Section 4: Dwelling-houses and flats formed by material change of use

Introduction

4.1 This Section gives guidance on dwelling-houses and flats formed by material change of use. For rooms for residential purposes formed by material change of use see Section 6.

4.2 It may be that an existing wall, floor or stair in a building that is to undergo a material change of use will achieve the performance standards set out in Section 0: Performance - Table 1a without the need for remedial work. This would be the case if the construction was generally similar (including flanking constructions) to one of the constructions in Sections 2 and 3 (e.g. concerning the mass requirement, the structure under consideration should be within 15% of the mass per unit area of a construction listed in the relevant section).

4.3 In other circumstances it may be possible to use the guidance in Section 2 or 3 (including flanking constructions) to determine the appropriate remedial treatment which will result in the construction achieving the performance standards in Section 0: Performance - Table 1a.

4.4 For situations where it is uncertain whether the existing construction achieves the performance standards set out in Section 0: Performance - Table 1a, this section describes one wall treatment, two floor treatments and one stair treatment as shown in Diagram **4.1. These constructions can be used to increase the sound insulation.

4.5 The guidance in this section is not exhaustive and other designs, materials or products may be used to achieve the performance standards set out in Section 0: Performance - Table 1a. Advice should be sought from the manufacturer or other appropriate source.

4.6 Wall treatment 1 Independent panel(s) with absorbent material
The resistance to airborne sound depends on the form of existing construction, the mass of the independent panel(s), the isolation of the panel(s) and the absorbent material.

4.7 Floor treatment 1 Independent ceiling with absorbent material
The resistance to airborne and impact sound depends on the combined mass of the existing floor and the independent ceiling, the absorbent material, the isolation of the independent ceiling and the airtightness of the whole construction.

4.8 Floor treatment 2 Platform floor with absorbent material
The resistance to airborne and impact sound depends on the total mass of the floor, the effectiveness of the resilient layer and the absorbent material.

4.9 Stair treatment 1 Stair covering and independent ceiling with absorbent material
To be used where a timber stair performs a separating function. The resistance to airborne sound depends mainly on the mass of the stair, the mass and isolation of any independent ceiling and the airtightness of any cupboard or enclosure under the stairs. The stair covering reduces impact sound at source.

4.10 In all cases it may be necessary to control flanking transmission in order to achieve the performance standards set out in Section 0: Performance - Table 1a. See Section 4: Junction requirements for material change of use.

4.11 Special attention needs to be given to situations where flanking walls or floors are continuous across separating walls or floors as a result of the conversion work. In such instances additional treatments may be required to control flanking transmission along these continuous elements. Specialist advice may be needed.

4.12 Significant differences may frequently occur between the construction and layout of each converted unit in a development. Building control bodies should have regard to the guidance in Section 1 when deciding on the application of pre-completion testing to material change of use.

4.13 For some historic buildings undergoing a material change of use, it may not be practical to improve the sound insulation to the performance standards set out in Section 0: Performance - Table 1a. In such cases refer to Section 0: Performance, paragraph 0.7.

4.14 Wall and floor treatments will impose additional loads on the existing structure. The structure should be assessed to ensure that the additional loading can be carried safely, with appropriate strengthening applied where necessary.

4.15 Floor or wall penetrations, such as ducts or pipes, passing through separating elements in conversions can reduce the level of sound insulation. Guidance on the treatment of floor penetrations is given below.

Work to existing construction

4.16 Before a floor treatment is applied appropriate remedial work to the existing construction should be undertaken as described in paragraphs 4.17 and 4.18.

4.17 If the existing floor is timber then gaps in floor boarding should be sealed by overlaying with hardboard or filled with sealant.

a. Where floor boards are to be replaced, boarding should have a minimum thickness of 12mm, and mineral wool (minimum thickness 100mm, minimum density 10kg/m³) should be laid between the joists in the floor cavity

b. If the existing floor is concrete and the mass per unit area of the concrete floor is less than 300kg/m², or is unknown, then the mass of the floor should be increased to at least 300kg/m².

Any air gaps through a concrete floor should be sealed. A regulating screed may also be required.

c. If there is an existing lath and plaster ceiling it should be retained as long as it satisfies Building Regulation Part B – Fire safety.

d. Where the existing ceiling is not lath and plaster it should be upgraded as necessary to provide at least two layers of plasterboard with joints staggered, total mass per unit area 20kg/m².

4.18 Extensive remedial work to reduce flanking transmission may also be necessary to achieve the performance standards set out in Section 0: Performance - Table 1a. This may involve wall linings, see Section 4: Junction requirements for material change of use, paragraphs 4.43 and 4.44.

