The layer nobody photographs, sold as one word — “padding” — when it is three unrelated products with three different sets of rules. It is also where the margin is, and where most flooring warranties are quietly voided.
It is the one thing almost every buyer gets wrong. Density carries the load; thickness only feels good for the moment you stand on the sample. The Carpet and Rug Institute puts residential cushion at no more than 7/16 inch, and mill warranties on cut-pile carpet generally set the same ceiling alongside a minimum density of 6 lb/ft³. Exceed the thickness and the carpet backing flexes over the cushion with every footstep until it breaks — so the upgrade shortens the life of the carpet and voids the warranty that would have covered it.
Bought by the square foot alongside the carpet, and specified by density in pounds per cubic foot. If a quote lists a thickness and no density, that is the question to ask.
6–10 lb/ft³ · 3/8″–7/16″ · $0.50–$0.90/sq ft installed
Shredded scrap foam bonded into a sheet — the multicoloured pad in almost every home in America. It is the default for good reason: at 8 lb density it meets nearly every mill warranty and costs a fraction of anything better.
Use it for: Cut-pile carpet in bedrooms, living rooms and hallways.
Watch out: Below 6 lb it fails early and voids most warranties. Retailers quote pad by thickness because it sounds like value; ask for the density in pounds.
2–4 lb/ft³ · 7/16″–1/2″ · $0.30–$0.50/sq ft installed
Virgin foam, not bonded scrap. Feels luxurious in the showroom because it is soft and thick, and that is exactly the problem.
Use it for: Light-traffic rooms on a tight budget, and not much else.
Watch out: Bottoms out under traffic within a couple of years, and its density is usually below the minimum the carpet warranty requires. The classic builder-grade pad.
12–20 lb/ft³ · 1/4″–3/8″ · $1.20–$2.00/sq ft installed
Chemically frothed rather than shredded and glued, producing a dense, uniform pad that is thin and firm rather than thick and soft.
Use it for: The upgrade worth paying for. Long-run hallways, stairs, and any carpet you want to outlive its warranty.
Watch out: Feels harder underfoot in the showroom than a cheap thick pad, which is why it loses the sale. It is the pad that keeps the carpet alive.
18–22 lb/ft³ · 1/4″–3/8″ · $1.50–$2.50/sq ft installed
Dense flat sheet rubber. The firmest common cushion and the longest-lived — frequently outlasts two carpets.
Use it for: Heavy traffic, stairs, commercial and institutional work, and rolling loads.
Watch out: Expensive, heavy to handle, and unnecessary in a bedroom. Do not confuse it with waffle rubber, which is a different product.
Hollow-domed sheet · $0.60–$1.10/sq ft installed
Rubber moulded with hollow air domes so it feels soft for its weight. Largely superseded by rebond and frothed urethane.
Use it for: Rarely specified new. You will meet it lifting an old floor.
Watch out: The domes collapse and the pad takes a permanent impression of the traffic pattern. Not a current recommendation.
Jute, hair or synthetic · firm · $0.60–$1.00/sq ft installed
Needle-punched natural or synthetic fibre. Firm, dense, and almost incompressible compared with foam.
Use it for: Berber and other loop constructions, patterned carpet, and over radiant heat where foam insulates too well.
Watch out: The right answer for loop carpet specifically — a soft pad lets the backing flex, and flexing is what breaks loop backings.
8–10 lb/ft³ + bonded film · 3/8″–7/16″ · $0.90–$1.40/sq ft installed
Standard rebond with an impermeable film bonded to the top face, so liquid stops at the pad instead of passing into the cushion and the subfloor. Some are sold with an added pet-urine treatment on top of the barrier.
Use it for: Households with animals or small children. The single most useful cushion upgrade a pet owner can buy.
Watch out: It stops liquid reaching the subfloor, not the carpet — the carpet still has to be cleaned, and quickly. Breathable versions block liquid while still letting vapour escape, which matters over a slab.
10 lb/ft³ slab + barrier · 3/8″–7/16″ · $1.50–$2.20/sq ft installed
Dense slab rubber with an integral barrier. The most durable moisture-resistant cushion made, and the only one that will comfortably outlive the carpet on top of it.
Use it for: Below-grade rooms, concrete subfloors, and pet households where the carpet will be replaced before the pad is.
Watch out: Roughly triple the price of plain rebond. Justified over a slab or with animals; hard to justify in a spare bedroom.
