Walls - Cavity Wall Insulation (CWI)

At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update has not happened yet and a significant methodology overhaul from RDSAP to the Hone Energy Model (HEM) is proposed for 2025.

During the assessment for an EPC, the energy assessor will determine the type of construction of the external walls of the property, for each building part - i.e. the main building plus any extensions.

The options available are:

• Stone - granite or whinstone
• Stone - sandstone or limestone
• Solid Brick
• Cob
• Cavity Wall
• Timber Frame
• System Build

In this section we are insterested in Cavity Walls and cavity wall insulation.

A Cavity Wall is generally where the external wall of a property is constructed as two walls very close to each other with a gap between them called the cavity. The external wall for example is often constructed with bricks, and the internal wall often constructed using larger blocks.

A cavity wall is determined in many cases by looking externally at the brick bond of the wall (how the bricks were laid when the wall was built) and considering the thickness of the wall, perhaps the age of the property and maybe the view of an end gable wall from inside the loft to support this choice.

The brick bond of a cavity wall is normally such that all the bricks are laid lengthways. The long side of a brick is called the 'stretcher', and you will only see stretchers across the face of the wall.

A solid brick wall in contrast though might be constructed with bricks laid both lengthways and also across the width of the wall. As you look at the external elevation of the property you'd therefore see a mixture of long faces of the bricks and also the shorter (end) faces of the bricks. These end faces are called the 'headers'.

A cavity wall with insulation installed in the cavity will result in a better EPC rating compared to the same property with un-filled cavities, so for us here where we are wanting to improve the EPC rating of our building, having CWI will usually be the goal.

In the following picture, a single brick has been removed from a cavity wall, just at the point where two terraced houses connect with each other. The resulting hole reveals the cavity:

This is an interesting photo because you can see several things here.

The house to the right has cavity wall insulation, and it is the soft cotton type fluffy insulation. There are other types available, and they all have their own pros and cons.

You can also see that the house to the left does not have cavity wall insulation.

There is also a barrier that has been installed to prevent the insulation from the house on the right passing across to the house on the left. This is the red brush like fitting, made from a pair of twisted metal cables wound round each other with short lengths of plastic strands inbetween them. This almost certianly runs the complete height of the cavity.

Also in the photo, we can see the cavity of the party wall which separates the two properties. This reveals that the party wall is a cavity wall and it is not insulated.

Firstly, just a quick mention that although we have said that for EPC rating purposes we would normally be looking to have cavity wall insulation fitted wherever possible in order to achieve a better rating, there are in fact some properties where cavity wall insulation is not appropriate (see notes later).

Newer properties with cavity walls have insulation installed during initial build, due to the more recent (tighter) building regulations that have come in over the years.

Older properties however did not have insulation installed at initial build because the building regulations at the time did not require such a level of thermal performance. It is possible however to have such insulation retrofitted - ie fitted afterwards.

This is usually achieved by drilling a pattern of 20mm or so holes in the mortar lines across a wall, injecting the insulation through the holes, and then capping the holes with mortar.

During an EPC assessment, the presence of cavity wall insulation is either determined by visual presence of the drill holes mentioned above, or it is 'assumed' one way or the other by the EPC software/methodology based on the build date of the property part to which the wall belongs.

How this works is as follows:

If the assessor can see a series of drill holes indicating the presence of retrofitted Cavity Wall Insulation, they will enter the insulation status of the wall in the assessment as: 'Filled Cavity' (There are also options for combined 'filled cavity' plus 'internal' or 'external' insulation too).

Otherwise, in the absence of any visible drill holes that would indicate the presence of retrofitted cavity wall insulation, the assessor indicates that the insulation status of a cavity wall is 'As Built' in the assessment.

Specific types of documentary evidence however can also be accepted if required, and are made available to the assessor. This might include the situation where the external wall has been rendered, or re-rendered, thus hiding the insulation fill holes, and suitable documentary evidence is made available to the assessor proving insulation is present.

In the case where the insulation status of a wall is described as 'As Built', whether the EPC assessment proceeds on the basis that the cavity wall has cavity insulation present or not, is determined by the construction date of that building part.

The energy assessor asigns a contruction date to each building part. This includes the 'Main Building' plus any 'Extensions' for example.

Generally, if the assessor has indicated the construction date of the building part that contains the wall is prior to 1983, then the system will generally indicate 'Not Insulated (assumed)' in the EPC Features table. If the contruction date is after 1983, the system will generally indicate 'Insulated (assumed)' instead.

Where a property is determined as having a cavity wall without insulation, a recomendation on the EPC will usually be triggered, advising it to be fitted because the improvement it makes to the EPC rating is usually large enough to mention.

In the example of my own 1950s terraced house, with Finlock concrete guttering (currently preventing me from having cavity wall insulation fitted), my EPC rating would improve from a D68 to a C73 by having Cavity Wall Insulation (CWI) fitted. Further information about this particular property is available on our Case Study 2 page.

That improvement of 5 SAP points for my property is pretty significant, and for properties where CWI is appropriate, a very useful improvement. Five SAP points would contribute considerably towards moving up a band from a D to a C for example.

