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Cavity walls

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Approved Document E contains the guidance for masonry cavity walls, which states that only butterfly ties should be used to connect the wall leaves. For structural reasons, these ties are generally only suitable for cavity widths of 50mm to 75mm.

However, there are many proprietary ties that fulfil the structural requirements for similar or wider cavity widths. The only reliable method of checking whether their use will result in worse sound insulation than with butterfly ties is to measure and compare their dynamic stiffnesses.

Knowledge of a wall tie's dynamic stiffness is vital if it is to be used in cavities greater than 75mm. Even if stiffer ties must be used for structural reasons, the mass-spring-mass resonance frequency can be chosen - to reflect that of a wall comprising a smaller cavity and butterfly ties - by increasing the mass of one or both of the leaves.

For external walls, thermal requirements leading to larger cavity widths are likely to make the need to identify equivalence between wall ties increasingly important.

Research is currently being conducted at the BRE Acoustics Centre into the effect that wall ties, cavity insulation and different inner leaves of external flanking walls have on the sound insulation between dwellings separated by a solid masonry wall. Initial results indicate that the measured sound insulation of separating walls is reduced if wall ties with a higher dynamic stiffness than butterfly ties are used in external cavity walls. This is due to increased flanking transmission across the wall ties to the outer leaf and back to the inner flanking leaf of the adjacent dwelling.

In conclusion, the use of dynamic stiffness as a specification parameter is less restrictive than using generic descriptors such as material, density and thickness and might stimulate the development and specification of alternatives to traditional materials and products.

Robin Hall and Carl Hopkins are researchers at the BRE Acoustics Centre, (01923) 664455. Research in association with Dr Fothergill at the DETR.

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