Acoustic insulation in residential construction is governed by Building Regulations Approved Document E (2003, amended 2004), which sets minimum sound transmission standards between dwellings. Getting below the targets — or, more precisely, getting above the required isolation values — requires understanding two distinct noise pathways, using the right materials for each, and ensuring workmanship is tight enough not to undermine the system.
Airborne vs Impact Sound
Airborne sound (speech, television, music) travels as pressure waves through the air and then into the structure. Performance is measured in decibels using the DnTw metric (weighted standardised level difference). A higher DnTw means better isolation. Part E requires separating walls and floors between dwellings to achieve a minimum DnTw of 45 dB (new build).
Impact sound (footsteps, dropped objects, furniture dragging) excites the structure directly. It is measured as L’nTw (weighted standardised impact sound pressure level). A lower L’nTw means better isolation — the target is a maximum of 62 dB L’nTw for floors in new dwellings. Impact insulation is, therefore, fundamentally about decoupling floor finishes from the structural deck and preventing vibration from transmitting into the ceiling below.
The two pathways require different solutions, and most real-world floor assemblies need to address both simultaneously.
Approved Document E Performance Targets
| Element | Airborne (DnTw) | Impact (L’nTw) |
|---|---|---|
| Separating floor (new build) | ≥ 45 dB | ≤ 62 dB |
| Separating floor (material change of use / conversion) | ≥ 43 dB | ≤ 64 dB |
| Separating wall (new build) | ≥ 45 dB | — |
| Separating wall (conversion) | ≥ 43 dB | — |
These are field test targets (measured in situ), not laboratory values; site conditions mean field performance typically runs 5–8 dB worse than the same assembly tested in a lab. Robust Details (a pre-tested construction register) offer an alternative to pre-completion testing if used exactly as specified.
Acoustic Mineral Wool
Acoustic-grade mineral wool (glass or rock fibre) has higher density than thermal insulation, typically 40–100 kg/m³, which improves its ability to absorb sound energy. Thermal mineral wool at 10–25 kg/m³ provides minimal acoustic benefit.
For separating walls, acoustic slabs at 50–75 mm between the twin leaf or within a resilient-bar ceiling treatment add meaningful airborne attenuation. For floors, an acoustic slab at 25–50 mm below a floating screed or floating board system addresses both airborne (mass and absorption) and impact (resilience) sound.
Typical products:
- Acoustic mineral wool slab, 50 kg/m³, 50 mm: £4–£7/m² supply
- Acoustic mineral wool slab, 80 kg/m³, 50 mm: £7–£12/m² supply
- Acoustic mineral wool roll (lighter grade, 40 kg/m³): £3–£5/m² supply (used in stud partitions)
Resilient Bars and Ceiling Treatments
For existing timber-joist floors where the structure cannot be altered, resilient bars (sometimes called resilient channels) are the primary tool. The bar is a light-gauge steel section fixed through the joist, with the plasterboard screwed to the bar but not touching the joist. The flexible web of the bar absorbs vibration, breaking the direct acoustic path.
Resilient bars are only effective if:
- Plasterboard screws do not accidentally penetrate through to the joist (short screw principle — use 25 mm screws maximum for 12.5 mm board)
- The perimeter of the ceiling is isolated with acoustic sealant or resilient strip
- No rigid bridging connects the plasterboard to the structure (electrical back-boxes, light fittings, pipes all need isolation)
Double layer 2 × 12.5 mm plasterboard on resilient bars with acoustic mineral wool above in the joist bay is the standard UK system for timber-floor upgrading. A well-executed assembly achieves DnTw 47–50 dB and L’nTw 60–64 dB — comfortably meeting Part E in most cases.
Resilient bar costs: £3.50–£6/m² for the bars + £15–£22/m² for 2-layer plasterboard and installation = total £20–£30/m² installed.
