What thickness underlay is best for carpet?

Around 10–11 mm for bedrooms, 9–10 mm for lounges and landings, and a firmer 8–9 mm for stairs and hallways. Thicker is not automatically better — density matters as much as depth.

What underlay should I use with underfloor heating?

A low-tog underlay of around 6–7 mm, keeping the combined tog of underlay and carpet at or below roughly 2.5 so the heat reaches the room rather than being trapped.

Can I reuse my old carpet underlay?

Sometimes, if it is recent, dry and still firm across the whole floor. Most underlay coming out with an old carpet is compressed under furniture and past its useful life, and reusing it shortens the life of the new carpet.

Is expensive underlay worth it?

Yes, more so than an equivalent upgrade to the carpet. Good underlay makes a mid-range carpet feel expensive, keeps the room warmer and quieter, and protects the carpet backing so it lasts years longer.

What underlay do I need?

Underlay is the part nobody sees and the part that decides how your carpet feels and how long it lasts. Spending a little more here does more for a room than spending the same money on a slightly better carpet.

Thickness and density in plain terms

Thickness is how deep it is; density is how firm. You want both. A thick, soft underlay feels lovely for a week and then flattens into a dead sponge that lets the carpet backing work loose.

For a bedroom, 10–11 mm of a decent density is ideal. For a lounge or landing, 9–10 mm firmer. For stairs and hallways, 8–9 mm and firm — soft underlay on stairs makes the nosings wear through quickly and feels unsafe underfoot.

The main types

PU foam — recycled foam, the usual choice now: warm, springy, good value, easy to fit
Crumb rubber — heavy and very firm, excellent for stairs and heavy-traffic areas
Sponge rubber — traditional, good under patterned and woven carpets
Felt or wool felt — firm and quiet, often specified under wool carpets
Low-profile 6–7 mm — for underfloor heating, where you want heat through, not held in

Underfloor heating

Here the rule reverses: you want low tog, not high. The combined tog of underlay plus carpet should stay at or below about 2.5, or the heating is fighting your floor. That usually means a thin 6–7 mm underlay designed for the job and a carpet with a shorter pile.

Do I need new underlay?

Usually yes. Existing underlay is often the same age as the carpet coming out, and it has spent that time compressed under furniture. If it is genuinely recent, firm everywhere and dry, we will tell you it can stay — we would rather do that than sell you something you do not need.

It has to come up if it is crumbling, patchy, damp, or a different thickness in different rooms.

What we fit as standard

Our carpet prices include a good-quality PU underlay suited to the room, along with grippers, door bars and taking the old floor away. If you want to step up to crumb rubber on the stairs or a low-tog underlay over heating, we will price both so you can compare.

Quick answers

What thickness underlay is best for carpet?: Around 10–11 mm for bedrooms, 9–10 mm for lounges and landings, and a firmer 8–9 mm for stairs and hallways. Thicker is not automatically better — density matters as much as depth.
What underlay should I use with underfloor heating?: A low-tog underlay of around 6–7 mm, keeping the combined tog of underlay and carpet at or below roughly 2.5 so the heat reaches the room rather than being trapped.
Can I reuse my old carpet underlay?: Sometimes, if it is recent, dry and still firm across the whole floor. Most underlay coming out with an old carpet is compressed under furniture and past its useful life, and reusing it shortens the life of the new carpet.
Is expensive underlay worth it?: Yes, more so than an equivalent upgrade to the carpet. Good underlay makes a mid-range carpet feel expensive, keeps the room warmer and quieter, and protects the carpet backing so it lasts years longer.

Want a straight answer for your own room? Call 07342 203294 or see our flooring prices and areas we cover.