binding.
- Finish floorLaminate, vinyl plank or engineered wood
- Cushion or acoustic padFoam, cork or rubber, often attached to the plank
- Moisture barrier6-mil polyethylene or an approved film, seams lapped
- Concrete slabTested for moisture before install
Over concrete, the moisture layer and the cushion are separate jobs, sometimes handled by one combination product. Source: [1] [3]
No single product does all of this well. That is why combination products exist and why the flooring maker’s list of approved underlayments matters.
Types of floor underlayment
| Type | Main job | Usual use |
|---|---|---|
| Foam pad | Cushion, some sound control | Floating laminate and engineered wood 1 |
| Combination pad (2-in-1 or 3-in-1) | Cushion plus moisture film, sometimes rated sound layer 1 | Floating floors over concrete |
| Polyethylene film (6-mil) | Moisture barrier | Over concrete under floating floors 13 |
| Cork | Sound and warmth | Often pre-attached to rigid-core vinyl 2 |
| Rubber | Impact sound control | Upper floors and multifamily units |
| Felt or paper | Separation and minor noise | Nail-down wood floors |
| Underlayment panels (plywood, cement board) | Smooth, stiff base | Tile, sheet vinyl, glue-down floors 3 |
Underlayment is not subfloor
The subfloor is structural and spans the joists. Underlayment is a thin layer on top of it. A soft pad cannot add stiffness to a bouncy floor; one vinyl maker calls for an extra layer of rated underlayment panel, fastened down, when the subfloor deflects 3.
Example: what one rigid-core vinyl guide requires below the floor
These requirements come from one brand’s published rigid-core vinyl installation guide (valid as of January 2024) 3. They show how specific underlayment and subfloor rules can be. Your product may differ.
| Condition | Requirement in the guide | Why it matters |
|---|---|---|
| Flatness | Within 3/16 in over 10 ft or 1/8 in over 6 ft | Planks click apart over dips |
| Over concrete | 6-mil plastic or approved roll-on moisture barrier | Stops trapped slab moisture |
| Plastic seams | Overlapped 6 in and taped with waterproof tape | Keeps vapor from leaking through laps |
| Slab moisture limits | 4 lb per 1,000 sq ft per 24 hr (ASTM F1869) or 70% RH | Wetter slabs need moisture abatement |
| New concrete | Poured at least 60 days, dry and fully cured | Fresh concrete releases water |
| Wood subfloor moisture | No more than 12% by moisture meter | Wet wood swells and moves |
| Bouncy subfloor | Add a layer of APA-rated underlayment panel | A pad cannot add stiffness |
- Read the flooring maker’s list of approved underlayments
- Check whether the plank already has an attached pad
- Test concrete for moisture and wood for moisture content
- Check flatness with a straightedge
- Fix bounce with structural or panel work, not a pad
- Plan seam laps and taping for any plastic film
Notice that only two rows are about the underlayment itself. The rest are about the layer below it. Most underlayment problems are really subfloor problems. Our flooring calculator helps plan material quantities once the base is right.
Where underlayment shows up in a flooring quote
As its own line item. A clear flooring bid names the underlayment product or says "attached pad, no added underlayment." If it only says "underlayment," ask which one and whether the flooring maker approves it.
In the prep scope. Leveling, patching, moisture testing and moisture barriers are often separate lines. Missing them is a common reason one bid looks lower than another. The quote comparison tool helps line them up.
In the warranty. Flooring warranties often depend on using an approved underlayment and meeting moisture limits. One vinyl guide says damage from mold, hydrostatic pressure or subfloor moisture is not covered 3.
In a condo’s rules. Many associations require an acoustic underlayment with a stated sound rating before hard floors replace carpet.
Check for an attached pad first
Many vinyl plank and some laminate products come with a pad already bonded to the back. Ask the maker whether any added underlayment is allowed before you buy one; stacking pads can make click joints flex and fail.
What underlayment adds, and what it cannot fix
What it adds
Limits of underlayment
Underlayment is thin by design. It hides very small flaws but not dips, humps or bounce.
One vinyl maker’s flatness tolerance is 3/16 inch over 10 feet, and the installer is responsible for meeting it before the floor goes in 3.
Anything worse needs patching or a self-leveling underlayment, which despite the name is a pourable leveling compound, not a pad.
Moisture barriers also have limits. They slow vapor; they do not stop liquid water from a high water table or a plumbing leak. The same guide says trapped moisture under the floor can damage the structure and distort the planks, even after a slab tested dry 3.
Sound pads help with footstep noise but are only one part of a floor-ceiling assembly. The ceiling below, insulation in the joists and how the layers are connected all change the result.
What drives the cost of underlayment, and how long it lasts
We do not publish flooring prices. These are the factors that move the underlayment part of a bid.