Floating Floor Deck Systems
For impact insulation on concrete or timber structural decks, a floating floor breaks the direct vibration transmission path. The finish floor is supported on a resilient layer rather than bonded directly to the slab.
Common systems:
Floating screed: 65–75 mm sand-cement or anhydrite screed on 25–50 mm acoustic mineral wool or polyurethane foam mat. The screed edges must not contact walls — a perimeter resilient strip (typically 10 mm) prevents flanking. This is the most acoustically effective system and suits ground-floor apartments or heavy-use areas. Additional cost over a solid screed: £12–£20/m² for acoustic mat and perimeter strip.
Floating board system (batten deck): 22 mm tongue-and-groove chipboard or OSB floated on resilient pads or battens on compressible mineral wool. Quicker to install, lower build-up (approx 50–75 mm total), suits refurbishment. Cost: £15–£25/m² installed, including boards and acoustic mat.
Specialist acoustic deck systems (e.g. Fermacell, GreenGlue compound sandwich panels): pre-engineered boards incorporating resilient layers, often 25–35 mm total depth. Typically £25–£40/m² supply only; suitable where floor build-up must be minimised.
| System | Build-up (mm) | Airborne improvement (dB) | Impact improvement (dB) | Supply cost (m²) |
|---|---|---|---|---|
| Acoustic mineral wool slab under screed | 25–50 | +5 to +8 | +8 to +12 | £4–£10 |
| Floating screed on acoustic mat | 75–100 | +8 to +12 | +12 to +18 | £12–£20 (mat only) |
| Floating board on batten/mat | 50–75 | +6 to +10 | +10 to +15 | £10–£18 |
| Specialist acoustic deck panel | 25–35 | +6 to +9 | +8 to +12 | £25–£40 |
Improvement figures are additive to the base structure. Actual site performance depends heavily on flanking transmission and workmanship.
Separating Wall Acoustic Treatments
For masonry separating walls, mass is the primary tool: dense aggregate blockwork at 1,350–2,200 kg/m³ substantially outperforms lightweight aerated block. A plastered 215 mm dense aggregate block wall typically achieves DnTw 53–57 dB on its own — well above Part E.
For new-build twin-leaf masonry with insulation, or for lightweight timber-frame party walls, the approach involves:
- Mass: double layers of 15 mm or 12.5 mm plasterboard (total ≥ 30 mm per face)
- Absorption: 25–50 mm acoustic mineral wool between the leaves
- Decoupling: no rigid ties or fixings crossing the cavity
Timber-frame party walls built exactly to a Robust Detail (e.g. E-WM-1 to E-WM-10) do not require pre-completion testing, which saves £400–£800 per test.
Flanking Transmission
Up to 40% of sound energy in a real building bypasses the primary element and travels via flanking paths — through floor/ceiling junctions, wall ties, services, or structural connections. A high-spec floor assembly can fail the test because of inadequately isolated floor-to-wall junctions. Key flanking mitigation measures:
- Resilient strip under all floating elements and partition bases
- Acoustic sealant at all perimeters and service penetrations
- Separate pipe boxing from structural elements with acoustic backing
- Avoid rigid connections between the party element and flanking elements
Relevant Standards and Guidance
- Building Regulations Approved Document E (2003/2004) — resistance to passage of sound
- BS EN ISO 717-1 — airborne sound rating (Rw, DnTw)
- BS EN ISO 717-2 — impact sound rating (Ln, L’nTw)
- BS EN ISO 10140 — laboratory measurement of acoustic performance
- BS EN ISO 16283 — field measurement in buildings
- Robust Details Handbook (Part E) — pre-tested construction details
- BRE BR 262 — avoiding acoustic failure in thermal/acoustic construction
Acoustic performance is one area where the gap between specification and site delivery is largest. A textbook assembly built with rigid connections or poorly sealed edges will fail the field test. Budget for workmanship quality and pre-completion testing (typically £350–£700 per test pair) alongside the materials themselves.
