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70–100mm Stone Wool: Part E Acoustic Insulation for England Extensions

Practical England guide to Approved Document E: buildable acoustic assemblies, testing requirements, and managed build options.

The Extension Works 2 October 2026
70–100mm Stone Wool: Part E Acoustic Insulation for England Extensions

70–100mm Stone Wool: Part E Acoustic Insulation for England Extensions

Stone wool fitted inside extension wall

For most single-storey rear extensions in England, a stone wool slab in a decoupled lining or floating floor construction, combined with sealed double linings, delivers the sound performance homeowners need and keeps the build compliant with Part E. Compliance runs through Approved Document E, which applies wherever separating elements exist between dwellings, and often calls for testing by a UKAS-accredited body before building control signs off the work.

TL;DR:

  • Single-storey rear extension soundproofing often relies on stone wool slabs combined with sealed double linings to meet Part E standards without extensive testing.
  • For separating walls and floors, 70mm stone wool slabs in timber frames and 100mm slabs between joists are typical configurations that meet resistance targets.
  • Internal partitions usually require 50 to 70mm mineral wool and a single layer of acoustic plasterboard to achieve around Rw 40 dB performance.
  • Pre-completion tests by UKAS-accredited bodies are mandatory for separating structures, and proper installation sealing is critical to meet specified performance.
  • Coordinating acoustic design early in the build process, rather than as an afterthought, reduces risks of failure and saves time and costs.
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Table of Contents

Which acoustic insulation materials suit an extension

Most extensions need a mix of products rather than a single fix, and the right combination depends on whether you are building a separating wall, an internal partition, or a floor.

  • Stone wool slabs such as ROCKWOOL Sound Insulation Slab are the usual choice for separating elements because they combine sound absorption with Euroclass A1 fire performance, which matters where a new extension sits close to a boundary or another dwelling.
  • Mineral and glass wool rolls, including Knauf Acoustic Roll, friction-fit neatly into stud partitions and joist voids, making them the practical option for internal walls and floors rather than separating structures.
  • Acoustic plasterboard and layered linings add mass without eating into floor area, which suits extensions where every centimetre of internal space has already been accounted for in the design.
  • Resilient bars, floating floors and acoustic sealants are the adjuncts that make the main insulation work: they stop rigid contact between layers and close the small gaps that otherwise let sound leak straight through.

None of these products performs to its datasheet figures on its own, and for practical guidance on reducing noise transmission in your home, see practical fixes for homeowners and renters. The assembly around it, the frame, the linings, the seals, decides whether the finished wall or floor meets the target.

How Part E applies to extensions: E1, E2 and E3 explained

Part E splits into three requirements, and only some of them touch a typical single-storey rear extension.

  • E1 governs separating walls and floors between different dwellings or between a dwelling and another building. If your extension shares a party wall or ceiling with a neighbour’s space, the stricter Table 1 laboratory values in Approved Document E apply, and this is the requirement most likely to trigger formal testing.
  • E2 covers internal walls and floors within the same dwelling, such as a partition between a new kitchen extension and an existing hallway. Local authority guidance, including a Lancaster Council leaflet summarising the same tables, points to a laboratory target of around Rw 40 dB for these internal elements.
  • E3 deals with reverberation in common areas of purpose-built flats and similar buildings. It rarely applies to a single dwelling extension, so most homeowners can set it aside.

Because a straightforward rear extension to a detached or semi-detached house often has no separating wall or floor with another dwelling, E1 may not apply at all. Where it might, for example an extension that wraps around a party wall, raise it with building control at the design stage rather than waiting for site inspection.

Build assemblies and thicknesses that meet Part E in practice

Manufacturer system sheets translate the Approved Document E tables into buildable constructions, and the examples below reflect the kind of specification a contractor would draw up for a typical rear extension.

  1. Separating timber frame wall: twin stud frames with no shared noggins, 70mm stone wool slab in each frame cavity, and two layers of gypsum board on each face. ROCKWOOL’s datasheet presents this as a standard system designed to meet Approved Document E separating wall values.
  2. Separating timber floor: a floating floor layer over a resilient RockFloor underlay, 100mm stone wool slab fitted between the joists, and double plasterboard linings on the ceiling below. The same datasheet gives this as the reference construction for separating floors between a ground and first-floor extension.
  3. Internal partition or mid-floor for E2: a single stud partition with 50 to 70mm mineral wool and one layer of acoustic plasterboard each side typically reaches the Rw 40 dB target referenced in local building-control guidance.
  4. Retrofit or change-of-use floors: where an existing floor needs upgrading rather than rebuilding, a platform floor with an added resilient layer and a second ceiling lining below often closes the gap to the required performance without disturbing the joists themselves.

