The Best of Both Worlds

A considerable amount of heat can escape through untreated cavity walls between connected buildings. With this, Part L1A of the Building Regulations now set out requirements to avoid this party wall bypass so that to achieve a u-value of 0.2 to zero. However, the recommendation to avoid this heat loss, by fully filling the cavity and having effective edge sealing, poses another challenge in that it provides a path for sound transmission. This does not meet the acoustic requirements as set out in Part E. Here, Lee Nichols, Product Development Manager at Icopal discusses the difficulties of fulfilling both Part L1A and Part E and how these can both be met with the right product.

Party cavity walls are commonly used to separate semi-detached properties, terraced houses and flats. It was previously thought that since properties on both sides of a party cavity wall are heated, any heat that went through the wall would be reciprocated back and therefore the heat loss would be zero. However, this was proved not to be the case by a study undertaken by the Buildings and Sustainability Group of the School of the Built Environment at Leeds Metropolitan University.

The study identified that the cavity in between the two wall leaves, actually acts as a chimney. This is because heat that passes through the wall heats up the air in the cavity. This heated air then moves up the cavity while drawing in colder air at the bottom of the cavity from both properties on either side of the party wall. This creates a heat loss mechanism by airflow in the wall’s cavity. In addition, a cold zone is created where outside air is able to flow into the cavity for the party wall. The result is heat flux or heat energy transfer through the wall section on either side. Furthermore, the extent of heat flux or airflow can be affected by external conditions such as wind and temperatures.

Heat loss through party cavity walls can be very significant. In fact, an untreated party cavity wall of a mid-terrace house can have a greater heat loss than all other external walls combined. It is therefore extremely important that steps are taken to restrict the flows and, in turn, reduce the amount of heat loss. This not only adheres to the requirements under Part L1A but also, for the end-user, this will reduce energy bills and the building’s environmental impact.

It is now generally accepted that fully filling the cavity will ensure that the needs under Part L1A will be met. However, in order to achieve the optimum u-value of zero, the study by Leeds Metropolitan University identified that in addition to fully filling the cavity an effective edge sealing is also a necessity.

Where the route of fully filling the cavity with an effective edge sealing is followed, a sound transmission path can be created from the neighbouring building and it is therefore important to be aware of the affects this can have on meeting Part E of the regulations, which relates to noise.

As part of the buildings design process, it is paramount that considerations are made for both the requirements under Part L1A and Part E. This will result in an improved thermal and acoustic performance of the party cavity walls. For this, a product such as Icopal’s Monarfloor Wall Cap and Wall Cap – Lamatherm, which that not only provides effective edge sealing for fully filled cavities but also answers the issues of acoustic performance should be specified. Icopal’s product is currently the only accepted and proven method, which will meet both Part L1A and Part E.

A solution which meets both the requirements of Part L1A and Part E should be considered for both new build projects where buildings will be joined in addition existing properties where an extension may be taking place. This provides the ideal opportunity to upgrade the party cavity wall.

The u-value of a party cavity wall is very significant in meeting Part L1A of the Buildings Regulations and, in turn, providing energy savings and reducing the building’s environmental impact. However, it is important that in meeting Part L1A, that sound transmission is not increased which will result in Part E of the regulations not being fulfilled. In the design and specification stages, this is a significant consideration and, for this, a product, which fulfils both requirements should be selected.