Underlay Does More Work Than The Carpet Above It

Carpet is chosen carefully and underlay is chosen last, often by price alone. That is the wrong way round, because the layer underneath does most of the mechanical work.

The carpet is protected rather than protecting

Every footstep compresses the pile, and the fibres recover only if something beneath them absorbs part of the load. Underlay provides that give, returning the pile towards its original height.

Laid on a hard floor with nothing between, the same carpet is crushed against an unyielding surface and the pile flattens permanently in the places that are walked on most.

This is why carpet in a hallway can look worn out while an identical piece in a bedroom looks new. The difference is often the underlay, not the traffic alone.

Density and thickness are separate properties

A thick, soft underlay feels luxurious underfoot and compresses easily, which suits a bedroom where the load is light and comfort is the point.

On a stair or in a hall the same product bottoms out under each step and stops cushioning anything. A denser, thinner product performs far better there.

Buying the thickest available is therefore not the same as buying the best, and the specification that suits one room may actively shorten the life of the carpet in another.

It changes how a room sounds

Soft layers absorb the mid and high frequencies that make a room echo, and the underlay contributes to that as much as the pile does because it is thicker and more open.

It also reduces impact noise transmitted downwards through the structure, which is the type of noise that carries best through floors and causes most complaints between dwellings.

Neither effect is dramatic on its own, but a soft floor with a proper underlay is measurably quieter than the same room with a hard covering.

Warmth is a side effect of trapped air

Insulation depends on still air held in a structure, and a fibrous or foamed underlay holds a great deal of it just above a cold subfloor.

Over an unheated space that is welcome, and it makes a solid ground floor noticeably more comfortable to stand on in winter.

Over underfloor heating it is the opposite problem. There the underlay must be chosen for low resistance so the heat actually reaches the room.

Replace it when you replace the carpet

Old underlay that has been compressed for years has lost most of its recovery, and putting new carpet on it wastes a good part of the new carpet's life.

The cost is small compared with the covering, and it is the only opportunity to fix a damp or uneven subfloor while everything is lifted and visible.