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Separating walls for new or existing buildings – timber frame

Since July 2003 it has been a requirement, under Part E of The Building Regulations 2000, that houses, flats, and rooms for residential purposes are designed and constructed to provide reasonable resistance to sound from other parts of the same building and from adjoining buildings.

This application note gives examples of partition wall constructions which, when built correctly, will achieve the performance standards set out in Approved Document E, ‘Resistance to the passage of sound’. For other construction elements please refer to Thermafleece application notes for separating floors, internal walls and internal floors.

Performance

The minimum value for airborne sound insulation of a separating wall is DnT,w+Ctr = 45dB.

DnT,w+Ctr is a single value figure that is calculated by measuring a test noise in one room, and comparing that to the noise measured through the separating wall. A correction factor is added for low frequency content. This means the separating wall must reduce sound by at least 45dB (although the complexities of the actual calculation take into account other factors in the overall sound insulation performance). A sound reduction of 45dB is reasonable for everyday living, but in some circumstances a higher figure, meaning a greater sound reduction, would be desirable.

Pre-completion testing is carried out on separating walls between habitable rooms.

Building a separating wall with independent timber frames

Approved Document E gives construction guidance on “Wall Type 4: Framed walls with absorbent material”. For further information please refer directly to Approved Document E - Resistance to the passage of sound.

The resistance to airborne sound in this construction depends on the mass per unit area of the leaves, the isolation of the frames, and the Thermafleece in the cavity between the frames.

Construction guidance

The construction consists of independent timber frames, with plasterboard linings on room surfaces, and with Thermafleece fixed to one frame.
• The minimum distance between inside lining faces should be no less than 200mm
• Plywood sheathing may be used in the wall for structural reasons such as to support wall cupboards
• Each lining should be two or more layers of plasterboard, each sheet of minimum mass per unit area 10 kg/m2, with staggered joints. Suitable plasterboards are 12.5mm British Gypsum Wallboard TEN or 15mm British Gypsum Wallboard. Standard 12.5mm (half inch) wallboard is not suitable for this construction. Denser plasterboards such as Soundbloc will give improved sound insulation
• Thermafleece can be friction fit between timbers on one frame. Staples may be used to ensure Thermafleece is secured in place and will not fall to the bottom of the cavity

Plasterboard type	Lining thickness mm	Thermafleece thickness mm	Wall thickness mm	Fire resistance mins	Sound Insulation Rw + Ctr dB
Wallboard TEN	2 x 12.5	50	250	60	49
Soundbloc	2 x 12.5	50	250	60	52
Soundbloc	2 x 15	100	260	90	55

Further improvements in sound insulation are possible if the cavity is increased to 240mm or greater.

Points to watch

Do
1. Ensure that where fire stops are needed in the cavity between frames that they are either flexible or fixed to only one frame
2. Stagger the position of sockets on opposite sides of the separating wall, and use a similar thickness of cladding behind the socket box
3. Ensure that each layer of plasterboard is independently fixed to the stud frame
4. Control flanking transmission from walls and floors connected to the separating wall

Do not
1. Where it is necessary to connect the two leaves together for structural reasons, do not use ties of greater cross section than 40mm x 3mm fixed to the studwork at or just below ceiling level and do not set them at closer than 1.2m centres
2. Do not locate sockets back to back. A minimum edge to edge stagger of 150mm is recommended
3. Do not chase plasterboard

Good design and construction are essential
Using a good design and applying the correct construction techniques are essential in achieving a high level of sound insulation.

Flanking sound

The design should take into account the effects of flanking sound transmission. This is because sound does not only go through the separating wall, but can also travel along other building elements such as the side walls, ceiling or floor, that are shared by adjacent rooms. Flanking transmission can exceed direct transmission through the separating wall which may result in the required sound insulation not be met.

How does Thermafleece improve sound insulation?

When you fit Thermafleece in a separating wall it improves sound insulation in three ways:
1. It lowers the resonant frequency of the wall by altering the characteristics of the air-gap in the cavity
2. It dampens sound waves as they are transmitted through the wall
3. It dampens sound waves that are contained in the cavity of the wall
The addition of Thermafleece in a wall can typically result in a 6-12dB improvement to the sound insulation.