Acoustic Underlay and Soundproof Flooring: A Trade Guide

Summary

This guide is written for the tradesperson pricing and fitting a soundproofing-motivated flooring job — most commonly a domestic customer who's had enough of hearing footsteps, TV, or a home gym above them, or who's converting a room into a home cinema, music room, or nursery next to a noisy area. It deliberately sits alongside, rather than repeats, acoustic underlay selection, which is the reference for Part E compliance certification, ΔLw/L'nT,w test data, and new-build/conversion pre-completion testing — read that article when the job is a genuine legal separating-floor obligation (a flat conversion, an HMO, a new-build). This article is for the far more common domestic scenario: a customer who wants their floor quieter and isn't necessarily building a legally separating floor at all.

The single biggest source of customer disappointment in this line of work is a mismatch between what "soundproofing" sounds like it promises and what a retrofit underlay can actually deliver. No underlay, however good, will silence footsteps from the room above to inaudibility — physics doesn't allow it without a full floating floor, mass, and decoupling system, which is a different scale of job entirely. A well-specified acoustic underlay under a sensible floor finish will make a noticeable, worthwhile difference — commonly described by customers as "the difference between hearing every step and hearing occasional movement" — but managing that expectation at the quoting stage prevents a comeback complaint once the job is finished and the customer is disappointed it isn't silent.

The other confusion worth clearing up early with any customer: soundproofing has two distinct problems — airborne sound (voices, music, TV, measured via DnT,w) and impact sound (footfall, furniture scraping, measured via L'nT,w). Underlay under a floor finish addresses impact sound at the source (the floor that's being walked on) far more effectively than it addresses airborne sound travelling between rooms, which is much more dependent on the mass and construction of the floor/ceiling structure itself and the walls around it. A customer asking to "soundproof the floor" because they can hear the TV from downstairs, rather than footsteps from above, may actually need ceiling or floor mass/construction work, not underlay — establishing which noise problem is actually being solved is the first job at survey.

Key Facts

• Two noise types — impact sound (footfall, objects dropped, furniture moved) travels through the floor structure itself and is what acoustic underlay primarily addresses; airborne sound (voices, music, TV) travels through both floor mass and flanking paths (walls, ceiling voids) and is much less influenced by underlay alone.
• Building Regulations Part E applies to separating floors only — floors between different dwellings, or between a dwelling and a common area (stairwell, corridor) in a converted building. It does not apply to floors within a single house (e.g. between a lounge and an upstairs bedroom in the same dwelling).
• New build separating floor target — L'nT,w ≤62dB (impact) and DnT,w+Ctr ≥45dB (airborne).
• Conversion separating floor target — L'nT,w ≤64dB (impact) and DnT,w+Ctr ≥43dB (airborne) — 2dB more lenient than new build, reflecting the practical limits of upgrading existing structures.
• Robust Details scheme — a registered, pre-tested set of floor/wall constructions that can be used instead of individual pre-completion testing on new-build separating floors, provided the exact specified build-up is followed without substitution.
• Realistic domestic retrofit expectation — a good acoustic underlay under carpet, LVT, or laminate typically delivers a perceived improvement equivalent to roughly 10-20dB, described by most customers as "significantly better" rather than "silent" — set this expectation before quoting, not after fitting.
• Underlay does not fix airborne sound alone — voices and music travelling through a floor/ceiling structure need mass (additional plasterboard layers, resilient bars, mineral wool in the void) far more than they need a thicker floor underlay; underlay alone is not the right answer to "I can hear the TV downstairs."
• Underlay thickness is not a reliable proxy for performance — a thicker underlay is not automatically better; a dense, correctly-specified thin product (e.g. 5mm rubber crumb) frequently outperforms a thick, low-density foam. Always specify against tested ΔLw/system data, not thickness alone.
• Common domestic underlay materials — rubber crumb (recycled tyre, dense, good all-round performance), cork (natural, good dimensional stability, mid-range cost), PU foam composite (budget, adequate under carpet, weaker under hard flooring), fibre/wool composite (mid-range, good under engineered timber and laminate), combined foam + mass-loaded layer (premium, best performance for hard flooring).
• Existing timber joist floors — the single most cost-effective retrofit soundproofing upgrade in an existing house is often mineral wool insulation in the joist void plus resilient bars/hangers to the ceiling below, combined with acoustic underlay above — treating both sides of the floor structure delivers a far bigger improvement than underlay alone.
• Skirting and threshold detailing matter as much as the underlay itself — a correctly specified underlay installed with the flooring tight against the skirting, or with a hard threshold strip bridging the acoustic gap, loses much of its benefit; a 1-2mm gap at the perimeter (sealed with flexible sealant, not filled solid) preserves the acoustic decoupling.
• Robust Details vs pre-completion testing — Robust Details registration must happen before construction starts and requires the exact registered build-up (no substitutions); pre-completion testing is more flexible on design but requires a UKAS-accredited test after completion, with remedial work and re-testing if it fails.
• Underfloor heating conflicts — acoustic underlay's insulating properties can reduce UFH heat output to the room; heat-spreader plates (for timber) or in-screed UFH (heating pipes above the acoustic layer, not sandwiched by it) avoid this conflict.
• VAT and grant funding — soundproofing work in a single dwelling for comfort reasons doesn't qualify for any specific VAT relief or grant; where it's part of a wider disability adaptation (e.g. reducing noise sensitivity as part of an autism-related home adaptation), it may occasionally fall within a wider DFG-funded scope, assessed case by case by an Occupational Therapist.

