Party Wall Cross-Scheme Guidance

In this context a ‘party wall’ (also known as ‘separating wall’) comprises any wall between the dwelling and another heated space which can be:

  • another dwelling;
  • commercial premises;
  • a heated corridor or stairwell in a block of flats;
  • a heated common area.



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Description automatically generatedThis is where the common walls between two separate properties containing a cavity can lose heat to the external environment i.e. where the heat from a dwelling escapes into the cavity space, rises to the top of the roof and escapes. Previously it had been assumed that there was no heat loss via party walls, but research undertaken by Leeds Metropolitan University has shown that where a cavity exists in a party wall the heat loss can be equivalent to the wall having a U-value of 0.50 W/m2K or higher.

Unlike heat loss walls, floors and roofs the party walls thermal performance is not based on the age of the property. The U-values remain static as there is currently no requirement in building regulations to insulate these walls, this is illustrated in S8B below.


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A party wall will contain warm air within the cavity. When cold air enters the cavity, it pushes the warm air upwards. Where a bypass is present, the cavity leads all the way up to the roof line and the warm air escapes to the outside or escapes into the loft space through gaps in the construction.


Where a property is divided into two or more building parts (i.e. main property and extension(s)) the party wall is assessed for each building part bounded by a party wall.

RdSAP 9.93 introduced a new improvement measure B4 in Appendix T. This recommendation is where there is a party wall type “cavity masonry unfilled” and the improvement U-value is 0.2 W/m2K.



The primary method of identification is from viewing the party wall within the loft space, and it is only the construction in the loft space that is relevant.

Note: The party wall should be indicated as filled cavity only where it is known to have been filled. Where post fill cannot be determined cavity masonry unfilled should be selected.

Examples of different types of party wall are shown below with the cavity types ticked.

Party wall types: 

In some situations, it will not be possible to conclusively identify the construction of a party wall in these cases select Unable to Determine and the software will assume a U-value of 0.25 W/m2K for houses and bungalows.

Situations where party wall construction is not visible could include:

  • Where a room in the roof is present;
  • Basement /Ground and mid-floor flats (especially Converted warehouses and factories);
  • No loft access hatch / inaccessible loft hatch / H&S hazard in loft space;
  • Too much clutter in the loft to see gable end/too far away;
  • Plastered / boarded loft (hobby rooms, etc.).


In all cases, the reason for being unable to identify the construction must be detailed in the Energy Assessor’s site notes.