Thicker and dearer underlay isn’t automatically better underlay. After fitting thousands of floors, we often tell customers in Bristol that a sensible mid-range underlay is the better choice—and that the thickest premium option can be wasted money or even the wrong product for stairs.

At Seamless Floors, we supply and fit carpet and underlay across Bristol, Bath and surrounding areas. We fit underlay every day, so our advice is based on what works under real carpets, on real stairs and in real homes—not on selling the most expensive roll.

The underlay advice you’ll read everywhere

Most flooring guides tell you to buy the thickest, softest and most expensive underlay you can afford. The usual number is 12mm premium underlay, presented as the safest route to comfort, insulation and a longer-lasting carpet. There is some truth in that advice, but much of it comes from retailers who profit when you buy more. The important question is not “What is the thickest underlay available?” It is “What underlay suits this carpet, this room and this subfloor?” If you are still weighing up carpet options, our guide to choosing the right carpet for your Bristol home explains why the carpet itself often deserves more of the budget.

What most guides tell you vs what we actually tell customers

What most guides say	What we tell customers
Always buy the thickest underlay you can afford.	Buy the right thickness and density for the room.
12mm premium underlay is best everywhere.	Around 10–11mm PU foam is the sweet spot for many bedrooms and lounges, but stairs usually need something firmer and thinner.
Softness is the main sign of quality.	Density, resilience, tog rating and the carpet manufacturer’s specification matter just as much.
Spend more on underlay before considering the carpet grade.	If the budget is limited, a better carpet over a suitable medium underlay is often the more durable choice.

Underlay is not optional. It cushions the carpet, helps with insulation and can reduce sound. The point is to choose the right underlay—not to assume the most expensive one is automatically the right one.

Why we say this

Underlay has three main jobs: cushioning, insulation and sound reduction. The improvement from a cheap, thin foam to a decent 10 or 11mm PU underlay can be dramatic. The improvement from that sensible mid-range product to the thickest option in the brochure is often much harder to notice. There are four practical reasons we do not recommend premium-thick underlay everywhere:

• The benefits flatten out. The first upgrade can transform how a floor feels, because you are moving away from a weak, thin base. After that, extra thickness often brings smaller gains in comfort, insulation and sound reduction.
• Too much give can work against the carpet. If the underlay compresses and flexes excessively, the carpet backing moves more than it was designed to. Over time, that can contribute to tufts loosening and traffic lines appearing sooner.
• Stairs need control rather than maximum softness. A thick, soft underlay can allow the carpet to roll over the stair nosing. The tread can feel spongy and the edge may not hold the clean shape you want underfoot.
• The carpet grade may be a better use of the money. Underlay is usually one of the smaller lines on the quote. Doubling its price may add less value than moving up to a better carpet that will be seen and walked on for years.

Density matters more than thickness

A thick underlay can feel impressive in a showroom because it compresses easily under your hand. That does not automatically make it the best foundation beneath a carpet. A denser medium-thickness underlay supports the carpet more consistently. It resists excessive movement, holds its shape and gives the carpet backing a stable platform. A very soft, low-density product can be thicker but less supportive. This is why we look at density and recovery as well as the number of millimetres. A good 10 or 11mm PU foam underlay can outperform a thicker, cheaper product that collapses under repeated foot traffic. The same principle applies when choosing a carpet. Our guide to carpet types, brands and fitting in Bristol covers the difference between common twist pile and other options, including how they cope with everyday use.

12mm everywhere causes real problems

Twelve millimetres is not inherently bad. It can be a comfortable choice in the right room. The problem is treating it as a universal rule. On stairs, thick soft underlay can make the carpet sit awkwardly over the tread and riser. It may roll around the nosing, feel unstable and make accurate fitting more difficult. For most staircases, we prefer a firmer underlay around 8–9mm, subject to the carpet manufacturer’s instructions. Extra thickness also raises the finished floor level. That can mean:

• doors catch on the carpet;
• cupboard doors need adjusting;
• thresholds no longer meet neatly;
• skirting and door bars sit at an awkward height; and
• transitions between carpet and vinyl, laminate or LVT become more difficult.