Quick Reference Table

Detailed Guidance

Establishing which noise problem the customer actually has

Before specifying anything, separate the complaint into impact or airborne, because the fix is different:

• "I can hear them walking around" — impact noise. Acoustic underlay under the floor finish upstairs is the right first response, and is something the customer occupying that floor can commission themselves without needing the downstairs occupant's floor/ceiling touched.
• "I can hear their TV/music/voices" — airborne noise. This needs mass and decoupling in the floor/ceiling structure itself (extra plasterboard layers on resilient bars, mineral wool in the void) far more than it needs underlay. If the customer only wants underlay fitted and the actual complaint is airborne noise, be clear at quoting stage that underlay alone will make limited difference to that specific complaint.
• "Both" — genuinely warrants treating both sides of the floor structure: joist void insulation and ceiling treatment below, acoustic underlay and finish above. This is a bigger job and should be quoted and explained as such.

Material comparison for domestic retrofit work

Prices vary by supplier and specification; always check the specific product's tested ΔLw or system data rather than choosing on price or thickness alone, particularly where the customer has an expectation of a specific noise-reduction outcome.

The retrofit floor build-up (existing house, no structural access)

Where the customer wants improvement without lifting the ceiling below (the most common domestic scenario — the downstairs occupant doesn't want their ceiling disturbed, or it's the same household but budget doesn't extend to joist-void work):

Existing floor finish removed
  |
Existing subfloor (boards/chipboard) — check condition, screw down
any loose boards (a loose board is itself a major noise source,
independent of the underlay)
  |
Acoustic underlay (selected per material table above, matched to
the new floor finish)
  |
New floor finish (carpet, LVT, laminate, engineered timber)
  |
Perimeter gap (1-2mm) at skirting, sealed with flexible sealant —
NOT filled solid, NOT bridged by a hard threshold trim

This is a genuinely worthwhile upgrade and the right scope for most domestic soundproofing enquiries, but it should be sold to the customer as "noticeably quieter," not "silent" — the floor structure itself (joists, subfloor) remains untreated and continues to transmit some impact energy regardless of what sits on top of it.

The fuller retrofit (access to the ceiling void below)

Where the downstairs ceiling can also be worked on — either because it's being replastered anyway, or the customer is committed to a bigger soundproofing budget:

1. Remove existing ceiling below (or access via loft/void if the floor is a ground/first floor separation with an accessible void)
2. Add mineral wool insulation into the joist void (100mm minimum, denser acoustic-grade wool performs better than standard loft insulation wool)
3. Fit resilient bars or independent ceiling system to decouple the new ceiling from the joists — this is the single highest-impact addition for airborne noise reduction
4. Fit two layers of acoustic plasterboard (e.g. 15mm sound-rated board) to the resilient bars
5. Above, fit acoustic underlay and new floor finish as per the retrofit build-up above

This combined approach — treating both the ceiling below and the floor above — delivers a materially bigger improvement than either alone, and is the appropriate answer when a customer's complaint includes airborne noise (voices, TV) as well as impact noise.

