Which carpet underlay should I choose?

Underlay is the layer that takes the impact of every footstep so the carpet does not have to. That is why a mid-range carpet on a good underlay outlasts and outperforms an expensive carpet on a poor one, and why fitters consistently say the underlay is where the money should go.

Step by step

1. Understand that thickness is not quality.A thick soft underlay feels luxurious in the shop and lets the carpet flex too much in use, which wears out the backing. Density matters more than thickness for durability.
2. Match it to the room.Living room and bedroom: comfort and warmth, so a good PU foam at around 10mm. Hall, landing and stairs: firmness and durability, so a denser and often thinner product. Flat with neighbours below: acoustic performance, so crumb rubber or a specified acoustic underlay.
3. Check what the carpet manufacturer specifies.Many carpet warranties state a maximum underlay thickness or a required class. Fitting outside that specification can void the warranty and, on a loop pile, will cause visible problems.
4. Use a firm underlay on stairs.A soft, thick underlay on a stair lets the carpet move over the nosing, which wears it out fast and makes the stair feel unsafe. Firm and dense is the rule, and some fitters use a heavier grade on stairs than on the landing.
5. Read the tog value if you have underfloor heating.Underlay insulates, which is exactly what you do not want over UFH. The combined thermal resistance of carpet and underlay has to stay within the system's limit, and low-tog underlays exist specifically for this.
6. Do not reuse old underlay.It is compressed where the traffic was, it holds the dust and dander of the previous carpet, and it costs a fraction of the job. Reusing it is the single most common way to make a new carpet feel and look like the old one.
7. Fit it correctly.Butted and taped at the joints, never overlapped, and butted up to the gripper rather than lapped over it. Underlay lapped over gripper produces a soft edge that lets the carpet release.
8. Buy for the room, not by the roll.It is entirely normal to use a comfortable underlay in the living room and a firmer one on the stairs and hall. Fitters expect this and it is not more expensive overall.

Alternatives

• PU foam: Recycled polyurethane foam, the most common domestic underlay. Excellent comfort and thermal performance for the money, light, easy to handle, and available in a wide range of thicknesses and densities. The default choice for most rooms.
• Crumb rubber: Dense recycled rubber. Heavy, firm underfoot and very durable, with good acoustic performance. The choice for heavy traffic, for stairs, and where impact noise to the room below matters.
• Sponge rubber: Traditional flat or waffle rubber. Firm, long-lasting, and still specified for contract and heavy domestic use. Heavier and generally more expensive than foam.
• Felt: Traditional fibre felt. Firm, breathable, good over old timber floors and with wool carpets, and often specified in period restoration. Less soft underfoot than foam.
• Combined underlay with a moisture barrier: Foam or rubber with an integral barrier layer, for use over concrete where a small amount of residual moisture is expected. Not a substitute for dealing with an actually damp slab.

Questions people ask