A door can sometimes be trimmed, but it is better to identify the clearance issue before fitting begins. A good fitter will check this rather than assume the thickest underlay will fit everywhere. For more detail on the practical fitting process, see our step-by-step carpet laying guide for Bristol homeowners.

The carpet and underlay work as a pair

Carpet and underlay are a system. A decent carpet on a stable, resilient underlay will usually perform better than a budget carpet placed over a very thick but overly soft base. That matters most in hallways, entrances and family rooms. These areas need the carpet to cope with repeated loading, not simply feel soft on day one. A firmer foundation can help the pile recover and reduce the excessive flexing that contributes to premature wear. Always check the carpet manufacturer’s specification before choosing underlay. Brands such as Cormar state recommended or maximum underlay requirements for their products. Going over the stated limit can create fitting problems and may affect the warranty. Manufacturer information is also available through Cormar Carpets.

When thick or premium underlay IS worth it

Our position is not that thick underlay is always wrong. There are homes and rooms where paying more genuinely earns its keep. Suspended timber ground floors are a good example. Victorian and Edwardian terraces in areas such as Bedminster, Southville, Bishopston, Cotham, Redland and Knowle can have cold air moving beneath the floorboards. A higher-tog underlay can make the room noticeably warmer. While the floor is exposed, it is also worth checking that airbricks are clear and that there is no damp or timber problem underneath. Upstairs flats and converted houses may need specialist acoustic underlay. If impact noise travels to the neighbour below, choose a product with a suitable acoustic rating rather than simply selecting the thickest foam. The specification matters more than the marketing language. Bedrooms and lounges are often where premium comfort makes sense. If the purpose of the room is to feel warm and luxurious underfoot, a quality 10–11mm underlay can be money well spent. You notice that comfort every day. Concrete subfloors can benefit from a suitable underlay alongside the correct damp protection. The underlay may help with warmth and comfort, but it cannot solve moisture coming through the slab. The subfloor must be assessed and prepared properly first. We would never recommend skipping underlay altogether or automatically reusing old underlay. Once compressed, contaminated or misshapen, it no longer provides the consistent support a new carpet needs. That saving is usually a false economy. Our carpet underlay guide for Bristol homes explains more about comfort, insulation and the conditions underneath the carpet.

What we actually recommend instead

Buy for the room, not from a thickness chart. We normally consider the room’s traffic, the carpet specification, the subfloor, door clearance and the customer’s budget together.

Room or situation	What we’d use	Why
Stairs and busy hallways	A firmer, denser underlay around 8–9mm	Better shape-holding, safer footing and less risk of the carpet rolling over the nosing
Bedrooms and lounges	Quality 10–11mm PU foam	A strong balance of comfort, support and insulation for most homes
Cold suspended timber floors	An underlay chosen for high tog, with the floor checked	Helps retain warmth; airbricks and the subfloor still need attention
Flats with neighbours below	Underlay with an appropriate acoustic rating	Designed to reduce impact noise rather than simply maximise softness

Then check the carpet maker’s specification. If the manufacturer gives a maximum thickness, that settles the argument on many jobs. Do not choose an underlay that creates a problem with the carpet warranty, doors or thresholds. If there is money left in the budget, we often suggest putting it into the carpet itself or into subfloor preparation. A smooth, sound subfloor and a better carpet can deliver more useful long-term value than moving from a good medium underlay to the most expensive product available.

Do you actually need premium underlay? A quick self-test

Ask yourself these questions before choosing:

• What room is it for? A main bedroom or lounge may justify extra comfort. A stairway needs firmness and control. A little-used spare room may not need the highest specification.
• How much traffic will it receive? Children, pets, shoes and repeated routes through a hallway favour a supportive, resilient underlay rather than a very soft one.
• What is beneath it? Concrete may need appropriate damp protection. A suspended timber floor may benefit from a higher-tog product. Neither situation is solved by thickness alone.
• Will doors and thresholds clear the new floor? Measure before ordering. A 12mm underlay plus carpet can raise the finished level enough to cause avoidable problems.
• What does the carpet manufacturer specify? Check the product data sheet, including maximum underlay thickness and any warranty requirements.
• Where will the next pound work hardest? If the choice is between premium underlay and a better carpet, compare expected wear, comfort and appearance over several years.
• Is the quote suspiciously cheap because the old underlay is being reused? New carpet deserves a clean, stable base. Old underlay may be compressed, contaminated or uneven.

