Internal / External Wall Insulation

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. 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 (MEES). Methods for improving EPC ratings will therefore change in the future.

Internal Wall Insulation (IWI) and External Wall Insulation (EWI) are two commonly used methods of improving the energy performance of solid brick or stone walls.

They can also be used to further improve the performance of cavity walls, but with these, the approach is normally to fill the cavity with insulation first.

Internal Wall Insulation involves applying insulation to the inside face of your external walls. You retain the original outer face of the building, but you build on the interior side — typically by fixing insulation boards or batts, often within wood battens, then putting up an insulated plasterboard finish.

External Wall Insulation covers your external walls from the outside. Insulation panels are fixed to the exterior brickwork, and then a weather-proof render or cladding is applied over them.

Under Approved Document L (Conservation of fuel and power), Building Regulations apply if you renovate a “thermal element” such as a wall, roof, or floor, over a significant area.

If you add, replace, or upgrade any insulation to a wall for example, whether internally or externally, it’s considered “building work” under the Building Regulations.

This means you must notify Building Control (either through your local authority or an approved inspector), unless the work is carried out by a registered installer under a competent person scheme.

A 25% rule applies - If you renovate more than 25% of the total surface area of a thermal element (for example, external wall), then you are required to notify Building Control and upgrade the insulation to meet certain thermal standards.

If you’re renovating less than 25% of that element, you usually don’t need to notify Building Control specifically for energy performance, although the work must still meet other relevant parts of the Building Regulations.

Contacting Building Control is often overlooked by homeowners upgrading insulation themselves, and also overlooked by many builders.

By operating through Building Control, you will ensure the works you conduct are compliant, and you'll avoid issues when you come to sell the property in the future.

Also a building control completion certificate can be used as documentary evidence that insulation has actually been added to the wall, therefore making it much easier for it to be taken into account in an EPC assessment.

I mention this information in passing only because it relates to solid stone/brick walls, which are the type of walls that we are interested in on this page.

The EPC assessment methodology for existing dwellings was updated on the 15th June 2025 from RDSAP 9.94 to RDSAP 10.

A change was made in the new methodology to how u-values for solid walls are calculated for the early construction datebands.

The following graphs show u-value reference curves for solid walls, comparing RDSAP 10 with the previous methodology, RDSAP 9.94:

There are graphs for Sandstone/Limestone, Granite/Whinstone and solid brick.

For both of the solid stone graphs, where walls are typically 400mm or more in thickness, you can see the newer methodology provides a u-value that is slightly more generous, ie less lossy, than the previous methodology.

For solid brick walls you can see that previously there was a fixed u-value, independent of the wall thickness, but now there is a step change at 280mm from a u-value of 1.7 to 1.4.

Further information to follow shortly.

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