Walls - Cavity Wall Insulation (CWI)

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 9.94. This updates to RDSAP 10 on 15th June 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consulations indicate future changes to the format of EPCs and minimum rating levels required for priate rental properties. Methods for improving EPC ratings may therefore change in the future.

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, if present.

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're interested in Cavity Walls and Cavity Wall Insulation (CWI).

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 outer wall is often constructed with bricks, and the internal wall often constructed using larger blocks.

Here is an example from my neighbour's modern extension which is currently in progress:

The external brick wall, and the internal block wall are clearly visible, with the gap (cavity) inbetween - in this case fully filled with insulation.

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 ordinarily you will only see stretchers across the external face of a cavity wall. Additionally, the thickness of the wall is likely to be around 275mm or slightly more.

In comparison, a solid brick wall is usually constructed with bricks laid both lengthways and also across the depth of the wall, with the wall being two (or possibly three) bricks thick. As you look at the external elevation of the property you'd expect to see a mixture of long faces of the bricks and also the shorter (end) faces of the bricks. These end faces are called 'headers'. The thickness of this type of wall is likely to be around 225mm or possibly 345mm (for a two brick thick solid wall and a three brick thick solid wall).

There are various patterns in which the bricks can be laid, and this example is called English Bond, where a whole row of stretchers is alternated with a whole row of headers.

Note that solid walls consisting of just a single skin of bricks laid end to end in stretcher bond can occasionally be found, but they are only about 115mm thick. Despite the external visual appearance, the thickness measurement would clearly indicate these are not cavity walls.

It's possible to find both cavity and sold brick walls on some earlier properties, and this is the case in this example, originally built sometime between 1905 and 1922:

Notice the brick bond of the main building. It's stretcher only, indicating a cavity wall. This was also supported by looking at the thickness of the wall at a window opening.

Notice though the brick bond of the extension at the rear. The brick bond is headers and stretchers. This indicates a solid brick wall, and this again was supported by looking at the wall thickness at a door opening.

'Modern' UK Building regulations came into force in 1966 (England & Wales), 1964 (Scotland) and 1972 (Northern Ireland).

In the methodology for EPC assessments of existing dwellings, the list of available construction datebands vary slightly across the nations due to the timing of changes in Building Regulations over the years, so in this section I'll stick with the England & Wales dates as this is where I am located.

Up until 1976, houses were almost entirely built without cavity wall insulation as they did not need it in order to meet building regulations.

From 1976, with slightly tighter requirements for thermal performance, it was more usual for builders to partially insulate cavities.

Then from 1983 onwards it was much more normal for cavities to be insulated at the construction stage due to further tightening of the thermal performance requirements.

Despite the above, some dwellings were actually still being built into the early 1990s without cavity wall insulation as their builders could meet the energy requirements by using thermal blocks on the inner skin of the wall instead of installing insulation.

For properties where cavity wall insulation was not installed at the time of construction, it is possible to have it 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.

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

Having said this however, there are some properties where cavity wall insulation is not appropriate (see notes later).

In the following picture, I've removed a single brick from a cavity wall, just at the point where two 1950s terraced houses join, and where one has had cavity wall insulation retrofitted:

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

The grey colour blocks visible through the hole are the inner wall of blocks.

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

We can't quite see enough in this photo to be sure, but the house to the left in fact does not have cavity wall insulation.

There is also a barrier that has been installed to keep the insulation in the walls in the house on the right. 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 caught inbetween them. This runs the complete height of the property.

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, rather than being a solid wall, and it is not insulated.

When a Domestic Energy Assessor visits your [existing] property, and determines that the external wall is a cavity wall, as part of the assessment they need to declare the insulation status of that cavity wall:

The standard options are:

• 'As Built'  - This means that the insulation status of the wall is the same as when it was originally constructed. (i.e. it has not changed).
• 'Filled Cavity' - This means that cavity wall insulation has been retrofitted to the wall since it was originally constructed. 

This choice, along with the construction dateband assigned to this building part, determines the u-value that the EPC methodology uses in the calculation of the final EPC rating.

A u-value is a numerical representation of the thermal performance (read: energy loss) of the wall, and is measured in Watts per metre squared, Kelvin (W/m2K).

