The Energy Saving Trust has updated the cost and savings figures in its Cavity Wall and Solid Wall guides, with the revised numbers dated May and June 2026. The figures are carried in a comparison of wall insulation types published by Uswitch, which states that the average costs and savings come from those two Energy Saving Trust guides and are based on a semi-detached dwelling with gas heating1.
The comparison sets average installation costs against annual savings and disruption for three wall types. Cavity wall insulation is the cheapest and the least disruptive, while internal solid wall insulation is the most disruptive of the three1.
| Measure | Average cost | Annual savings | Disruption |
|---|---|---|---|
| Cavity wall | £2,700 | £200 | Low |
| External solid wall | £15,000 | £280 | Low |
| Internal solid wall | £12,000 | £280 | High |
Average costs based on a semi-detached dwelling, from the Energy Saving Trust's Cavity Wall and Solid Wall guides, updated May/June 2026, based on gas heating1.
The same page notes that savings may be higher where a home is heated by something other than gas1. It also states that around a third of heat in most homes is lost through the walls, and that cavity walls lose less heat than solid walls and are much easier to insulate1.
Separate cost estimates from Checkatrade, also cited for 2026, are higher than the Energy Saving Trust averages for cavity wall work: £1,800 for a mid-terrace rising to £4,600 for a detached house1. For solid walls, Checkatrade estimates external wall insulation on a mid-terrace starts at £7,200 including VAT and can exceed £36,000 for a large detached house1. The page does not reconcile the two sets of figures, and the Energy Saving Trust averages are given for a semi-detached dwelling rather than by property type.
On payback, the page says the cost of cavity wall insulation should be recovered in around 9 to 12 years depending on property type, based on the Energy Saving Trust estimates1. It adds that reputable cavity wall insulation should carry a 25-year guarantee, backed by the Cavity Insulation Guarantee Agency or an equivalent independent insurance-backed scheme1.
"Whichever type of walls you have, always find a registered and insured insulation expert and ask for a 25-year guarantee."
Why it matters for households
Wall insulation is one of the few home energy measures where the saving is largely independent of how the home is heated, because it reduces the rate at which heat leaves the building rather than changing the fuel used. The updated figures give a clearer picture of the trade-off: cavity wall insulation costs a fraction of solid wall work and saves £200 a year on the stated basis, while solid wall measures save £280 a year but cost between four and six times as much1.
For a household weighing up how much insulation saves on energy bills, the practical point is that the type of wall a home has is not a choice. Solid walls are usually found in homes built up until the 1930s, cavity walls in those built after, and the two need different approaches1. Homes built in the last couple of decades are likely to have had cavity insulation installed when they were built, so the first step for many owners is establishing whether the work has already been done1. The page lists physical checks, including drill holes refilled with mortar at roughly waist height and insulation spilling into the loft at the tops of walls1.
The figures also bear on how much could be saved by improving a home's energy efficiency overall, since wall losses are the largest single fabric loss cited at around a third of heat in most homes1. A 25-year guarantee matters for cavity wall insulation problems, guarantees and claims, because it is the mechanism that covers defects long after the installer has left.
What happens next
No further update dates have been reported beyond the May and June 2026 revisions to the Energy Saving Trust guides1.
Sources1 cited
- Solid and cavity wall insulation - Uswitch, uswitch.com