Cavity Party Wall Insulation

If your property is attached to another property in some way, such as in the case of a terraced house or a semi-detatched house, then the wall that they share in common is called a Party Wall.

A mid-terraced house like mine for example will have two party walls, one with each neighbour.

This also applies to flats where another flat is located alongside with a shared wall joining them.

Where possible, an energy assessor determines the construction type of the party wall and enters this into the EPC assessment.

The options are (including the associated U-values):

The information in the table above is from Table S8B in the RDSAP Appendix S document on the BRE website.

Unlike 'Heat Loss Walls', ie the external walls of a property, a Party Wall's thermal performance is not dependent on the age of the property. The u-values are consistent as there is currently no requirement in building regulations to insulate these walls.

Notice that solid masonry party walls are assumed to have zero heat loss to the neighburing property (the u-value is zero). Other party wall types have a slight heat loss.

For a house, the energy assessor will normally deduce the construction type of the party wall, if possible, by looking in the loft at how the blocks are laid in the party wall.

Blocks laid flat in a stretcher bond fashion would indicate a solid masonry party wall, and also bricks in a mixed stretcher/header bond would too.

Blocks laid on edge with their large face facing into the loft might ordinarily indicate a cavity party wall, however, there is a recently introduced Convention that must be followed for this latter case (Convention 2.24a - introduced on 1st January 2024):

The reason I thought I'd mention party walls is partly to show the U-values in the table above but also to mention a Recommendation that was introduced to EPC certificates for a while, and which is no longer incuded on new EPCs.

The current version of EPC methodology for existing dwellings is RDSAP 10. In an earlier version, 9.93, a recommendation was introduced whereby if an un--filled cavity party wall was declared, a recommendation would likely appear on your EPC advising to have that cavity party wall insulated.

This recommendation would have needed to improve the EPC rating by at least 1 SAP point in order to appear on the EPC and would have looked like this:

This recommendation has been removed in RdSAP 10 and will no longer appear on any newly conducted EPCs.

For those that are interested however, under RDSAP 9.94, the rating for my property - a 1950s mid-terrace house, the party wall type used to have the following effect on my EPC rating:

So for me, it doesn't make much of a difference.

From 'unable to determine' (which is the type that the new convention instructs me to declare) to Cavity Masonry Filled, there is no improvement to the integer value of my EPC rating.

My property however, although being a mid-terraced property and therefore having two party walls, is not very deep so those party walls are not very long. The house is fairly wide instead, and the unfilled cavity walls on the front and rear elevations are of more interest to me for the EPC rating.

For a house that is perhaps fairly narrow, and deeper, and therefore has longer party walls, the difference can be more significant.

Consider for a moment the situation where you have a mid-terraced property that is fairly narrow and deep, and constructed from 1966 onwards. You might have a previous EPC issued before Convention 2.24a was issued, and under that EPC the assessor was allowed to declare the party wall as 'Unable to determine'. You commission a new EPC and the assessor now has to follow Convention 2.24a and the party wall is described instead as 'Cavity - Masonry unfilled'. Due to the lower U value, the EPC rating is reduced. I have seen such a property where that difference in choice made a difference of 3 SAP points on a new EPC under RDSAP 9.94. On that same property, installing party cavity wall insulation would have added 4 points from the rating with unfilled cavity.

Although the costs and effort required with neighbours to effect cavity fill of a party wall might be high, it might be worth looking into the cost if it would lead to a useful increase in your rating.

As always, check with a local energy assessor what difference Party Wall insulation might make to your EPC rating, before going ahead with any works. This is because the EPC rating is calculated using all the information gathered during an assessment, and it is therefore not possible to rely on any gains for one property being at all representative of what could be achieved on another property.