Function. A plain moisture film is simpler than a combination pad with a rated sound layer 1.
Material. Rubber and cork usually cost more than foam.
Prep. Moisture testing, leveling and extra underlayment panels add labor 3.
Building rules. Condo sound requirements can narrow the choice to tested products.
Attached pads. Floors with a pre-attached pad may need no separate underlayment, which moves the cost into the flooring itself 2.
Underlayment usually lasts as long as the floor on top of it, since it is replaced when the floor is. Moisture damage is the main thing that cuts that short. See our cost guide for how flooring fits a remodel budget.
How underlayment goes in on a remodel
Most flooring jobs do not need a permit. Structural repairs to the subfloor or joists may, and should be done by a contractor.
- Remove the old floor. In older homes, old resilient flooring and its adhesive may contain asbestos; one maker tells installers to assume it does unless tested 35.
- Check the subfloor for flatness, moisture and bounce, and fix problems 3.
- Confirm the underlayment the flooring maker allows, or that the attached pad is enough 2.
- Over concrete, lay the moisture film with seams lapped and taped as the instructions direct 3.
- Roll out the cushion or acoustic layer with seams butted, not overlapped, unless the maker says otherwise.
- Install the finish floor, leaving the perimeter expansion gap the maker specifies. See our expansion gap entry.
Protecting what is under the floor
You cannot reach underlayment once the floor is down, so care is about keeping water and damage away from it.
- Wipe up spills quickly. One vinyl maker warns liquids can migrate through seams into the substrate 3.
- Fix plumbing leaks fast. Water trapped under a floating floor can grow mold.
- Keep indoor humidity steady. Big swings make floating floors move.
- Use felt pads under furniture. Heavy point loads can crush soft pads over time.
- Keep the expansion gap clear. Do not caulk or pin trim into the floor.
Signs the underlayment or the layer below it is failing
- Musty smell or dark stains at seams, often moisture trapped under the floor.
- Cupping or gapping planks over a concrete slab.
- Click joints that separate or a floor that feels spongy.
- Hollow, crunching sounds when walking.
- New complaints of noise from neighbors below after a hard floor replaced carpet.
Old floors and asbestos
Older resilient flooring, backing, lining felt and cutback adhesive may contain asbestos.
One flooring maker tells installers to assume it does unless tested 3, and EPA advises not to touch suspect material and to use a trained, accredited professional if it will be disturbed 5. Do not sand or scrape old flooring before it is tested.
Moisture problems need the source found before the floor is relaid. A flooring contractor can test the slab or subfloor, and a plumber can check for leaks.
Rules and standards that touch underlayment
Single-family homes. Building codes rarely regulate underlayment under a finish floor in a house. The flooring maker’s instructions and industry guidance govern it 2.
Industry guidance. The National Wood Flooring Association (NWFA) guidance, as reported by Hardwood Floors magazine in 2017, calls for at least 6-mil construction-grade polyethylene or another material rated 0.15 perm or less over concrete under a floating wood subfloor, and notes that films meeting ASTM E1745 are tougher 1. Check the current NWFA guidelines.
Multifamily sound. Building codes do regulate sound between dwelling units.
California’s 2025 Building Code, Section 1206.3, requires floor-ceiling assemblies between units to reach an impact insulation class (IIC) of at least 50 in lab tests under ASTM E492, or 45 when field tested under ASTM E1007; Section 1206.2 sets airborne sound at STC 50, or 45 in the field 4.
Other states adopt similar rules from the International Building Code. In condos, underlayment is often how a new hard floor keeps the assembly in compliance.
Moisture tests. Concrete moisture limits in flooring instructions often cite ASTM F1869 (calcium chloride) or relative humidity tests 3.
Underlayment vs related flooring layers
| Layer | What it is | Main difference from underlayment |
|---|---|---|
| Floor underlayment | Thin layer between subfloor and finish floor | — |
| Subfloor | Structural panels or slab | Carries load; underlayment does not |
| Self-leveling underlayment | Pourable cement compound | Flattens a floor; not a pad |
| Vapor retarder | Material that slows vapor | A function some underlayments provide |
| Attached pad | Cushion bonded to the plank 2 | Comes with the floor; may replace a separate underlayment |
| Tile backer board | Cement-based panel for tile | A stiff tile base, not a cushion |
Underlayment myths
- Myth Thicker underlayment is always better.
- Reality Too much cushion can let click joints flex. Follow the maker’s approved list 2.
- Myth A pad will hide an uneven floor.
- Reality Flatness is a subfloor job; one maker’s limit is 3/16 in over 10 ft 3.
- Myth You always need separate underlayment under vinyl plank.
- Reality Many rigid-core products have an attached pad 2.