Density is what a quote leaves out and thickness is what it advertises. This is the number to ask for.
| Density | What it is | Where it belongs | Note |
|---|---|---|---|
| 6 lb/ft³ | The floor, not a target | Guest rooms and light-traffic bedrooms. | The minimum most mill warranties on cut pile will accept. It works, and it will not last as long as 8. |
| 8 lb/ft³ Default | The residential default | Living rooms, bedrooms, hallways — most of a house. | At 7/16 inch this is the configuration that satisfies essentially every warranty. If a quote does not state a density, this is what to ask for. |
| 10 lb/ft³ | Traffic and stairs | Stairs, long hallways, and any run that gets walked every day. | Stairs in particular want density rather than softness — a soft pad on a nosing wears the carpet through at the edge. |
| 12–22 lb/ft³ | Frothed urethane and slab rubber | Commercial work, rolling loads, and carpet you intend to keep. | Thin and firm rather than thick and soft, which is why it loses the showroom comparison and wins over ten years. |
Bought separately and laid as a continuous sheet with taped laps. It does three unrelated jobs at once — smoothing minor subfloor irregularity, reducing impact sound, and keeping slab moisture out — and a product can be excellent at one and useless at another.
2–3 mm polyethylene · $0.20–$0.35/sq ft installed
The thin white or blue roll that comes with the cheapest quotes. It levels minor subfloor irregularity and does very little else.
Use it for: Laminate or floating engineered over a dry wood subfloor where noise is not a concern.
Watch out: No vapour retarder. Over concrete it must be paired with 6-mil poly or a film-backed product, and skipping that is one of the most common causes of a failed floating floor.
2–3 mm foam + film, taped lap · $0.30–$0.50/sq ft installed
The same foam with a moisture film bonded to one face and an adhesive lap so the seams seal into a continuous membrane.
Use it for: The default over any concrete slab, and over a crawl space.
Watch out: The film only works if the laps are actually sealed. An unsealed seam is a hole in the vapour retarder.
3 mm recycled fibre, dense · $0.45–$0.75/sq ft installed
Dense needle-punched fibre. Noticeably quieter underfoot than foam and firmer, so the floor feels more solid.
Use it for: Over wood subfloors where the hollow click of laminate is the complaint.
Watch out: Absorbs moisture. Not for concrete without a separate vapour retarder underneath it.
3–6 mm natural cork · $0.70–$1.20/sq ft installed
The best-regarded natural acoustic underlayment, and genuinely effective at impact sound between floors.
Use it for: Upstairs rooms in a house, and anywhere footfall noise below matters.
Watch out: Cork is NOT a vapour barrier — this is the most common misunderstanding about it. Over concrete it needs 6-mil poly beneath it, every time.
2–5 mm recycled rubber · $1.00–$1.80/sq ft installed
The heaviest and the best acoustic performer of the common underlayments, and the one most often specified in condominium acoustic requirements.
Use it for: Multi-family construction with a written IIC requirement, over concrete.
Watch out: Do not put rubber under vinyl. Plasticizers migrate out of the vinyl into the rubber and produce yellow-to-amber staining that comes up through the plank, and many vinyl manufacturers void the warranty over it. Rubber is for laminate and engineered wood.
Any floating floor over a slab needs 6-mil polyethylene or a film-backed underlayment, with the laps sealed. Cork and felt do not do this job — cork in particular is widely assumed to and does not. Getting this wrong does not fail immediately; it fails eighteen months later, from below, and the floor comes up.
Laminate and rigid-core vinyl both come two ways: bare, or with the underlayment already bonded to the back. It is not something you buy separately and not something you can change later — it is a specification on the plank, and it varies more between products than anything else printed on the box.
1–2 mm · on Laminate
Laminate is sold both bare and with underlayment already bonded to the back. The attached version skips a whole step on site, which is most of its appeal.
Use it for: Straightforward rooms over a sound, dry, flat subfloor where speed matters.
Watch out: The attached layer stops at every board edge, so unlike a rolled sheet with taped laps it is not continuous — there is a seam wherever two planks meet, and moisture can travel through it. It is also usually thinner and less dense than anything you would buy on a roll.
1–1.5 mm · on SPC, WPC
The standard attached pad on mid and upper-tier rigid core. Closed-cell, so it does not absorb water, and it holds its thickness.
Use it for: Most rigid-core planks worth buying.
Watch out: Thin by design. It takes the hollow ring off the plank; it is not there to make the floor feel soft.
1–2 mm · on SPC, WPC
The cheaper attached foam. Similar on day one, softer and quicker to compact under traffic lanes and furniture.
Use it for: Budget rigid core.
Watch out: Once it compresses it does not recover, and you cannot replace it without replacing the floor. Worth knowing which one you are buying.
1–2 mm · on SPC, WPC, some laminate
A genuine acoustic upgrade over foam and the reason some planks carry a much better impact number than their neighbours on the shelf.
Use it for: Premium rigid core, upstairs rooms, multi-family.
Watch out: Adds real cost. On a ground floor slab, where nobody is underneath, you are paying for something you cannot hear.