If you answer “comfort and warmth” for a lounge, “cold floor” for a suspended timber room or “noise from upstairs” for a flat, premium underlay may be justified. If the answer is simply “because 12mm must be best,” pause and compare the full specification.

Underlay basics, briefly

Underlay sits between the subfloor and carpet. It cushions footsteps, helps the carpet feel more comfortable, can improve thermal insulation and may reduce airborne or impact sound depending on the product and construction. Common thickness bands are roughly 7–9mm, 10–11mm and 12mm. The thickest band is not automatically the best. Thickness should be considered alongside density, resilience, tog, acoustic performance and the carpet manufacturer’s requirements. PU foam is light, comfortable and widely used in homes. A good dense PU product around 10–11mm suits many bedrooms and living rooms. Crumb rubber is heavier and can offer useful resilience and acoustic performance, although the right product still depends on the room and carpet. Tog indicates thermal resistance. It can be particularly relevant over cold suspended timber floors, but a high tog rating is not the same thing as maximum thickness. Acoustic ratings should be checked separately where neighbour noise is a concern. On stairs, the usual rule is to prioritise a firmer, thinner product—often around 8–9mm—so the carpet can be shaped securely around the tread, riser and nosing. Always follow the carpet manufacturer’s instructions and have the finished floor level checked against doors and adjoining surfaces. For technical fire performance information, including relevant carpet and flooring requirements, consult the British Standards guidance for BS 5808, as well as the manufacturer’s own specification for the carpet and underlay.

Underlay FAQ

Do I need expensive underlay?

Not necessarily. You do need suitable, good-quality underlay, but the most expensive option is not automatically the best. For many homes, a dense 10–11mm PU underlay gives an excellent balance of support, comfort and value.

Is thicker underlay always better?

No. Thick underlay can be worthwhile in lounges, bedrooms, cold rooms or some acoustic applications. Too much thickness or softness can create problems with stairs, carpet stability, doors and thresholds.

What thickness is best for stairs?

A firmer underlay around 8–9mm is a common choice, subject to the carpet manufacturer’s specification. Stairs need the carpet to hold securely around the tread, riser and nosing, so maximum softness is not the goal.

Will thick underlay stop my doors closing?

It can. The carpet and underlay together raise the finished floor height. Check door clearance before fitting, especially where the existing carpet is thin or where the new carpet is particularly deep.

Does underlay double the life of a carpet?

Good underlay can help a carpet perform properly by supporting it and reducing stress on the backing. It cannot guarantee a particular lifespan, though. Carpet quality, traffic, cleaning, fitting and subfloor condition all matter.

Is premium underlay worth it in a rental property?

It depends on the room and expected tenancy. A durable, appropriately dense medium underlay is often a sensible choice. Premium acoustic or high-tog underlay may be worthwhile where noise or cold floors are a genuine issue, but fitting the thickest product everywhere may not give the best return.

Is density more important than thickness?

Often, yes. A dense, resilient medium-thickness underlay can support a carpet better than a thick product that compresses too easily. Look at both measurements, along with tog, acoustic performance and the carpet maker’s requirements.

Can I reuse old underlay?

We generally advise against it. Old underlay may be compressed, uneven, dusty or affected by previous spills. It will not provide the consistent foundation a new carpet needs, and any apparent saving can be lost through premature wear or an uneven finish.

Honest underlay advice in Bristol, Bath and surrounding areas

Seamless Floors gives straight advice on carpet and underlay across Bristol, Bath and surrounding areas. We will tell you when to save, when premium underlay is genuinely worthwhile and when your budget would work harder on the carpet or subfloor. Call 07901751755 to discuss your rooms and flooring requirements, or use our carpet fitting service information to arrange an enquiry. If you would prefer to talk through the options first, call 07901751755—we are happy to recommend the right specification rather than simply the most expensive one.