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Internal floors for new or 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 an internal floor 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, separating floors and internal walls.

Performance

The minimum value for airborne sound insulation of an internal floor (i.e. a floor between two rooms within the same dwelling) is Rw = 40dB.

Rw is a laboratory rating for sound reduction and field testing of internal walls and internal floors is not normally carried out. However a building inspector, or acoustic specialist carrying out pre-completion testing on other elements of the construction, will check that the design specification for internal walls and floors is based on construction methods that should achieve Rw = 40dB.

Building an internal floor with a timber joist
Approved Document E gives construction guidance on “Internal floor type C: Timber or metal joist”. 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 structural floor base, the ceiling and the Thermafleece between the joists. A soft covering will reduce impact sound at source.

Construction guidance

The construction consists of a timber joist, with wood based floor board, plasterboard ceiling and Thermafleece in the void.

• The floor should be a timber or wood based board of minimum mass per unit area 15 kg/m2 such as 22 mm flooring grade chipboard.
• The ceiling should be one layer of plasterboard of minimum mass per unit area 10 kg/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.
• 100mm Thermafleece should be laid in the cavity
• Insulation against impact sounds can be improved by adding a soft covering (e.g. carpet)

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.

How does Thermafleece improve sound insulation?

When you fit Thermafleece in a internal 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.