1–2 mm · on Commercial rigid core
Rare, heavy and the best acoustic attached pad made. Mostly seen in commercial-spec product.
Use it for: Commercial rigid core with an acoustic requirement.
Watch out: The rubber-under-vinyl staining problem does not apply here — this rubber is bonded by the manufacturer and tested against their own plank. It is loose rubber underlayment that causes it.
Attached underlayment saves a step on site, which is most of its appeal and a real saving on a large job. But it stops at every board edge — unlike a rolled sheet with taped laps, it is not continuous, so there is a seam wherever two planks meet and moisture can travel through it. It is also usually thinner and less dense than anything sold on a roll, so it compacts sooner in the traffic lanes. Over a dry, flat, sound subfloor in an ordinary room, attached is fine. Over concrete, or where you want acoustics, roll is the better product.
Stacking cushion lets the click joint flex under load, and the joint is the one part of a floating floor that cannot tolerate movement. Manufacturers are unambiguous about it and it voids the warranty. The only thing that goes under a pre-padded plank on concrete is a vapour retarder — a sheet of poly, not another cushion.
IIC and STC describe an entire floor-and-ceiling assembly — structure, cavity, ceiling finish and floor together. The mass of the structure does most of the work, and underlayment contributes almost nothing to STC at all.
The figure most often printed on acoustic underlayment is measured over six to eight inches of concrete. That test is not conducted on wood-frame assemblies. If your floor is joists and plywood, a headline number earned on a slab does not transfer.
A bare six-inch slab sits around IIC 28 and a bare wood-frame floor around IIC 30 — but their STC figures are roughly 52–55 and 40–43 respectively. Two structures that are nearly identical on impact sound are a world apart on airborne sound, which is why one number never describes a floor.
If you live in a condominium with an acoustic requirement, ask for a test report on an assembly that resembles yours, not a product datasheet. If you simply want the room quieter, cork or rubber under laminate does more than any number on a box will tell you.
No, and this is the most expensive misunderstanding in a carpet showroom. Density carries the load; thickness just feels good underfoot for the thirty seconds you stand on the sample. The Carpet and Rug Institute puts residential cushion at no more than 7/16 inch, and most mill warranties on cut-pile carpet set the same ceiling with a minimum density of 6 lb/ft³. Go thicker and the carpet backing flexes over the cushion every time someone walks on it, which breaks the backing from underneath — so the pad that felt like an upgrade shortens the life of the carpet and voids the warranty at the same time.
Eight pounds per cubic foot of bonded urethane at 7/16 inch is the residential default and satisfies essentially every mill warranty. Six pounds is the floor, not a target. If you want to spend more, spend it on density rather than thickness — a 1/4 inch frothed urethane at 15 lb outperforms a 1/2 inch prime foam at 3 lb by every measure that matters, including how it feels after two years.
No, and adding one is a warranty problem rather than a bonus. Stacking cushion under a click floor lets the joint flex under load, and the joint is the one part of a floating floor that cannot tolerate movement. Manufacturers are explicit about this: attached pad means no additional underlayment. The exception is a vapour retarder over concrete, which is a sheet of poly rather than a cushion and does not add compressible thickness.
You should not. Plasticizers migrate out of the vinyl into the rubber and produce a yellow to amber stain that telegraphs up through the plank, and it is permanent. Many vinyl manufacturers void the warranty specifically over rubber underlayment and rubber-backed mats. Rubber is an excellent underlayment for laminate and engineered wood — it is the combination with vinyl that fails.
No. This is the most common misunderstanding about cork, and it is an expensive one over a slab. Cork is a very good acoustic layer and it is not a moisture retarder — it will happily pass slab vapour into the floor above it. Over concrete, cork needs 6-mil polyethylene underneath it, taped at the laps.
Less than the packaging implies. Both are ratings of a whole floor and ceiling assembly, not of the product in the roll, and the mass of the structure does most of the work. Underlayment contributes almost nothing to STC. Worse, Delta IIC — the number most often quoted on acoustic underlayment — is tested over six to eight inches of concrete, and that test is not conducted on wood-frame assemblies at all. A headline number earned on a concrete slab tells you very little about a timber floor in a house.
IXPE is cross-linked polyethylene: closed-cell, holds its thickness, and standard on mid and upper-tier rigid core. EVA is the cheaper foam and compacts under traffic lanes and furniture without recovering. They feel identical in the store and you cannot replace either without replacing the floor, so it is worth asking which one is bonded to the plank you are being quoted.
More than most upgrades. Standard cushion passes urine straight through into the subfloor, which is where the smell lives and why the smell survives cleaning the carpet. Moisture-barrier cushion bonds an impermeable film to the top of standard rebond so liquid stops at the pad. It does not clean the carpet for you, and it is only as good as its seams, but it keeps the problem in a layer you can replace.