Where you live changes the underlayment you need
Two regional factors matter most. First, basements and slab-on-grade homes, common in some regions, put floors directly on concrete, which raises the moisture barrier question 1.
Second, multifamily sound rules come from the building code your state adopts; California’s 2025 code sets IIC 50 lab and 45 field between units 4. Condo associations often add their own rules on top.
When underlayment should shape your flooring choice
Underlayment decisions matter most over concrete, on upper floors where noise travels, and with floating floors that have no attached pad. In those cases, choose the underlayment when you choose the floor, from the maker’s approved list.
If the subfloor is uneven or bouncy, budget for that work first; no pad will save a floor laid over a bad base. Our whole-home renovation guide covers floors in larger projects.
- If Floating floor over concrete
- Then Use a moisture film or combination pad the maker allows
- If Plank already has an attached pad
- Then Check whether added underlayment is allowed before buying any
- If Upper floor or condo
- Then Choose a tested acoustic underlayment that meets building rules
- If Subfloor is uneven or bouncy
- Then Fix the subfloor first; a pad will not solve it
Questions about Floor Underlayment
What is floor underlayment?
Floor underlayment is a thin layer between the subfloor and the finished floor. Depending on the type, it cushions the floor, blocks moisture from below, reduces footstep noise or gives a smooth base. Common types include foam, cork, rubber, plastic film, felt and thin plywood panels. The flooring maker decides which kinds are allowed.
Do I need underlayment for vinyl plank flooring?
Often not a separate one. Many rigid-core vinyl planks come with a pad already attached, and adding another layer may not be allowed. Over concrete, makers commonly still require a 6-mil plastic or approved moisture barrier. Check the plank maker’s instructions before buying any underlayment.
Do I need a moisture barrier over concrete?
Usually yes for floating wood, laminate and vinyl floors. NWFA guidance, as reported in 2017, calls for at least 6-mil polyethylene or a material rated 0.15 perm or less over concrete, and one vinyl maker requires 6-mil plastic over all concrete with seams overlapped 6 inches and taped.
Can underlayment level an uneven floor?
No. Underlayment is too thin to fix dips or humps. Flooring makers set flatness limits, such as 3/16 inch over 10 feet, that the subfloor must meet first. Low spots need patching or a self-leveling compound, and bouncy floors need structural or panel work.
What is the best underlayment for soundproofing?
Dense rubber, cork and rated acoustic foams usually reduce footstep noise the most. In multifamily buildings, the floor-ceiling assembly may need an impact rating; California’s 2025 code requires IIC 50 in lab tests between units. Choose a product tested in an assembly like yours and approved by your flooring maker.
Can I put underlayment on top of an attached pad?
Usually not. Most makers of floors with attached pads say not to add more cushioning, because too much softness lets the click joints flex and break. A thin moisture film over concrete is often allowed. Read the maker’s instructions, since breaking them can void the warranty.
What is the difference between underlayment and subfloor?
The subfloor is the structural layer, plywood, OSB or concrete, that spans the joists and carries weight. Underlayment is a thin layer on top of it that cushions, controls moisture or sound, or smooths the surface. Underlayment cannot make a weak subfloor stiffer.
Which way does the plastic side of underlayment go?
Follow the product’s instructions. On many combination pads the film side faces down toward the concrete and is lapped over the next row and taped. Some products face differently. When separate plastic is used, one vinyl maker requires seams overlapped 6 inches and taped with waterproof tape.
Does hardwood flooring need underlayment?
Nail-down solid hardwood is often laid over a felt or paper layer, while floating engineered wood needs a cushion pad and, over concrete, a moisture barrier. Glue-down wood usually goes without a pad. The wood flooring maker and NWFA guidance set the rules for each method.
Sources
- Hardwood Floors magazine (NWFA), Subfloor Focus Part 3: Vapor Barriers and Pads for Floating Floors (Roger Barker, June 1, 2017), retrieved .
- Resilient Floor Covering Institute, Rigid Core (WPC and SPC), retrieved .
- RW Supply+Design, Rigid Core Vinyl Installation Guidelines (valid as of January 2024), retrieved .
- California Building Code 2025 (Title 24 Part 2), Section 1206 Sound Transmission, via GoCodeBook, retrieved .
- U.S. EPA, Protect Your Family from Exposures to Asbestos, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Underlayment functions and NWFA concrete guidance come from a Hardwood Floors magazine article (2017) that quotes the NWFA guidelines; the current NWFA edition was not freely available.
Attached pads and maker authority come from the Resilient Floor Covering Institute. Flatness, moisture and lap figures come from one rigid-core vinyl maker’s guide and are labelled product-specific. Sound ratings were read in the 2025 California Building Code Section 1206. No prices are given.
Suggest a correction. We fix errors and say what changed.