Will Cavity Wall Insulation Improve My EPC?

By Ian on Saturday 6th January 2024

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We complete hundreds of EPC assessments per year and see every kind of home construction. You may be thinking of adding cavity wall insulation to your home to save a bit of money on your energy bills. How much difference will this make though? We’ll have a look at the technical side of cavity wall insulation as well as how it affects your EPC.

Table of Contents

What is a Cavity Wall

Lets start by looking at the two most common type of wall constructions, solid and cavity. Although you would hope all of the walls in your house are pretty solid, when we say ‘solid’ or ‘cavity’ wall, we’re really talking about how many layers of brick your walls have.

Solid Walls

A solid wall is just one layer of brick. In a typical solid wall construction, one brick connects the external and internal surfaces of the wall. This was a really common method of building walls prior to 1930. When we do an EPC, we identify this kind of wall by looking at the brick pattern. We’re looking for a combination of stretchers (the long side of the brick) and headers (the short side of the brick).

‘English Garden Wall Bond’ solid brick wall. Will be noted as solid brick on the EPC.

As there is no cavity in this wall, cavity insulation is not possible, but you can add either external solid wall insulation or internal solid wall insulation.

Cavity Walls

A cavity wall differs as it is made with two different layers of wall that create a void in the middle. This was originally two layers of brick, introduced in the early 1900s and becoming common building practice in the late 1920s. More recently the internal layer is constructed from an aerated concrete block. This void helps keep houses warmer because it creates a buffer between the ambient air inside a house and the cold air on the outside.

Basic diagram of a cavity wall showing the external layer of brick, the cavity and the internal layer.

When completing EPCs, we identify this type of wall from the brick pattern again. This wall is made up of stretchers only, no headers. It is by far the most common wall construction in use today for dwellings.

This wall would be entered as a cavity during the EPC. Identified from the stretcher pattern.

What difference does a cavity wall make compared to a solid wall on the EPC? We recently completed an EPC on a semi detached house in Paddington, Warrington. It was constructed with an uninsulated cavity wall and came out with 55 points, a band D. If we go back and tweak the construction to a solid wall, the house drops to 53 points and falls down to a band E.

What is Cavity Wall Insulation

With a cavity wall, insulation can be added to the void between the two layers of brick further improving its thermal resistance (U Value). Modern cavity wall insulation is usually made of small polystyrene balls and is pumped into the cavity through drilled holes. Modern houses will have cavity wall due to building regulations. Sometimes this insulation is in the form of rigid insulation boards like Kingspan.

If you’ve had cavity wall insulation retrofitted to your older cavity walls, its important to keep hold of the paperwork. The first reason for this is that some of the older cavity insulation retrofits cause damp problems in peoples homes. This is when a mineral wool style insulation was used which soaked up any ambient moisture or water ingress through leaks and transferred this to the internal layer of the wall. Keeping a record of this for any potential insurance claim is important. The second reason to keep your paperwork is because of your EPC. If we can’t evidence the upgrade, we can’t put it in to the EPC. Sometimes we can see the injection holes that we’re drilled and patched, but sometimes its a little trickier, especially if any repointing work has been done.

This is the pattern of drill holes we’d expect to see. If we see these, we’d mark this as having cavity wall insulation.

What difference does having cavity wall make to the EPC? Lets revisit the EPC from earlier, a semi detached that has no cavity wall insulation that got 55 points on the EPC. If we tweak the inputs to mark the walls as having insulation, this boosts the score to 62 points and much closer to a Band C. With other energy upgrades, they may get there. There is not many other areas on the EPC where these kind of score boosts occur. This may not be a typical increase for all homes though. Many factors contribute to EPC scores. Remember this is and old semi with three heat loss walls so its effects will be elevated on a home like this.

How to Get Cavity Wall Insulation

There have been problems in the past with cavity wall insulation causing damp in peoples homes with insurance companies forking out for repair costs.

Its really important that you choose a reputable installer that provides and insurance backed guarantee. CIGA (Cavity Insulation Guarantee Agency) has a list of these installers that tick all the boxes.

You need to get planning permission to install cavity wall insulation unless you go through a member of the CIGA Scheme.

Cavity Wall Building Regs

When it comes to the currently building regulations and energy saving, they are focused around thermal resistance. This is measured by something called a U Value. The lower the U Value a wall has, the less heat will be lost through it. The standard U Values on the EPC are as follows for walls

  • Uninsulated Solid Wall – 1.7 W/m2/K
  • Uninsulated Cavity Wall (1930s) – 1.5 W/m2/K
  • Filled Cavity Wall (1930s) – 0.7 W/m2/K

Current building regulations require all new build homes to have walls that do not exceed 0.18 W/m2/K. Developers will achieve this through not only insulation but the thermal properties of the bricks and blocks themselves.


The type of walls your house is made of can make a difference to your EPC. The biggest difference will come if that wall is insulated or not. One one EPC the difference between a solid and cavity wall was 2 points. The difference between an uninsulated and an insulated cavity wall was 7 points. It’s an upgrade that will save you money, reduce your carbon footprint and potentially increase the value of your home.

About the Author

About Ian Kay

Ian is a seasoned energy assessor helping his customers, reduce energy usage and save money on their bills. He holds qualifications from both City & Guilds and ABBE for Domestic EPCs and Commercial EPCs (Level 3 NDEA and Level 4 NDEA). With a background in the building trade he can provide his customers with a unique perspective and advice. Ian combines his deep industry knowledge with practical advice to create blog posts that help visitors understand the complexities of energy performance certificates and reducing operating costs. When not immersed in the world of energy assessments, Ian enjoys exploring the great outdoors and spending quality time with his family.

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