Corridor walls and doors

4.19 The separating walls described in this section should be used between dwelling-houses, or flats formed by material change of use, and corridors in order to control flanking transmission and to provide the required sound insulation. However, it is likely that the sound insulation will be reduced by the presence of a door.

4.20 Ensure that any door has good perimeter sealing (including the threshold where practical) and a minimum mass per unit area of 25kg/m² or a minimum sound reduction index of 29dB Rw (measured according to BS EN ISO 140-3:1995 and rated according to BS EN ISO 717-1:1997). The door should also satisfy the Requirements of Building Regulation Part B – Fire safety.

4.21 Noisy parts of the building should preferably have a lobby, double door or high performance doorset to contain the noise. Where this is not possible, nearby flats should have similar protection. However, there should be a sufficient number of them that are suitable for disabled access, see Building Regulations Part M - Access and facilities for disabled people.

Diagram 4.1 Treatments for material change of use

Wall treatment 1: independent panel(s) with absorbent material

4.22 The resistance to airborne sound depends on the form of existing construction, the mass of independent panel(s), the isolation of the panel(s) and the absorbent material.

Construction

4.23 The independent panel may be used on one side of the existing wall only where the existing wall is masonry, and has a thickness of at least 100mm and is plastered on both faces. With other types of existing wall the independent panels should be built on both sides.

4.24 Independent panel(s) with absorbent material (see Diagram 4.2)

• minimum mass per unit area of panel (excluding any supporting framework) 20kg/m²;

• each panel should consist of at least two layers of plasterboard with staggered joints;

• if the panels are free-standing they should be at least 35mm from masonry core;

• if the panels are supported on a frame there should be a gap of at least 10mm between the frame and the face of the existing wall;

• mineral wool, minimum density 10kg/m³ and minimum thickness 35mm, in the cavity between the panel and the existing wall.

4.25 Points to watch:

Do

a. Do ensure that the independent panel and its supporting frame are not in contact with the existing wall.

b. Do seal the perimeter of the independent panel with tape or sealant.

Do not

Do not tightly compress the absorbent material as this may bridge the cavity.

Diagram 4.2 Wall treatment 1

Floor treatment 1: independent ceiling with absorbent material

4.26 The resistance to airborne and impact sound depends on the combined mass of the existing floor and the independent ceiling, the absorbent material, the isolation of the independent ceiling and the airtightness of the whole construction.

4.27 Independent ceiling with absorbent material (see Diagram 4.3)

• at least 2 layers of plasterboard with staggered joints, minimum total mass per unit area 20kg/m²;

• an absorbent layer of mineral wool laid on the ceiling, minimum thickness 100mm, minimum density 10kg/m³.

The ceiling should be supported by one of the following methods:

• independent joists fixed only to the surrounding walls. A clearance of at least 25mm should be left between the top of the independent ceiling joists and the underside of the existing floor construction; or

• independent joists fixed to the surrounding walls with additional support provided by resilient hangers attached directly to the existing floor base.

Note This construction involves a separation of at least 125mm between the upper surface of the independent ceiling and the underside of the existing floor construction. However, structural considerations determining the size of ceiling joists will often result in greater separation. Care should be taken at the design stage to ensure that adequate ceiling height is available in all rooms to be treated.

4.28 Where a window head is near to the existing ceiling, the new independent ceiling may be raised to form a pelmet recess. See Diagram 4.4.

4.29 For the junction detail between floor treatment 1 and wall treatment 1, see Diagram 4.5.

4.30 Points to watch:

Do

a. Do remember to apply appropriate remedial work to the existing construction.

b. Do seal the perimeter of the independent ceiling with tape or sealant.

Do not

a. Do not create a rigid or direct connection between the independent ceiling and the floor base.

b. Do not tightly compress the absorbent material as this may bridge the cavity.

Diagram 4.3 Floor treatment 1

Diagram 4.4 Floor treatment 1 - high window head detail

Diagram 4.5 Floor treatment 1 - wall treatment 1

Floor treatment 2: platform floor with absorbent material

4.31 The resistance to airborne and impact sound depends on the total mass of the floor, the effectiveness of the resilient layer and the absorbent material.

Platform floor with absorbent material (see Diagram 4.6)

4.32 Where this treatment is used to improve an existing timber floor, a layer of mineral wool (minimum thickness 100mm, minimum density 10kg/m³) should be laid between the joists in the floor cavity.

The floating layer should be:

• a minimum of two layers of board material;

• minimum total mass per unit area 25kg/m²;

• each layer of minimum thickness 8mm;

• fixed together (e.g. spot bonded or glued/ screwed) with joints staggered.

The floating layer should be laid loose on a resilient layer. The resilient layer specification is:

• mineral wool, minimum thickness 25mm, density 60 to 100kg/m³;

• the mineral wool may be paper faced on the underside.

