What Goes Under a Carpet? Felt Compared With Rebonded Foam

The layer under a carpet does four jobs at once, and which of them matters most is what decides the material. The first is protecting the carpet's backing: laid straight onto a hard floor, the weave rubs against that floor at every step and starts to open up first along the busiest walking lines.

The second is comfort underfoot. The same carpet feels noticeably softer with a compressible layer beneath it, because the energy of each step is taken by that layer rather than by the carpet itself.

The third is damping impact sound. Footsteps, dropped objects and moving chairs all reach the floor as impact, and a resilient layer in between absorbs part of that energy before it enters the structure. The fourth is hiding small irregularities in the floor — a slight ridge on a screed, or the line of a joint, telegraphs through a thin carpet.

Felt is a compressed fibre mat: thin, dense and distinctly firmer than foam. Those three properties together make it a good friction and slip barrier. The carpet's backing bears on the felt rather than on the floor, the abrasion collects on the felt's surface, and the carpet stays where it was laid.

Felt's real advantage is that it is thin. It raises the finished floor level barely at all, which often makes it the only workable option in refurbishment work where door clearance is tight and thresholds and transition profiles have to stay put. Where the floor is already flat and quiet, there is little reason to ask for more than that.

Alongside standard felt there is a thermobonded type, Termowat, in which the fibres are held to each other by heat rather than adhesive, giving a more stable surface. Both are supplied in a range of thicknesses, as roll or as sheet.

Rebonded foam is a dense sheet made by pressing reclaimed foam pieces together, and under a carpet it does a different job from felt. It is thicker, it compresses noticeably under load, and it comes back when the load lifts. The softness felt underfoot and the damping of impact are two faces of the same behaviour.

That is why foam goes beyond what felt can do on hard, reverberant floors — screed, concrete, tile. It is the usual choice on upper floors where footfall reaching the room below is the complaint, in heavy-traffic office and retail areas, and wherever the texture of the floor would otherwise read through a thin carpet.

The parameter that governs foam is density: the DNS figure decides how far the layer compresses under load and how much permanent deformation it takes on over time. Which grade suits which application varies, so it is more useful to look at the options on the product page than to reduce them to a single number here.

The thicker the underlay, the better the floor feels underfoot — but that improvement has a limit. A thin, flexible carpet over a very soft layer starts to feel unstable: each step sinks and stretches the carpet locally, the backing works at every footfall, and seams and edges move.

The second limit is dimensional. Underlay thickness adds directly to the finished floor level, and door leaf clearance, threshold profiles, lift and terrace transitions and furniture heights all feel those few millimetres. In refurbishment this is often a tighter constraint than the acoustic wish that started the conversation.

The practical approach is to judge the whole build rather than the underlay on its own: carpet, underlay and floor behave as one. Laying the candidate combination over a small area and walking on it tells you more than anything that can be read off a specification sheet.

Both felt and rebonded foam are supplied as roll or sheet and calculated by the square metre. Roll width is the first dimension to ask about, because it decides how many joins the installation will have: a width that suits the room's short side reduces both the number of joins and the offcut.

The calculation starts by measuring floor area, then adds a cutting allowance, extra for recesses and column returns, and an offset so the underlay joins do not land on top of the carpet joins. That offset is not a detail: where two joins coincide, that line becomes the weakest part of the floor.

YAYLA supplies these materials in bulk to businesses; it does not sell retail and does not install. Rebonded foam is produced in its own plant, while felt is sourced and supplied. For manufacturers laminating underlay to carpet, what usually governs is roll width and how the line is fed — dimensions settled per order.