This is a representation of how much power (Watts) is lost for each square metre of wall, per degree Kelvin (same unit size as Centigrade) temperature difference between one side of the wall and the other.

As we will see in a moment, this value is not actually measured by an energy assessor onsite, but the assessment methodology takes it from a fixed reference table of u-values.

First though, we need to talk about how the decision is made between 'As Built' or 'Filled Cavity', because I want to address a couple of misconceptions that many people have about the textual description of cavity walls in the Features table on EPCs.

So, to determine the insulation status of a cavity wall, the assessor will stand outside the property and look at the wall. If they can see a series of drill holes indicating the presence of retrofitted Cavity Wall Insulation, they will enter the insulation status of the cavity wall in the assessment as: 'Filled Cavity'.

I'll repeat once more, 'Filled Cavity' means that cavity wall insulation has been retrofitted to the property, since it was originally constructed.

(There are also options for combined 'filled cavity' plus 'internal' or 'external' insulation too, if that has also been fitted, but we'll proceed here to consider only 'Filled Cavity').

When 'Filled Cavity' is selected, the Features table on a resulting EPC will then contain a line that reads:

Wall - Cavity wall, filled cavity

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.

I'll repeat that once more as well, 'As Built' means the cavity wall has the same thermal energy performance right now as it had when it was originally constructed.

(Note that specific types of documentary evidence for retrofitted CWI can however also be accepted if required, and are made available to the assessor. This might include the situation where it is not possible to visually determine that such insulation has been installed due to the wall having been rendered, re-rendered, or re-pointed)

In the case where the insulation status of a cavity wall is described by the assessor as 'As Built', the EPC methodology will show a different description in the Features table of the EPC certificate. This description will also include text that visually indicates to the reader an assumption about the presence of insulation in the cavity wall.

The asumption reflects the most common construction approach in use at the time that the wall was originally built, considering the construction dateband assigned to the building part.

If the dateband of the building part is prior to 1976, then the EPC Features table will indicate:

Wall - Cavity wall, as built, no insulation (assumed)

If the dateband of the building part is 1976-1982, then the EPC Features table will indicate:

Wall - Cavity wall, as built, partial insulation (assumed)

If the dateband of the building part is 1983 onwards, then the EPC Features Table will indicate:

Wall - Cavity wall, as built, insulated (assumed)

This choice of 'assumed' insulation is made by the EPC methodology. The Energy Assessor does not control this themselves, it is purely controlled by the assessment methodology and is based on the construction dateband applied to the building part when 'As Built' is chosen for a cavity wall.

This is how that text originates, despite many forum postings you might come across on the Internet from ill-informed souls thinking that the assessor makes the assumptions.

In the background as we have mentioned earlier, calculation of the EPC rating actually proceeds using a numerical U value for the wall, taken from a reference table.

The u-value used depends on the dateband, wall type (cavity in our case here), and insulation/build status ('As built' versus retrofitted 'Filled Cavity'), unless it is overidden manually by a calculated U value, backed up with documentary evidence provided by the property owner.

For those that are interested, the table of default U values are available to view in Table S6 (England & Wales), S7 (Scotland) and S8 (Northern Ireland) of the RDSAP Appendix S document on the Building Research Establishment (BRE) website.

To locate this doucment, visit the folllowing website:

https://bregroup.com/expertise/energy/sap/standard-assessment-procedure-2012

Scroll down and click on the link for 'RdSAP 2012 v9.94' (this document will be replaced with the release of RDAP 10 in June 2025 and the introduction of the Home Energy Model in 2026 or later).

Here are the cavity wall U values for England & Wales (my location) from table S6 with the E&W datebands, for 'As Built' and Filled Cavity':

* = assumed 'As Built'

So the EPC calculation proceeds with a chosen u-value from this table unless this value is manually overidden by the assessor (which requires specific documentary evidence).

For recent datebands, the values tend to be based on the minimum U values required by Building Regulations at the time.

For older datebands they are likely based on research by the BRE of the performance of the most commonly built walls at the time.

With the size of the U value from this table being so dependant on the dateband assigned to the building part, it is therefore important that the correct dateband is assigned to each building part by the assessor, and I think that this particular point is the cause of much mis-understanding in online forums.

