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Climate, Environment, Buildings & Energy
Study to investigate Solutions to the Cavity Party Wall Thermal Bypass


Leeds Metropolitan University's Centre for the Built Environment has undertaken a study of heat loss through typical cavity party walls in dwellings. The work has been funded by Eurisol, the UK Mineral Wool Manufacturers Association. This project followed on from the Stamford Brook Field Trial which identified significant heat losses via party wall cavities in terraced and semi-detached masonry houses, and proposed various techniques to eliminate or minimise the effect.

The purpose of this study was to extend the work carried out by Leeds Met at Stamford Brook, and help in the development of practical low cost solutions to mitigate of heat loss via party wall cavities.

Project Objectives

This project attempts to quantify the bypass effect in more detail and also to evaluate whether filling the party wall cavity with mineral fibre insulation will eliminate or significantly reduce the heat loss due to the thermal bypass that operates in most cavity party walls. In addition, the project intends to:

  • Establish a robust methodology for quantifying the effect of the thermal bypass on the energy and CO2 performance of dwellings.
  • Obtain evidence that the proposed solution (in this case insulating the party wall cavity) can provide a measurable effect on the thermal performance of buildings.
  • Increase understanding and inform CLG of how this data might be handled by SAP, so that there is a mechanism for accounting for heat losses via untreated cavity party walls and other identifiable thermal bypasses. This may eventually provide a methodology for allowing for technical solutions, such as insulating the party wall cavity, that might reduce or even eliminate such heat loss.

Project Methodology

The experimental programmes were carried out over the winters of 2008/9 and 2009/2010 at sites in Bradford and Darlington. The experimental methodology used coheating tests in combination with pressurisation testing, acoustic tests, thermal imaging and a range of other analytical techniques in order to understand the thermal and acoustic performance of the separating party walls.

Coheating tests were carried out before and after the party wall was insulated with mineral wool. The change in the measured heat loss coefficient could then be attributed to the effect of the insulation on the thermal bypass.

Thermal imaging of the party wall   Thorough monitoring of the party wall
Thermal imaging of the party wall shows a variation in surface temperature, with the lowest temperature at the junction of the party wall with the ground floor and external wall.   Typical sensor array on party wall.

Throughout the coheating tests, heat flow through the party wall and adjacent external walls were measured by the research team. Measurements included cavity and surface temperatures, air flows inside cavities, heat flux densities and selected pressure differentials provided valuable information on the heat transfer and air movement mechanisms in operation during the test period. In additional measurements were taken of internal conditions (room temperatures, RH, CO2 concentration) and the external environment (temperature, RH, wind speed and direction, rainfall, solar insolation.

Ventilation rates were estimated from pressurisation tests at critical stages during the test programme and in addition, ventilation rates of the dwellings were also determined by gas decay rates using CO2 as a tracer gas.

Acoustic testing of the party wall was performed before and after filling the party wall cavity in order to determine the effect of the insulation on acoustic performance of the separating party wall.

Project Partners

Cavity masonry dwelling report
Timber frame dwelling report