Carpet Cushion Types and the Maker's Cushion Spec
Why this matters
Customers buy cushion by squeezing a sample in the showroom, and the softest, thickest one always wins the squeeze. Under a loop berber that choice voids the carpet's warranty, and within a year it shows: seams that peak, backing that grins open between rows, ripples that come back after a restretch. The carpet maker's cushion specification is the document that decides which pad is acceptable under which carpet, and it is a short read. The shop that checks it before writing the order avoids a claim it cannot win.
What the cushion is doing under a stretched carpet
A stretch-in carpet is a membrane held in tension by tackless strip at the walls. Every footstep pushes it down into the cushion, and the cushion pushes it back. Two properties of the cushion decide what that does to the carpet:
- How far it lets the carpet deflect. A thick, soft cushion lets the backing bend sharply around every heel. The secondary backing flexes against the latex that bonds it to the primary, the seam tape flexes with it, and over thousands of cycles the bond fatigues.
- How much support it gives the face. A firm cushion carries the load so the pile fibers are not doing all the work, which helps texture retention. A cushion that bottoms out under a heel gives no support at that point at all.
Comfort and support pull opposite ways. The maker's spec sets the limit where comfort starts costing the carpet its life.
The cushion families
| Type | How it is made | How the spec usually states it | Where it fits |
|---|---|---|---|
| Bonded urethane (rebond) | Chopped urethane foam bonded into a slab, often with a film face | Thickness (in) and density (lb per cu ft) | The common residential pad under cut pile |
| Prime urethane | Foam made as a slab, not reprocessed | Thickness and density | Residential cut pile where the maker lists it |
| Frothed foam (densified urethane) | Urethane mechanically frothed and cured, very uniform and dense | Thickness and density | Higher-end residential, attached cushion goods |
| Rubber | Flat sponge or slab rubber; waffle rubber has ribs on the underside | Thickness and weight (oz per sq yd) | Firm support; check whether the maker accepts waffle |
| Fiber | Synthetic or natural fibers needle-punched into a felt | Thickness and weight (oz per sq yd) | Firm, thin support; often listed for berber, woven and some commercial goods |
Two things in that table matter more than they look. Density and weight are not interchangeable. A urethane density in pounds per cubic foot and a fiber weight in ounces per square yard measure different things, so a pad cannot be shown to comply by converting one into the other; check it against the line of the spec written for its own type. And thickness alone means nothing. A thick, low-density pad and a thin, high-density pad can feel similar in a hand squeeze and behave completely differently under a carpet.
Why loop and berber goods want thin and firm
A loop carpet's face is a field of closed loops sitting in rows, with the rows set apart by the gauge. Bend the backing sharply and the rows spread at the top of the bend, so the backing shows between them (installers say the carpet grins), and the loops on the outside of the bend are pulled tighter at their base. Loop goods also tend to be denser and stiffer than cut pile, so the backing resists bending more and transfers more of the stress into the latex bond and the seams. A thin, firm cushion keeps each deflection shallow. That is why carpet makers commonly publish a separate, stricter line for loop and berber goods, with a lower maximum thickness and a higher minimum density than for cut pile. Woven carpets are frequently stricter again.
Reading the spec
The carpet maker publishes cushion requirements in its installation instructions or warranty, often in a table by carpet style. Read three things on it:
- The line that matches the carpet, by construction (cut pile, loop, cut-and-loop, woven) or by named style.
- Every limit on that line. Typically a maximum thickness and a minimum density or weight per cushion type; some lines add a minimum thickness, a required moisture barrier, or a statement that certain types (waffle rubber is a common one) are not acceptable.
- The consequence. Most residential warranties exclude texture retention, delamination, wrinkling or seam failure when the cushion does not meet the spec.
CRI 105, the Carpet and Rug Institute's residential installation standard, also addresses cushion, but where the carpet maker states a requirement for its own product, the maker's statement is the one the warranty is judged against. All numbers below are the maker's, and they change by product.
Worked example: one cushion, two carpets
A house: two bedrooms in a cut pile saxony and a family room in a level-loop berber, both from the same maker. The customer squeezed samples and picked a 7/16 in, 6 lb bonded urethane for the whole house.
Say the maker's cushion table reads (illustrative values; read your own maker's table):
- Cut pile: bonded urethane, maximum 7/16 in thick, minimum 6 lb density.
