The evolution of Party Walls


Masonry party walls have seen many changes over the years, driven by the need to improve acoustic performance, prevent thermal bypass and reduce the cost of installation. Tom Foster, senior product manager at Saint- Gobain Isover, looks at the evolution of the masonry party wall and how Isover have supported the industry in developing a better performing construction.


During the first half of the twentieth century the majority of homes were built with a solid brick party wall. The acoustic performance was relatively good due to the high level of mass, and restriction in air movement also ensured no heat could be lost from the party wall via thermal bypass, a concept that would not be identified for many years.

Despite good acoustic and thermal performance, the cost of materials and speed of installation were too high, which resulted in the industry gradually moving to a cavity wall construction in the 1950s and 1960s.


The acoustic twin leaf concept was introduced to reduce mass from the construction by around a third, without negatively impacting acoustic performance. The introduction of the cavity allowed the industry to build the same standard of wall more cheaply and quickly.

The unforeseen consequence was that, by introducing an empty cavity, the industry had created a way for heat to escape from the building – a concept that would later be known as ‘party wall bypass’.


Developments in the past ten years have primarily been driven by the introduction of Approved Document E 2003, which set out a minimum 45dB requirement for party walls, as well as on-site pre-completion testing (PCT). The requirement for PCT, while effective at enforcing the regulation, proved a burden to house builders and so Robust Details Limited was established as an alternative route of compliance in 2004.

In 2005, Isover were the first to market a series of Robust Detail compliant proprietary party walls that removed the requirement for PCT and the labour-intensive parge coat, without negatively impacting the acoustic performance. This was achieved with a partial- fill insulation product called Isover RD35 and was the first time an insulation product had been used in the party wall, something that has now become an industry norm.

Despite this leap forward for the industry, the partial-fill construction still didn’t fully address heat loss through thermal bypass, an issue that was gaining momentum within the industry.


After work was carried out by Leeds Metropolitan University to prove the concept of party wall bypass, steps were taken to address the issue in the update of Approved Document L in 2010.

Once again, Isover were the first to market in 2009, a year before the regulations were introduced, with Isover RD Party Wall Roll. This full-fill roll restricts air movement within the cavity and when installed with effective edge sealing, helps the house builder to claim a zero heat loss party wall.


Since the introduction of Approved Document E 2003, Saint-Gobain Isover has been at the forefront of maximising acoustic performance, reducing cost of installation, and removing thermal bypass from party wall structures. Isover offer the widest choice of proprietary full-fill Robust Details on the market. E-WM-17, E-WM-20 and E-WM-24 all deliver three credits towards the Code for Sustainable Homes, remove the requirement for parge-coating and help to deliver a zero U-value party wall.

Tom Foster

Senior Product Manager

Saint-Gobain Isover

Tel: 0115 969 8005


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