Setting expectations at the quoting stage

This is worth treating as a formal part of the quote, not an afterthought conversation on the day:

• Explain the difference between impact and airborne noise, and which the proposed work addresses
• Give a realistic, non-numeric description of expected outcome ("noticeably quieter, particularly for footsteps and dropped objects; voices and TV noise will improve less unless we also treat the ceiling below")
• Avoid quoting a specific dB figure to a domestic customer for a non-regulated retrofit job unless a proper acoustic survey and tested system has been specified — a dB number implies a guaranteed, measurable outcome that a retrofit underlay-only job cannot reliably promise
• Where the customer's real complaint is airborne noise and the budget only extends to underlay, say so clearly rather than accepting the job and risking a comeback ("I paid for soundproofing and I can still hear the TV")

When to bring in a specialist acoustic consultant

For anything beyond a domestic underlay-and-finish job — a home cinema build wanting a genuine low-noise-floor rating, a commercial change-of-use where Part E compliance is a hard legal requirement, or a dispute situation where a completed floor has failed a neighbour noise complaint and needs remedial work with guaranteed performance — bringing in a specialist acoustic consultant to specify and, ideally, commission post-completion testing is the right call. This protects the tradesperson from being held to an outcome (a specific dB figure) that wasn't independently verified, and gives the customer the certainty a specialist survey provides.

Frequently Asked Questions

Do I legally need acoustic underlay in my house?

Only if the floor in question is a separating floor under Building Regulations Part E — meaning it separates your home from a different dwelling or from a shared common area (as in a converted house split into flats). Within a single house, from one floor to another, there is no legal requirement — it's entirely a comfort and lifestyle choice.

Will acoustic underlay stop me hearing my upstairs neighbour's TV?

Underlay fitted on your neighbour's floor above will meaningfully reduce impact noise (footsteps, dropped items) transmitted into your ceiling, but has much less effect on airborne noise like TV or music, which travels through the floor/ceiling structure's mass and any flanking paths (walls, service risers) more than through the floor covering. If TV/voice noise is the main complaint, ceiling treatment (mass and decoupling) on your side is usually more effective than underlay on theirs.

What's a realistic budget for retrofit soundproofing on one room's floor?

For underlay-and-finish-only work on a typical bedroom (12-15m²), expect materials plus labour broadly in line with a standard flooring job plus a premium for the acoustic underlay itself — the underlay/system cost is the main additional line item over a standard re-floor, typically adding a modest amount per m² depending on the material chosen from the comparison table above. Adding ceiling-void treatment below roughly doubles the scope and cost, since it's effectively two separate jobs (ceiling plus floor) rather than one.

Is Robust Details cheaper than pre-completion testing for a new-build project?

Often yes, because Robust Details avoids the cost and programme risk of a failed test and re-test cycle — but it requires committing to a specific registered build-up before construction starts, with zero tolerance for substitution. Pre-completion testing gives more design flexibility but carries the financial and programme risk of a failed test requiring remedial work and re-testing.

Can I just add a thicker underlay if the first one didn't work well enough?

Not reliably — thickness alone is a poor predictor of acoustic performance, and simply adding a second layer of underlay under a floor finish can create instability, trip hazards at doorways, and doesn't address flanking transmission or ceiling-side issues that are often the real limiting factor. Diagnose why the current build-up underperforms (loose subfloor boards, missing perimeter isolation, hard threshold bridging, or genuinely needing ceiling-side treatment) before assuming "more underlay" is the fix.

Regulations & Standards

• Building Regulations Part E (Resistance to the passage of sound) — mandatory for separating floors/walls between dwellings or between a dwelling and a common area; not applicable within a single dwelling

• Approved Document E — statutory guidance on achieving Part E compliance, including target dB values and testing regime

• BS EN ISO 717-2 — single-number rating method for impact sound insulation (the "w" in L'nT,w and DnT,w)

• BS EN ISO 10140-3:2021 — laboratory measurement of impact sound reduction by floor coverings

• BS EN ISO 16283-2 — field measurement of impact sound insulation in completed buildings

• Robust Details Handbook — pre-tested, registered floor/wall constructions offered as an alternative to individual pre-completion testing

• NHBC Standards Chapter 6.1 — separating walls and floors guidance for residential new build

• GOV.UK — Approved Document E: Resistance to sound — statutory guidance and target values

• Robust Details Ltd — registered separating floor/wall constructions

• Association of Noise Consultants — UK acoustics industry guidance and consultant directory

• BRE Acoustics Centre — UK acoustic research, testing, and technical guidance

• acoustic underlay selection — Part E compliance detail, ΔLw/L'nT,w test data, pre-completion testing, and floating floor system build-ups for regulated separating floors

• part e sound — full Part E overview for separating walls and floors

• building regs part e acoustic — Part E acoustic compliance from the dry lining/ceiling perspective

• acoustic ceiling systems — resilient bar and independent ceiling systems for airborne noise reduction

• subfloor preparation guide — subfloor condition checks that affect acoustic performance regardless of underlay choice