At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update is due in Q1 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026.
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:
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 outside 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 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.
A solid brick wall in contrast is usually constructed with bricks laid both lengthways and also across the width of the wall, with the wall being two bricks wide. 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 the 'headers'. The thickness of this type of wall is likely to be around 225mm or possibly 345mm (for a two brick wide solid wall and a three brick wide solid wall).
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 clearly indicates these are not cavity walls.
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.
Having said this however, there are some properties where cavity wall insulation is not appropriate (see notes later).
In the following picture, a single brick has been removed from a cavity wall, just at the point where two 1950s 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 grey colour blocks visible through the hole are the inner wall of blocks.
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 certainly 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.
'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 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 they could meet the thermal requirements by using thermal bocks 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.
When a Domestic Energy Assessor visits your property, and determines a cavity wall exists, the following process is followed to determine whether the EPC assessment proceeds on the basis that cavity wall insuation is present or not:
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 cavity wall in the assessment as: 'Filled Cavity' (There are also options for combined 'filled cavity' plus 'internal' or 'external' insulation too, if that is the case).
On a resulting EPC, the Features table will then contain a line that indicates:
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.
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, re-rendered, or re-pointed, thus hiding the insulation fill holes, and suitable documentary evidence is made available to the assessor proving insulation has been retrofitted.
In the case where the insulation status of a cavity wall is described by the assessor as 'As Built', the EPC methodology proceeds by considering the construction dateband of the wall, and what was the more common construction approach at the time.
The energy assessor asigns a contruction date to each building part. This includes the 'Main Building' plus any 'Extensions' for example.
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 based on the construction databand applied to the building part.
This is despite many forum postings you might come across on the Internet from ill-informed souls thinking that the assessor makes the assumptions. In reality, the souls are making the assumption about the assessors.
In the background, calculation of the EPC rating actually proceeds using a numerical U value for the wall. This is a representation of how fast heat is lost through the wall, and it is expressed in Watts/m2K. The U value used depends on the dateband, wall type, and insulation/build status (unless it is overidden manually by a calculated U value provided by the property owner).
For those that are interested, these 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.
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' (or whichever is now marked as the 'CURRENT' version).
Here are the cavity wall U values for England & Wales (my location) from table S6 with the E&W datebands:
Dateband |
Cavity Wall 'As Built' |
Cavity Wall 'Filled Cavity' |
<1900 | 1.5 | 0.7 |
1900-1929 | 1.5 | 0.7 |
1930-1949 | 1.5 | 0.7 |
1950-1966 | 1.5 | 0.7 |
1967-1975 | 1.5 | 0.7 |
1976-1982 | 1.0 | 0.40 |
1983-1990 | 0.6 | 0.35 |
1991-1995 | 0.6 | 0.35 |
1996-2002 | 0.45 | 0.45* |
2003-2006 | 0.35 | 0.35* |
2007-2011 | 0.30 | 0.30* |
2012 onwards | 0.28 | 0.28* |
* = assumed 'As Built'
With the variation of U values evident within this table dependant on the dateband assigned, it shows the importance of the correct databand being assigned to a building part. The correct dateband ensures a rating is calculated using the most approriate energy performance information.
The 'assumed' cavity wall status as displayed on the EPC is just a noticeable 'visual' indicator of the choice of dateband around the 1976-1982 area.
Where a property is 'determined' as having a cavity wall without 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:
The U value listed here of 0.6 was the minimum requirement introduced in the E&W 1983 building regs.
The recomendation 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 table above.
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:
Wall Type | EPC Rating |
1950-1966 Cavity Wall - As Built | 68D |
1950-1966 Cavity Wall - Filled Cavity | 72C |
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 significant, 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 there are many characteristics of a property that contribute to the amount of gain that will be 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 unecessarily. 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, 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, particularly around the 1976 to 1983 area. 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 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 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 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 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):
Party Wall Type | U-value |
Solid masonry / timber frame / system built | 0 |
Cavity masonry unfilled | 0.5 |
Cavity masonry filled | 0.2 |
Unable to determine, house or bungalow | 0.25 |
Unable to determine, flat or maisonette | 0 |
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):
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 to EPC methodology - version 10 due in Q1 of 2025 (unless it gets delayed again), 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:
Party Wall Type | My EPC Rating |
Cavity - Masonry, Unfilled | 67D |
Unable to determine | 68D |
Cavity - Masonry, Filled | 68D |
Solid Masonry / Timber / System Build | 69C |
So, not a lot.
From 'unable to determine' (which is the type that the new convention allows me to declare) to Cavity Masonry Filled, there is no improvement to the integer value of my EPC rating.
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 you were keen to get another point on your rating.
Always check with a local energy assessor what difference this might make to your EPC rating though, before going ahead with any works.
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.
There is an independent body that provides 25 year guarantees for Cavity Wall Insulation fitted by registered installers in the UK, which might have a certificate available for your property. They are called the Cavity Insulation Guarantee Agency (CIGA).
There is no online search facility on their website like the one on the FENSA website for glazing installations, but they indicate that you can enquire through them whether a certificate is available for your property, and for a fee they can provide a copy of that certficate to you. Their website is at:
Otherwise, if documentary evidence is still required, it may be possible in the case of a wall that has been rendered or re-pointed for documentation to be provided by such persons as a chartered surveyor. This could be via invasive inspection of the wall, which could then be presented to 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.
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