A new research project on cavity party wall heat loss is being undertaken by the Leeds Sustainability Institute at Leeds Beckett University – in collaboration with Cambridge Architectural Research Ltd – commissioned by the Department of Business, Energy and Industrial Strategy. 

The research aims to discover how much heat is lost through cavity party walls, and look at the effectiveness of insulation on these figures. It used to be assumed that with heated dwellings on both sides, there was no cavity party wall heat loss to speak of, but back in 2009 another study by Leeds Beckett (then Leeds Metropolitan) significantly changed this view. The report’s findings were, in a nutshell, that a thermal bypass could double the fabric heat loss coefficient of a mid-terraced house, and led to the effects of party walls being included in the 2010 update of Part L of the Building Regulations.

After the 2010 changes to Part L of the Building Regulations, assessors had to note whether the party walls were solid, fully filled cavity walls, or unfilled cavity walls with equivalent U-values included in the SAP calculation for each situation. The difference in the SAP calculation between a fully insulated party cavity wall and an unsealed, unfilled party cavity wall could be at least 12% for mid-terraced houses. The minimum requirement for compliance is currently to have effective edge sealing around all exposed edges, and in line with insulation layers in abutting elements, to minimise thermal bypass.

It’ll be interesting to see the results of this new study in due course. Of course, we think further research on the effects on thermal bypass is very welcome indeed, particularly as the increase in insulation standards and air tightness elsewhere in dwellings make this more of a significant issue relatively speaking. If we are also having to oversize heating systems to compensate for heat losses through cavity party walls, then this particularly reduces the efficiencies and cost effectiveness of renewable energy systems such as heat pumps.

Of course, the results of the new study are unlikely to influence the next versions of SAP and RdSAP, as the consultation was announced by the BRE last year, and did not suggest any amendments to the current way of assessing thermal bypass to cavity party walls. We suspect it will be another four years until the next SAP update!