Separating floors for existing buildings – timber joist
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 a separating floors construction 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 walls, internal walls and internal floors.
Performance
The minimum value for airborne sound insulation of a separating floor in a dwelling-house or flat formed by material change of use is DnT,w+Ctr = 43dB.
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 43dB (although the complexities of the actual calculation take into account other factors in the overall sound insulation performance). A sound reduction of 43dB is reasonable for everyday living, but in some circumstances a higher figure, meaning a greater sound reduction, would be desirable.
Separating floors must also be able to resist impact sound. This is tested by using a standardised tapping machine on the floor and measuring how much sound the machine makes in the room below. The machine has five weights, or hammers, which repeatedly hit the floor at a set speed. The maximum noise level allowed in the room below is L’nT,w = 64dB.
L’nT,w is a single figure value calculated to indicate the sound level generated by the tapping machine. The calculation is complex as it also takes into account other factors in the sound insulation performance, but in simplified terms it means the tapping hammer must not cause a noise level greater than 64dB in the room below. If the sound insulation of the separating floor is better than the minimum standard then the noise level in the room below will be lower, so the number will be smaller.
Pre-completion testing is carried out on separating floors between habitable rooms.
Upgrading a separating floor with an independent ceiling
Approved Document E gives construction guidance on “Floor Treatment 1: Independent ceiling with absorbent material”. For further information please refer directly to Approved Document E - Resistance to the passage of sound.
The resistance to airborne and impact sound in this construction depends on the combined mass of the existing floor and the independent ceiling, the absorbent material, the isolation of the independent ceiling and the air tightness of the whole construction.
Construction guidance
The construction consists of an independent ceiling containing a Thermafleece lining. This is added underneath an existing floor. It is important that the existing floor is in good condition and some remedial work may be required to the existing structure before the independent ceiling is fitted.
• If the existing floor is timber then gaps in floor boarding should be sealed by overlaying with hardboard or filled with sealant
• Where floor boards are to be replaced, boarding should have a minimum thickness of 12mm, and 100mm of Thermafleece should be laid between the joists in the floor cavity
• 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
• If there is an existing lath and plaster ceiling, it should be retained as long as it satisfies Building Regulation Part B – Fire safety
• 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 such as 12.5mm British Gypsum Wallboard TEN or 15mm British Gypsum Wallboard. Standard 12.5mm (half inch) wallboard is not suitable for this construction.
• Extensive remedial work to reduce flanking transmission may also be necessary
Independent ceiling may be supported by one of the following methods:
Note 1: Independent ceiling joists fixed only to surrounding walls. A minimum clearance of 25mm must be left between the underside of the existing floor structure and top of the new independent joists.
Note 2: Independent ceiling joists fixed to surrounding walls with additional support provided by resilient hangers attached to the existing floor base. A minimum clearance of 125mm must be left between the underside of the existing floor structure and the upper surface of the new independent ceiling.
Points to watch
Do
Remember to apply appropriate remedial work to the existing construction
Seal the perimeter of the independent ceiling with tape or sealant
Do not
Do not create a rigid or direct connection between the new ceiling and the existing floor base
Do not tightly compress the Thermafleece as this may bridge the cavity
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 floor, but can also travel along other building elements such as the walls that are shared by adjacent rooms. Flanking transmission can exceed direct transmission through the separating floor which may result in the required sound insulation not be met.
How does Thermafleece improve sound insulation?
When you fit Thermafleece in a separating floor it improves sound insulation in three ways:
1. It lowers the resonant frequency of the floor by altering the characteristics of the air-gap in the cavity.
2. It dampens sound waves as they are transmitted through the floor.
3. It dampens sound waves that are contained in the void of the floor.
The addition of Thermafleece in a floor can typically result in a 6-12dB improvement to the sound insulation.