RockFloor resilient layers typically run within a moderate thickness range suitable for acoustic performance as per manufacturer guidance, and ROCKWOOL’s product guidance pairs this range with specific slab thicknesses depending on the floor build-up, which is worth checking against your own joist depth before ordering materials. Structural depth matters here too. If your extension needs steel beams to open up the space, coordinate the structural design with the floor build-up early, since resilient layers and services often compete for the same zone.

Pre-completion testing: when it happens and who carries it out

Testing is the point where a good specification either proves itself or reveals a weak junction, so it pays to plan for it rather than treat it as a formality.

  • Building control will normally require pre-completion sound testing on separating walls and floors between dwellings, unless the design uses Robust Details, a pre-approved construction that exempts the project from individual testing.
  • Tests must be carried out by a UKAS-accredited testing body, and accreditation matters because it is what building control relies on to accept the result without question.
  • Approved Document E notes that testing between a living space and a circulation space, such as a hallway or stairwell, is treated differently and is often excluded from the standard test regime.
  • Build in a contingency window. If a test fails, remedial work such as re-sealing junctions or adding a lining takes time, and chasing a failed result against a fixed completion date creates unnecessary pressure.

Practical installation tips and common pitfalls to avoid

Datasheet performance only shows up on site when the installation matches the drawing, and small errors are the usual cause of underperformance.

  • Seal every gap and service penetration with acoustic sealant. It is inexpensive compared with the cost of a failed test.
  • Fill cavities completely with insulation. Compressed or gapped mineral wool loses much of its rated performance.
  • Keep decoupled layers decoupled. A single rigid fixing bridging an isolator can undo the benefit of the whole system.
  • Track flanking paths, the ceiling voids, ducts and loft hatches that carry sound around a wall rather than through it, and coordinate finishes so they do not reopen a sealed junction.
  • Schedule acoustic works before final finishes go on, and leave room in the programme for testing before plasterboard and decoration make remedial access difficult.

Pro Tip: Fit acoustic sealant and check for gaps before the second lining goes on, since that is the last point where a junction is easy to reach.

Why this matters when you’re planning the build

Why this matters when you're planning the build — overview diagram

Part E compliance is often treated as a paperwork exercise that gets sorted out once the shell is up, but that approach causes most of the failures I come across. The performance figures in Approved Document E are laboratory values, achieved under controlled conditions with a specific, unbroken assembly. Every gap, every shortcut on decoupling, every skirting board nailed straight through a resilient layer chips away at that number before a tester ever arrives on site.

The homeowners who avoid trouble are the ones who treat acoustic detailing as part of the structural and thermal design from day one, not as an afterthought bolted on before completion. At The Extension Works, our in-house design and build process for single-storey rear extensions folds acoustic detailing into the same drawings as the structural calculations and building control liaison, so nothing gets specified in isolation.

— Esskay

Getting a compliant, well-insulated extension without managing it yourself

Coordinating stone wool specifications, Part E testing and structural design across separate contractors is where a lot of extension projects lose time and money. The Extension Works handles design, planning, building control liaison and construction as one in-house service, so acoustic detailing gets specified alongside the structural drawings rather than argued over on site.

The Extension Works

  • Get an instant price estimate through our online quote tool before committing to anything.
  • See the build-up of a typical project on our single-storey rear extension page, including live 3D modelling of your own layout.
  • Every build carries a 12-month workmanship warranty, and the £499 booking fee secures your project slot with a dedicated team managing design through to sign-off.

Start with the quote tool to see what a compliant, acoustically sound rear extension would cost for your home.

Where to check the technical detail yourself

Where to check the technical detail yourself — overview diagram

For the regulatory position, read Approved Document E directly. For construction detail, check ROCKWOOL’s and Knauf’s system sheets, alongside your local authority’s building control guidance.

Sources

FAQ

Part E of the Building Regulations, set out in Approved Document E, sets the legal performance requirements for resistance to the passage of sound in England. It applies mainly to separating walls and floors between dwellings, with different but related targets for internal walls and floors within the same home.

What is the best acoustic insulation in the UK?

There is no single best product. Stone wool slabs such as ROCKWOOL Sound Insulation Slab suit separating walls and floors because of their combined fire and acoustic performance, while mineral wool rolls like Knauf Acoustic Roll work well in internal partitions.

How thick should acoustic insulation be?

Thickness depends on the element and the target. ROCKWOOL’s datasheet sets out 70mm slabs for separating timber frame walls and 100mm slabs between joists for separating floors, while internal partitions built to the Rw 40 dB target referenced in local building-control guidance typically use 50 to 70mm.

Is acoustic insulation any good?

Acoustic insulation works when it is specified correctly and installed without gaps, compression or rigid bridging across decoupled layers. Poor installation, rather than poor materials, is the usual reason a finished wall or floor underperforms its datasheet figures.