Note The lower figure of density for the resilient layer gives the best insulation but a 'softer' floor. In such cases additional support can be provided around the perimeter of the floor by using a timber batten with a foam strip along the top attached to the wall.

4.33 For the junction detail between floor treatment 2 and wall treatment 1, see Diagram 4.7.

4.34 Points to watch

Do

a. Do remember to apply appropriate remedial work to the existing construction.

b. Do use the correct density of resilient layer and ensure it can carry the anticipated load.

c. Do allow for movement of materials e.g. expansion of chipboard after laying (to maintain isolation).

d. Do carry the resilient layer up at all room edges to isolate the floating layer from the wall surface.

e. Do leave a small gap (approx. 5mm) between skirting and floating layer and fill with a flexible sealant.

f. Do lay resilient materials in sheets with joints tightly butted and taped.

g. Do seal the perimeter of any new ceiling with tape or sealant.

Do not

Do not bridge between the floating layer and the base or surrounding walls (e.g. with services or fixings that penetrate the resilient layer).

Diagram 4.6 Floor treatment 2

Diagram 4.7 Floor treatment 2 - wall treatment 1

Stair treatment: stair covering and independent ceiling with absorbent material

4.35 Stairs are subject to the same sound insulation requirements as floors where they perform a separating function.

4.36 The resistance to airborne sound depends mainly on the mass of the stair, the mass and isolation of any independent ceiling and the airtightness of any cupboard or enclosure under the stairs. The stair covering reduces impact sound at source.

Stair covering and independent ceiling with absorbent material

4.37 Lay soft covering of at least 6mm thickness over the stair treads. Ensure it is securely fixed (e.g. glued) so it does not become a safety hazard.

If there is a cupboard under all, or part, of the stair:

a. line the underside of the stair within the cupboard with plasterboard of minimum mass per unit area 10kg/m² and an absorbent layer of mineral wool (minimum density 10kg/m³), within
the space above the lining; and

b. build cupboard walls from two layers of plasterboard (or equivalent), each sheet of minimum mass per unit area 10kg/m²; and

c. use a small, heavy, well fitted door for the cupboard

Where there is no cupboard under the stair construct an independent ceiling below the stair (see Floor treatment 1).

4.38 For fire protection where a staircase performs a separating function refer to Building Regulation Part B - Fire safety.

Diagram 4.8 Stair treatment

Junctions with abutting construction

4.39 For floating floors, carry the resilient layer up at all room edges to isolate the floating layer from the wall surface.

4.40 For floating floors, leave a small gap (approx. 5mm) between the skirting and floating layer and fill with a flexible sealant.

4.41 The perimeter of any new ceiling should be sealed with tape or caulked with sealant.

4.42 Relevant junction details are shown in Diagrams 4.5 and 4.7.

Junctions with external or load-bearing walls

4.43 Where there is significant flanking transmission along adjoining walls then improved sound insulation can be achieved by lining all adjoining masonry walls with either

a. an independent layer of plasterboard; or

b. a laminate of plasterboard and mineral wool. For other drylining laminates, seek advice from the manufacturer.

4.44 Where the adjoining masonry wall has a mass per unit area greater than 375kg/m² then such lining may not be necessary, as it may not give a significant

Note Specialist advice may be needed on the diagnosis and control of flanking transmission.

Junctions with floor penetrations

4.45 Piped services (excluding gas pipes) and ducts which pass through separating floors in conversions should be surrounded with sound absorbent material for their full height and enclosed in a duct above and below the floor.

Do

a. Do seal the joint between casings and ceiling with tape or sealant.

b. Do leave a nominal gap (approx. 5mm) between the casing and any floating layer and fill with sealant.

Construction

4.46 Pipes and ducts that penetrate a floor separating habitable rooms in different flats should be enclosed for their full height in each flat.

4.47 The enclosure should be constructed of material having a mass per unit area of at least 15kg/m².

4.48 Either line the enclosure, or wrap the duct or pipe within the enclosure, with 25mm unfaced mineral wool.

4.49 The enclosure may go down to the floor base if floor treatment 2 is used but ensure isolation from the floating layer.

4.50 Penetrations through a separating floor by ducts and pipes should have fire protection to satisfy Building Regulation Part B - Fire safety. Fire stopping should be flexible and also prevent rigid contact between the pipe and floor.

Note There are requirements for ventilation of ducts at each floor where they contain gas pipes. Gas pipes may be contained in a separate ventilated duct or they can remain unducted. Where a gas service is installed, it shall comply with relevant codes and standards to ensure safe and satisfactory operation. See The Gas Safety (Installation and Use) Regulations 1998, SI 1998/2451.

Diagram 4.9 Floor penetrations