- Loop and berber: bonded urethane, maximum 3/8 in thick, minimum 8 lb density.
Bedrooms, saxony. Thickness 7/16 in against a 7/16 in maximum: equal, so it passes (the limit is not exceeded). Density 6 lb against a 6 lb minimum: equal, passes. The customer's pad is acceptable here.
Family room, berber. Thickness 7/16 in against a 3/8 in maximum: 7/16 is 1/16 in over, fails. Density 6 lb against an 8 lb minimum: 2 lb short, fails. The same pad fails both limits under the berber, and either failure alone voids the warranty on that carpet.
The options. A 3/8 in, 8 lb bonded urethane passes the berber line on both counts (3/8 at its 3/8 maximum, 8 at its 8 minimum). Checked against the cut pile line it also passes: 3/8 is under 7/16, and 8 is over 6. So one pad that clears the stricter line clears both, and the house gets a single cushion with no thickness step under the bedroom doorways. If the cut pile line had also stated a minimum thickness, 3/8 in would have to be checked against it before this works. A fiber or rubber cushion listed for loop goods would be the other route, checked against the weight line written for its own type.
The conversation. "Under this berber, the thick pad you liked will let the backing flex every step, and the maker won't cover the carpet if it's installed over it. This firmer pad feels a little harder in your hand and makes the carpet last. It works under the bedroom carpet too, so the whole house matches." Write the cushion type, thickness and density on the order next to each carpet style so the warehouse loads the right rolls.
Cushion specs that change the job
- Moisture barrier cushion. A film face keeps spills from soaking into the pad, and some makers require a specific barrier cushion for a stain or pet warranty to apply. It is not a slab vapor retarder; slab moisture is tested and judged per the flooring and adhesive maker's limits, the same as any other floor (see related: Slab Moisture Testing for Flooring).
- Stairs. Stairs flex carpet more than any floor, so makers often want the firmer end of the range there. The stair method is in How to Install Carpet on Stairs.
- Radiant heat under the floor. Cushion adds thermal resistance on top of the carpet's. The heating system designer or maker may state a ceiling on the combined value of carpet plus cushion, and that limit decides the pad before comfort does.
- Commercial glue-down. Much commercial carpet goes down without cushion, or double glue-down with the cushion glued to the floor and the carpet glued to the cushion. Rolling loads and the maker's method decide it, not the stretch-in rules above.
What a wrong cushion looks like later
These are what the callback reports, and each traces to deflection rather than to the install:
- Ripples that return after a restretch. A soft pad lets the carpet relax under traffic; restretching treats the symptom. The six-month case is worked in The Carpet That Rippled After Six Months.
- Seam peaking. The seam is the stiffest line in the carpet, so it rides up as the soft cushion compresses on both sides.
- Grin in loop goods, especially at the top of stairs and in traffic lanes.
- Secondary backing delamination, felt as a soft, crunchy area where the face has separated from the secondary.
When a claim inspection finds any of these, the pad gets measured and identified before the carpet is condemned. Many pads are printed on the film or the back with type, thickness and density, so look first at a closet corner where a flap can be lifted. If nothing is printed, cut a 12 in by 12 in square from a hidden spot and measure:
- Thickness, with calipers under no load, at several points.
- Density, for urethane, as weight over volume. At 7/16 in thick, one square foot is 1 sq ft times 7/16 in divided by 12 in per ft, or about 0.0365 cu ft. A sample weighing 0.22 lb works out to 0.22 divided by 0.0365, about 6.0 lb per cu ft. Check the other way: 6 lb per cu ft times 0.0365 cu ft is 0.22 lb.
- Weight, for fiber or rubber, in ounces for that square foot, times 9 for ounces per square yard.
The result goes on the inspection report beside the maker's line for that carpet, and the hole gets a patch of matching pad before the carpet goes back down. The pad install itself, including where its seams fall relative to the carpet seams, belongs to How to Install Tackless Strip and Carpet Cushion.
References
- Carpet manufacturers' installation instructions and warranty cushion requirements (the governing document)
- CRI 105, Standard for Installation of Residential Carpet (Carpet and Rug Institute, current edition)
- See related: Carpet Construction for Installers; How to Install Tackless Strip and Carpet Cushion; The Carpet That Rippled After Six Months