Carpet Underlay Explained

Underlay is the layer fitted between the floor and the carpet. It absorbs the impact of footfall, which is what stops the carpet pile from being crushed, and it insulates against cold and sound. Thickness is measured in millimetres and insulating performance in tog. For most domestic rooms, 8mm to 11mm is standard and 12mm is a luxury specification.

Underlay is the part of a flooring quote people most often try to save on, and it is usually the wrong place to do it.

What Underlay Actually Does

Three things, in order of how much they matter.

First, it protects the carpet. Every footstep compresses the pile against whatever is beneath it. On a hard floor with thin underlay, the pile is crushed between a shoe and concrete and it breaks down. On a good underlay the impact is absorbed, and the carpet keeps its appearance for years longer.

Second, it insulates. A suspended timber floor over a ventilated void is a significant route for heat loss in an older house, and underlay is the cheapest meaningful thing you can put in the way of it.

Third, it reduces sound, both within the room and between floors.

Thickness: What the Millimetres Mean

Underlay is sold by thickness. Thin products around 6mm to 7mm exist and are generally a false economy in a living space. Between 8mm and 11mm covers most domestic requirements. At 12mm you are into luxury specification, which is noticeably softer underfoot and insulates better.

Thicker is not automatically correct everywhere. On stairs, an over thick underlay can make the carpet difficult to fit neatly around the nosing, and a slightly firmer specification often gives a better and safer result. This is a case where the right answer differs room to room, which is why we specify it per room rather than quoting one product for the whole house.

We fit a 12mm luxury underlay beneath our heavier carpet ranges, including our deep pile saxony, because that is where it makes the biggest difference.

Tog Rating and Warmth

Tog measures thermal insulation. A higher tog means more resistance to heat passing through, so more warmth retained in the room.

This matters most in older properties with suspended timber ground floors, which describes a great deal of the Victorian and Edwardian housing across Mansfield, Alfreton, Ripley and Nottingham. Customers in those houses tend to notice the difference within the first week, and it is the part of the job we get thanked for most often.

If the room has underfloor heating, the requirement inverts. You want a low tog underlay so the heat can get through rather than being insulated away from the room. Tell your fitter if you have underfloor heating, because the wrong underlay will leave an expensive heating system working against a blanket.

Can You Reuse Old Underlay?

Almost never worth it. Underlay works by compressing, and once it has spent fifteen years being compressed it has little left to give. Fitting a new carpet on dead underlay wastes most of what you have just paid for.

Replacing it also means the fitter can see and deal with the subfloor underneath, which is the right time to find a problem.

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