Uninsulated cavity party walls are now included in energy calculations with the updated 9.92 version of RdSAP

By Steve Smith: Market Development Manager (Knauf Insulation Northern Europe)

We look at the reasons for inclusion and the potential savings which stand to be realised.

With the new version of RdSAP now including heat loss via party walls in existing homes, I thought it would be worthwhile providing a little background on the reasons for the inclusion, and also the potential savings the inclusion could have on both the economy and the environment.

Previous versions of RdSAP did not take account of heat lost through uninsulated party cavity walls – mainly because it was traditionally believed that if the two homes either side of the party wall were heated, then there would be no heat transfer between the two dwellings and into the party wall cavity – sounds logical right?

However, since 2005 a detailed research programme into real energy performance of buildings identified that, contrary to popular belief, party walls with uninsulated cavities were actually subject to significant amounts of heat loss – for example, in a mid-terrace dwelling which was monitored as part of the research programme, it was found that the heat lost through the untreated party cavity wall was greater than that which was lost through all of the other external walls combined!

But, the reason so much heat is lost through a party wall is slightly different to the way in which heat is lost through an external wall (now for casting my mind back to that physics lesson!).

Heat lost through an external wall is mainly due to conduction through the wall as the heat passes through the various material layers, whereas heat loss through party walls is due to a phenomenon called party wall thermal bypass.

Sounds scary and more like an ailment my doctor might diagnose…

Party wall thermal bypass isn’t so scary (unless your home is a victim of it and you’re looking at your latest energy bill as a result!) – it is a process whereby cold air enters an uninsulated party wall cavity at the exposed edges of the wall and results in heat loss via convection (the old adage ‘heat rises’ springs to mind). The classic idea is that cold air enters the cavity at the bottom of the wall, and rises like in a chimney and then takes heat from the adjoining homes. All of that nice warm air it has taken from the homes on either side of the wall is dispatched when it escapes the party wall – either into the loft space or out through the roof. In reality of course it’s more complicated than that and cold air can enter and leave the cavity at all exposed edges depending on wind direction and strength.

Now the clever bit – and the reason it is being included in the new version of RdSAP …

The research carried out by Leeds Beckett University (formerly Leeds Metropolitan University), MIMA and Knauf Insulation didn’t stop at identifying the problem – i.e. that heat is being lost at an alarming rate through the uninsulated walls across the country – it went one step further to make sure that the heat loss could be stopped.

After measurements were taken to quantify the heat loss, the next step was to treat the party walls using Knauf Insulation’s Supafil blown mineral wool, which is currently used to insulate external cavity walls. Measurements taken post-installation confirmed that party wall thermal bypass was prevented and heat loss through the party wall was stopped.

Research from the BRE estimates that there are over 5 million homes which could be affected – having wall cavities which extend all the way from the ground to the underside of the roof, thus increasing the amount of heat lost compared to party cavity walls which may have a solid section to the underside of the roof.  If all of these walls were insulated it would save approximately £465 million per year on energy bills – a welcome reduction given the current economic climate.

On top of the economic benefit comes the environmental benefit – the BRE also estimate that insulating all affected homes would reduce carbon emissions by 2.5million tonnes each year. That’s roughly the equivalent of an individual taking 2.5million return flights from London to New York, driving 5 billion miles in a 4×4 vehicle, or travelling 27.5 billion miles on national rail! *


So what happens now?

First of all, the energy assessor who is using the new version of RdSAP will need to identify the construction type of the party wall. Knauf Insulation, who have been involved in the detailed research programme since 2008, will be providing guidance on this but it’s fairly straightforward and only needs a quick look in the loft. Traditionally, to identify the construction method of the external walls, the assessor would be looking at the brick bonds on the outside of the home. The problem with party walls is that they are more than likely covered by a decoration layer such as wallpaper, paint or tiling, but not in the loft. A simple look in the loft with a torch or a light will allow the assessor to see the brick bonds and quickly identify the wall type, whether there is a cavity, and allow the correct option to be selected in the new version of RdSAP.

If a home is deemed to be suffering from party wall thermal bypass, the next step will be arranging installation of a blown insulation solution by a trained and approved installing technician. One example would be Knauf Insulation’s Supafil Party Wall solution which has been developed specifically for use in party walls to ensure both thermal and acoustic performance.

On top of that, and to help with the installation cost, party wall insulation is now eligible for funding under the Energy Company Obligation, and is recognised as a primary measure – a welcome message to help increase uptake and begin to reduce fuel bills and carbon emissions across the estimated 5million affected households.


*(Aviation Environment Federation)

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