Carpet Underlay Explained: Does It Really Make a Difference?

Yes, and more than most people expect. Underlay supports the carpet backing, absorbs the pressure of footsteps and furniture, adds cushioning, and puts an insulating layer between the carpet and a cold subfloor. Fit a good carpet over a tired or unsuitable underlay and it will neither feel nor last the way it should.

But the thickest underlay is not the best underlay. Density matters more than depth, and getting that the wrong way round is the most common and most expensive mistake in a carpet order.

This guide covers what underlay does, how the different materials compare, and which specifications matter for bedrooms, living rooms, stairs and homes with underfloor heating. We have been fitting carpet across Liverpool and Merseyside since 1996. You can browse our underlay range alongside the full carpet range, or ask us to specify the right combination for your room.

What underlay actually does

Four jobs, and they pull in slightly different directions, which is why one product does not suit every room.

Comfort

Underlay creates the cushioning layer beneath the carpet, and it is what makes a bedroom or living room feel soft underfoot rather than merely carpeted. Thickness contributes here, but not on its own.

Warmth

Carpet and underlay together resist heat moving through the floor. Over cold concrete, or the suspended timber floors found throughout Liverpool’s terraces, the right underlay noticeably reduces the cold feeling at floor level. It will not fix a damp floor or substitute for proper insulation. Our guide to flooring for Victorian and terraced homes explains why cold floors, draughts and underfloor ventilation have to be considered together.

Sound

Underlay reduces impact noise: footsteps, dropped objects, chairs moving. It matters most in upstairs rooms, on landings and in flats. Manufacturers publish impact sound reduction in decibels, though figures are only comparable when products have been tested the same way. No underlay soundproofs a room - voices and television travel through walls and ceilings regardless.

Carpet lifespan

Every step compresses the pile and the backing. Underlay absorbs part of that and helps the carpet recover, which is why it matters most on the routes people actually walk: hallways, stairs, and the line between the sofa and the door.

Why thicker is not always better

Thickness tells you the depth of the underlay. It tells you nothing about how much material is packed into that depth, or how well it recovers after being compressed a few thousand times. That is density, and density is what determines how long an underlay keeps doing its job.

A low-density 12mm foam feels wonderful on the day it goes down. In a lightly used bedroom that may be exactly right. On a staircase or in a hallway, it will flatten considerably faster than a firmer, denser product that felt less impressive underfoot in the showroom.

When comparing underlay, look at density, thickness, usage classification, tog rating, sound reduction, and whether it suits stairs and your particular carpet. The best underlay is not the thickest or the dearest. It is the one matched to the room.

Understanding tog ratings

A tog rating measures thermal resistance. The higher the tog, the more the material resists heat passing through it.

Over a cold subfloor, a higher-tog underlay helps hold warmth in the room, which is useful in living rooms, bedrooms and older properties with suspended timber floors.

When the heat is coming from below, the logic reverses entirely.

Underlay and underfloor heating: the mistake to avoid

The common and costly error is fitting a thick, highly insulating underlay in a room with underfloor heating. A high-tog underlay does exactly what it is designed to do: it resists heat transfer. Which means it insulates the heat away from the room it was installed to warm.

Industry guidance puts the practical limit at a combined carpet and underlay thermal resistance of 2.5 tog or below for underfloor heating to work efficiently. Many heating manufacturers specify lower, and some underfloor heating specialists recommend an underlay rated around 0.8 tog or less on its own.

Before ordering, confirm five things:

• The tog rating of the carpet
• The tog rating of the underlay
• The combined figure for both together
• The maximum permitted by your heating manufacturer
• That both carpet and underlay are approved for use over that system

Do not assume a thin underlay is automatically suitable. Use a product specifically rated for underfloor heating.

Types of underlay compared

Type	How it feels	Best for	Watch out for
PU foam	Soft, light, easy to handle	Bedrooms and living rooms	Low-density versions flatten quickly in busy areas
Crumb rubber	Firm and dense	Hallways, stairs, heavy traffic	Heavier to handle, and firmer than some people expect
Sponge rubber	Varies widely by construction	General domestic use, good acoustics	Compare by specification, not by material name
Felt	Firm, with strong thermal insulation	Woven carpets including Wilton and Axminster	Confirm suitability for your carpet and fitting method

Recycled PU foam is the most common domestic choice and comes in a wide range of densities. Crumb rubber, made from granulated recycled rubber, resists compression well and is the sensible specification wherever traffic concentrates.

Which underlay for each room?

Bedrooms

Comfort leads, because traffic is light and people walk barefoot. A good foam underlay gives the softer feel. Density still matters under beds, wardrobes and the route to the door.

Living rooms

A balance of comfort and durability. The underlay carries furniture weight and repeated movement between seating and doorways, so a dense foam, sponge rubber or crumb rubber usually works better than the softest option available.

Hallways and landings

Density and recovery beat softness. Specify something rated for general domestic, luxury or heavy use rather than a light domestic product.

Stairs

Stairs want firm, stable support, and this is where most people get it wrong. A thick, soft underlay compresses around the nosing and makes the carpet feel unsteady where each foot lands. A thinner, denser product is safer and lasts longer. Many light domestic underlays are explicitly not approved for stairs, so check the usage classification rather than assuming. If you are considering a stair runner, have both carpet and underlay measured and specified professionally.

Can you reuse your existing underlay?

Rarely worth it. Old underlay has usually flattened under doorways, seating and furniture, and once compressed it cannot give a new carpet consistent support. Manufacturers generally advise against it, and reusing it may affect performance or warranty cover.

Lift it and check for flattened or crumbling sections, tears, damp, odours and uneven joins. If you cannot establish its age or specification, replace it. While the floor is up, check the carpet grippers too - damaged or badly positioned grippers need replacing before the new carpet goes down.

What does underlay add to a carpet quote?

It depends on the product, the room size and how many rooms you are doing. The important thing is to compare the full installed price rather than the carpet price on its own. Our guide to what a fitted carpet costs in Liverpool breaks down how underlay, accessories, uplift and labour build into a final quotation.

Choosing the right underlay with us

The right underlay depends on your carpet, your room, your subfloor and what you care about most. A soft bedroom needs a different product from a busy staircase. A cold Victorian terrace benefits from higher thermal resistance, while a home with underfloor heating needs a low-tog system that lets the heat through.

We will help you compare thickness, density, tog rating and room suitability before you order, rather than adding whatever is cheapest to the quote. Visit the showroom, browse our underlay range, or ask us for advice.

West Derby Carpets & Blinds Ltd

269 Blackmoor Drive, West Derby, Liverpool L12 3HA

Call 0151 254 2322 to speak to our flooring team.