If the wrong dateband is applied then the EPC rating can be skewed one way or another, but it is the textual description of the wall in the Features table on the EPC certificate that people actually see and comment about.

Also, if the Energy Assessor mistakenly records the wall as 'Filled Cavity' even though there is no evidence of the insulation having been retrofitted rather than having been there since original construction ('As Built'), then the EPC calculation will use better U values than it should for construction dates prior to 1995, and the rating will come out overly positive.

This could happen where the property owner presents proof to the assessor that the wall has insulation in it (from original construction), perhaps from core drill holes through the wall from fitting exhaust appliances, and the assessor forgets that 'Filled Cavity' means retrofitted CWI.

Finally, just to reiterate , the 'assumed' cavity wall status as displayed in the Features table on an EPC is just a noticeable 'visual' sign which can indicate the choice of a construction dateband either side or around the 1976-1982 range for a cavity wall with insulation status of 'As Built'.

CIGA is an independent body estabished in 1995 that provides 25 year guarantees for retrofitted Cavity Wall Insulation fitted by registered installers in the UK.

If your property has had CWI retrofitted since 1995, it is quite possible that you could purchase a duplicate certificate from CIGA confirming the presence of CWI, if you need to prove it.

In most cases it would not be necessary to present such a certificate to your energy assessor because the insulation fill holes would be visible on the outside of your property, but there are some circumstances where a certificate would definitely help.

This would be the case where since having retrofitted CWI, your property has been:

• re-pointed - where the mortar between the bricks is ground out and replaced with fresh mortar
• rendered - where the brickwork has had a smooth coat of render applied to the outside
• re-rendered - where an external render coat has been removed and a fresh one applied

In each of these cases, the once visible fill holes would no longer be present, and an energy assessor would otherwise have to describe the insulation status of the wall as 'As Built'.

There is no online search facility on the CIGA website (such as the one present on the FENSA website for glazing installations), but they indicate that you can contact them directly to enquire whether a certificate is available for your property. If a certificate is available, then for a fee they can provide a copy of it to you. Their website is at:

https://ciga.co.uk/

Consider for a moment the situation where:

• You home was built with an external cavity wall in 1976 or later, perhaps even up into the 1990s as described above (England & Wales).
• From invasive inspection, perhaps using a borescope, you identify that the cavity is empty - because the builder met the thermal performance requirements of building regulations at the time by using thermal blocks to construct the inner wall, rather than installing CWI.
• Although the wall met building regulations at the time, you would like to install cavity wall insulation now to make the thermal performance of the wall even better.
• You would like to apply for a grant of some sort to help pay for the cavity wall insulation.
• As part of the requirements of your chosen grant scheme, you need a currently valid EPC, and the scheme would ordinarily expect the EPC to identify that the cavity wall is not filled.
• You either commission a new EPC, or perhaps have an existing one, and the assessor has followed the rules properly and recorded the insulation status of the wall as 'As Built', but due to the construction dateband of the building, the RdSAP methodology indicates on the EPC that the wall is 'insulated (assumed)' or 'partially insulated (assumed)', even though it isn't.

This is an interesting situation because firstly, you have an outstanding opportunity. You have a cavity wall that met the building regs at the time, and the EPC rating was calculated using the building regs minimum U value for that dateband. In terms of the modelled performance of the wall in the EPC, this will match that of another building constructed at the same time but where CWI was originally fitted. As a result, the EPC rating is 'correct', but you now have the ability to relatively cheaply add CWI to increase the thermal performance of the wall, wheras the other property with CWI already fitted would have to have revert to Internal or External wall insulation to make any improvement, and that would come at a much greater cost.

Secondly though, you might have a dilema because, the form of grant funding you want to apply for is expecting your EPC to say your cavity wall is 'uninsulated (assumed)'.

This situation is described in a technical bulletin from the accreditation scheme I am a member of (Elmhurst Energy), and they indicate:

"Where a cavity wall has been built post 1976, but inspection by borescope reveals that the cavity is void, a Structural Engineer's Report or report from a qualified body is needed for submission with the EPC when applying for some forms of grant funding. The supplementary report is required to prove suitability of the dwelling for CWI in view of the automated EPC text which cannot be overwritten. The actual build-date must be input accurately and NEVER back-dated to indicate a void cavity".

