Sound insulation between flats and rooms: voices, footsteps and pipes, and why a strong wall alone is not enough

Noise from neighbours is one of the most common complaints in flats and apartment buildings. Many products sold as "sound insulation" do not address the real path the sound takes. This guide explains how sound travels, what the code requires, and what has been shown in tests to make a real difference.

1) Two kinds of sound

Source: the 2021 International Building Code12. The higher the number, the better the insulation.

2) What does the code require?

• Airborne sound: walls, partitions and floor-ceilings separating dwelling units from each other or from public or service areas (such as corridors): STC of at least 50 in the lab, or NNIC of at least 45 if field tested1.
• Impact sound: floor-ceilings between dwelling units: IIC of at least 50 in the lab, or NISR of at least 45 in the field2.
• Penetrations: openings for pipes, electrical boxes, recessed cabinets, bathtubs and HVAC or exhaust ducts must be sealed, lined or insulated so the required rating is maintained1.
• Entrance doors: exempt from the number, but must fit tightly to the frame and sill1.

3) Why does sound get through when the wall is good?

Sound does not only cross the separating wall; it shakes the floor, ceiling and adjoining walls, and the vibration travels through the junctions into the next room. This is flanking transmission, and the result the occupant actually hears is rated ASTC4.

In National Research Council Canada tests on wood-frame buildings4:

• A wall rated STC 52 in the lab gave only ASTC 44–45 between the rooms, because the floor carried more sound than the wall itself.
• Adding a layer of gypsum board raised the wall to STC 57, but the real result only reached ASTC 43, because the main path (the floor) was not treated.
• Adding a topping over the floor raised the result to ASTC 50 (with a panel layer) and 56 (with gypsum concrete over a resilient mat).
• Fixing flanking after construction is much more expensive than addressing it in the design.

4) Block walls

From the National Research Council Canada guide on concrete block walls5:

• Weight matters most: the higher the mass per square metre, the better. A single-leaf block wall gives about STC 45–55 when its surface is sealed with paint or plaster.
• Porous (lightweight) block: sealing the surface with plaster or block sealer improves insulation significantly; gains of 5 to 10 STC points or more are not uncommon for some lightweight blocks.
• Joints: mortar joints must be complete with no gaps; a gap in a joint leaks sound.
• Gypsum board glued directly to the block does not seal it as well as plaster or paint.
• Filling block cores with absorbent material does not help; sound passes through the block body. Filling with sand or grout only adds weight.

Representative values for 50% solid blocks sealed on at least one side, from the same guide5. Note that thickness alone raises the figure only a little.

5) Adding gypsum board: how to make it work

The effective way to upgrade an existing block wall is to add gypsum board not rigidly connected to the block, with a cavity and absorbent material5:

1. Support: best is independent studs that do not touch the block, or resilient metal channels. Rigid furring carries sound straight through.
2. Cavity depth: the deeper the better. A shallow cavity can reduce insulation instead of improving it.
3. Absorbent material in the cavity: fibres such as rock wool, glass fibre or cellulose. Closed-cell foam such as polystyrene does not absorb much sound.

In lightweight-wall research, mounting gypsum board on resilient channels increased the attenuation of a flanking path by at least 10 dB, while adding a second directly fixed layer added only 1 to 2 dB4.

6) Double walls: excellent on paper, sensitive in execution

Two separate leaves with a cavity can give very high insulation, but any bridge between them ruins it. In one test a double wall expected to exceed STC 70 gave only STC 60 because mortar droppings in the cavity connected the two leaves; such errors are hidden inside the wall and cannot be fixed after construction5.

Practical advice: ask the contractor to keep the cavity clean during construction, and photograph it before closing. See the handover checklist.

7) Footsteps from the flat above

• The code requires IIC 50 for floor-ceilings between units2.
• In NRC research, the most effective improvement between units one above the other was adding layers over the floor, best a heavy topping over a resilient mat; improving the ceiling alone without treating the walls gave limited benefit4.

Practical advice: carpet and resilient underlays reduce footstep noise. Tiles bonded directly to the concrete slab with no underlay transmit impact noise easily. In a new building this is decided at design stage, because a floating floor raises the floor level.

8) Pipes, AC units and doors

• The code requires openings for pipes and HVAC or exhaust ducts through a separating wall or floor to be sealed, lined or insulated1.
• The entrance door must fit tightly to the frame and sill1.

Practical advice (not regulatory text): support drain pipes with rubber-lined hangers, avoid running them inside a bedroom wall where possible, put the AC outdoor unit on anti-vibration mounts, and seal around back-to-back electrical boxes on either side of a separating wall.

9) New or existing building?

10) Before you buy "soundproofing"

1. Ask for a test report for the complete assembly (the wall or floor with all its layers) to ASTM E90 or E492, not a figure for a single material.
2. Note: foam and decorative absorbent panels reduce echo inside the room; they do not stop sound reaching the neighbours.
3. There is no "total" soundproofing; any promise of complete silence is an exaggeration.
4. For sensitive projects (plant rooms, studios, hotels) ask for an acoustic engineer and field measurement.

See also: gypsum board · blocks · insulation materials compared.