When a house or commercial building is converted into flats, the walls and floors between the new homes have to meet the sound insulation standards in Approved Document E. Building control will usually ask for pre-completion sound testing, so it pays to get the build-up right before the boards go on.
What the standards ask for
For conversions (a "material change of use"), the performance targets are slightly lower than for new-build flats:
- Airborne sound (voices, TV) through separating walls and floors: at least 43 dB DnT,w + Ctr.
- Impact sound (footsteps) through separating floors: no more than 64 dB L'nT,w.
Purpose-built flats need 45 dB and 62 dB respectively. For airborne sound a higher figure is better; for impact sound a lower figure is better.
The tests measure the finished building, so workmanship and flanking paths (sound travelling around a wall or floor by another route) matter as much as the products.
Upgrading a timber separating floor
Most conversions keep the existing timber joists between flats. A typical upgrade adds absorption, decoupling and mass:
- Absorb: fill the joist void with acoustic partition roll or a higher-density mineral wool slab. At least 100mm is a common minimum.
- Decouple the ceiling: fix resilient bars across the underside of the joists so the ceiling isn't rigidly connected to the floor above, or hang a new ceiling on an MF ceiling system.
- Add mass: use two layers of acoustic plasterboard with staggered joints on the ceiling below.
- Reduce footstep noise at source: lay an acoustic underlay or resilient layer on the floor deck. See our acoustic layers.
Building separating walls
For new separating walls, the best results come from two independent frames with a gap between them, so there's no solid path for sound. A common approach is twin metal stud or timber frames, acoustic roll in both cavities, and two layers of sound board on each outer face.
Where an existing masonry wall becomes a separating wall, an independent lining can lift its performance without rebuilding it. That means a free-standing stud, not fixed to the wall, filled with acoustic roll and faced with sound board.
Watch the details
- Seal every perimeter gap with acoustic sealant. A small gap lets a lot of sound through.
- Use acoustic putty pads or rated back boxes behind sockets and switches on separating walls.
- Don't run services straight through separating walls or floors without sealing around them.
- Take separating walls right up through ceiling and loft voids. Flanking through these spaces is a common reason for failed tests.
Fire as well as sound
Separating walls and floors between flats also need fire resistance. Many acoustic plasterboards are fire-rated too, but check the datasheet, or combine them with fire-rated plasterboard where the system calls for it. Our HMO fire separation guide covers fire boards in more detail.
Typical materials list
- Acoustic partition roll or mineral wool slab
- Resilient bars or MF ceiling components
- Acoustic (sound) plasterboard for two-layer linings
- Metal stud and track for new walls
- Acoustic underlay for floors
- Acoustic sealant, tape and fixings
Flat conversions near you
Flat conversions are common in London's Victorian terraces, Manchester's former mills and large period houses in Liverpool. See those pages for local delivery details.
Send your acoustic specification or drawings to sales@nextdayinsulationltd.co.uk and we'll quote the full package.