To learn what difference could be made to your EPC rating, check with a local energy assessor before proceeding with any work, as there are many characteristics of a property that contribute to the amount of gain that will be experienced.

You need to make choices from a position of certainty when spending sums of money in trying to improve your EPC rating, and a local DEA is the best person to work with to help work out the best changes to make.

If you have a previous EPC for your property, look in the Features table on that EPC, and see what how it describes your walls.

If you have cavity walls, you would expect the Walls Feature to say 'Cavity Wall'.

If the assessor can see drill holes for retrofitted CWI and has manually described the wall as a filled cavity wall, then the Feature entry will also include the word 'Insulated' (and nothing afterwards - the word 'assumed' will not be present).

If the assessor has not been able to see CWI fill holes and therefore entered 'As Built' for the Insulation status of the wall, then it will say either 'Insulated (assumed)' or 'not insulated (assumed)' depending on the date band assigned to that building part by the energy assessor.

Roughly, for properties built from 1983 onwards, the system will assume that cavity wall insulation was installed on construction due to the building regs in place at the time. For date bands prior to 1983, it will assume that CWI is not present. (The trigger is actually the assumed U value of the walls being less than or greater than 0.6).

Check the entry in the Features table and see if the insulation status of your cavity wall matches your expection of what it should be. Mistakes in EPCs can occur - see one of the later pages in this website about EPC mistakes, particularly in relation to construction dates for building parts.

Do you know the exact year of contruction of your property and any extensions? If the construction dates are entered incorrectly by an assessor, then the EPC rating can be affected, particularly around the 1983 threshold. Consider presenting proof of the contruction age to your asessor when they arrive, to prevent any mitakes. They will usually be very grateful for the information, as dating the contrustion of a propertty can be tricky.

It's possible for there to be multiple entries in the Features table for Walls. This can occur if: you have extensions that have different wall types to the main part of the building, or, you have 'Alternative Walls' (parts of a wall that are different to the rest of the wall in that building part).

Retrofitting Cavity Wall  Insulation (CWI) is not appropriate for all properties.

Advice should be sought when seeking to have cavity wall insulation installed.

My property that I mention above has concrete Finlock guttering which has a propensity for water leakage into the cavity. We therefore will have the Finlock replaced with PVC guttering before we have Cavity Wall Insulation fitted.

There are other situations too. Properties that are exposed to significant weathering such as on the coast, particularly in the South West of Britain might not be suitable.

This is due to the liklehood of moisture penetration into the cavity through the external brickwork.

Another cause for caution might be a property built around a steel frame. Many such buildings were constructed shortly after the second world war. In EPC terms, this is one form of 'System Build' property, and with a steel frame within the cavity wall, specialist advice should be sought.

Some precautions may need to be taken when installing CWI, to mitigate/prevent unexpected condensation within the property afterwards. This may include the adequate provision of trickle vents in windows, or if necessary a form of Mechanical Ventilation. You may also want to consider removing any rubble from the bottom of your cavities to prevent unwanted thermal bridging.

In this section, we talk about Party Walls, and in particular Cavity Party Walls. I'll get some text together to put in here shortly, but a short summary is that during an EPC assessment, the energy asssessor has to determine the type of party wall if your property has one. This is the wall that you share with another directly connected property. A detached property does not have a party wall because it stands on its own without any directly connected neighbours. A semi-detached property and an end-terraced property will have one party wall, and a mid-terraced house will have two party walls, one with each neighbour either side. We'll cover how the type of party wall is determined and what difference this might make to your EPC rating. In a recent update to the EPC methodology, a recommendation was introduced, whereby if you had an un--insulated cavity party wall, a recommendation would appear on your EPC to have that cavity party wall insulated. In the next update to EPC methodology - version 10 due in Q1 of 2025 (unless it gets delayed yet again), this recommendation will no longer appear.

Just a precautionary note now: If you render over the outside of your property, and cover over the retrofit CWI fill holes which were previously visible, then a DEA will not be able to visually confirm that cavity wall insulation was installed after the property was built.

Unless documentary evidence is available and presented, they will have to enter the walls as 'As Built' and the system will assume the presence of cavity insulation as per the building regs in force at the time the building part was built.

If you have cavity wall insulation installed whilst you currently own a property, make sure you keep the documentation safe, and make that documentation available to any future buyer of the property.

Also if you are purchasing a house, enquire whether such documentation is availabe from the vendor, particularly if the property was contruscted prior to 1983 and there are no cavity fill holes visible.

It may be possible in the case of a wall that has been rendered over, for documentation to be provided either proactively, or reactively by such persons as a chartered surveyor. This could even be via invasive inspection of the wall after the fact, which can then be relied upon by an energy Asessor as documentary evidence when conducting an EPC assessment.

Lastly, there are a few properties where wall ties have been retro-fitted to a cavity wall for one reason or another. These can be implemented via drill holes which appear to be very similar to those for Cavity Wall Insulation fill, although the holes are usually narrower. Care should therefore be taken when checking for the presence of CWI fill holes.

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.