Party Wall Bypass and Part E

Referred to as Thermal Bypass, the phenomenon is now taken into consideration within the new Part L and potentially represents a major concern to housebuilders and designers in terms of achieving compliance for semi-detached and terraced property types. This is because on typical floor plans, where the footprint is quite narrow, the party wall presents a larger area than the front or back, to which the Building Regulations now assign a U-value of 0.5 W/m²K unless specific action is taken to improve its performance.

Leeds Metropolitan showed how, once heat has escaped into the cavity, it is carried upwards to the roof space; primarily due to wind drift at the junctions with the outside walls creating a stack effect. No developer will want to accept the onerous U-value penalty, but filling the cavity could also compromise the acoustic insulation afforded by the original arrangement.

A U-value of zero can be claimed if the cavity is filled with an appropriate insulation material and the zone is also edge sealed. If the specifier opts for edge sealing only to prevent the wind drift, then the tables direct a U-value of 0.2 W/m²K. This could still cause a design to fail its SAP assessment.

Fortunately mineral wool is not only a very good thermal insulant, but also has well documented acoustic benefits. Therefore, in cooperation with both Building Regulations and Robust Details Limited, members of MIMA led the way by beginning a series of trials within both new and existing dwellings to demonstrate a practical solution which would save clients from conducting pre-completion testing for sound performance.