Where a property is 'determined' as having a cavity wall without full cavity wall insulation, a recommendation 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 for it to be included in the certificate.

The trigger required is:

• Wall Type: Cavity wall, assessed as 'As Built'
• Wall U value (from the above table) is > 0.6 W/m2K

The U value listed here of 0.6 was the minimum requirement introduced in the E&W 1983 building regs. Therefore any 'As Built' cavity wall prior to 1983 will likely trigger the recommendation.

The recommendation suggested is for the wall to become a Cavity Filled Wall.

The improved EPC rating displayed against the recommendation is calculated using the U value from the Cavity Filled wall column from the above table, rather than the 'As Built' column.

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 as follows if I were to have cavity wall insulation installed:

Further information about this particular property is available on our Case Study 2 page.

This improvement of 4 SAP points for my property is pretty reasonable, and for properties where CWI is appropriate, a very useful improvement.

Four SAP points would contribute considerably towards moving up a band, and in my case I would go from a D comfortably into the C band 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 various characteristics of a property affect the amount of gain experienced.

It is always best to make choices from a position of certainty when trying to improve your EPC rating, as this will avoid spending money unnecessarily. A local Domestic Energy Assessor 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, or even a new one, 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 as described in the section above, it will say either 'Insulated (assumed)' or 'partial insulation (assumed)' or 'not insulated (assumed)' depending on the date band assigned to that building part by the energy assessor.

So for properties built from 1983 onwards (England & Wales), 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 1976, it will assume that CWI is not present. Inbetween these dates, it will assume partial insulation.

Check the entry in your 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 datebands applied to 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. Could you possibly provide any proof of the contruction date to your asessor when they arrive? They will usually be very grateful for the information, as dating the contruction of a property 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 a building part).

Retrofitted 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 down into the cavity. We therefore need to have the Finlock cut off and replaced with PVC guttering before we have Cavity Wall Insulation fitted.

There are other scenarios 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 from driving rain.

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 a 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 prior to the retrofitting of CWI.

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 Recomendation that was recently introduced to EPC certificates and which will be removed in the near future.

The current version of the RDSAP methodology is 9.94. In the previous 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.

In the next update in EPC methodology - RdSAP version 10 coming into force on 15th June 2025, this recommendation will be removed and no longer appear on new EPCs.

The rating for my property - a 1950s mid-terrace house, the party wall type has the following effect on my EPC rating:

So for me, not a lot.

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.

For a house that is perhaps fairly narrow, and deeper, and therefore has longer party walls, the difference could 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. 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.

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

Similarly, if you have your property with a cavity wall re-pointed, perhaps due to the property being in a high weather exposure area, then the assessor will also not be able to confirm the presence of retrofitted CWI.

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 dateband of the building as we have already described above.

If you have cavity wall insulation installed whilst you currently own a property, it would be a good idea to 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 contructed prior to 1983 and there are no cavity fill holes visible.

Otherwise, if documentary evidence is still required to prove the presence of CWI, it may be possible to contact CIGA (as mentioned above) to see whether a certificate is already available.

As a last resort, in the case of a wall that has been rendered or re-pointed, you could potentially obtain documentation via someone suitably qualified, conducting an invasive inspection of the wall. This could then be presented to an energy Asessor as documentary evidence when conducting an EPC assessment.

A penultimate thought now, it is possible for a cavity wall to have been built with a better U value that the one that would be chosen from the table above in an RDSAP EPC assessment. The only way I can think of this being evident might be in the case of a new-build property built (or converted) from late 2008 onwards which had a full SAP EPC conducted and where the U value is indicated in the EPC Features table. If this is the case, then it is unlikely that the homeowner will have any documentary evidence to support the value for use in a new EPC using RDSAP. It is possible for the energy assessor to override the U value for the wall in the assssment, subject to specific documentary evidence being available, and it is possible for a person qualified in calculating U values to be able to calculate a value for the owner, based on results from invasive inspection of the property.

Lastly, there are a few properties where wall ties have been retro-fitted to a cavity wall for one reason or another. This could be where existing wall ties were missing, or perhaps decayed over time. Retro-fitted wall ties 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, and the pattern of the holes is different. Care should therefore be taken when checking for the presence of CWI fill holes.

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