How much does cavity wall insulation improve an EPC

How much does cavity wall insulation improve an EPC rating?

Filling an empty cavity usually improves an EPC rating because the assessment gives a filled cavity wall a much lower heat-loss value. How far the rating moves depends on the home's size, heating system and other insulation, so the EPC's own recommendations are the best guide.

By the Cavity Wall Insulation UK editorial team · checked against the official sources below · last reviewed 19 October 2026

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Check what your EPC assumed

Find your certificate on the government register and look at the walls line. If it says the cavity is filled but you are not sure, a borescope check settles it. If it says 'as built, no insulation (assumed)', a fill should be recorded on the next certificate once your paperwork is shown to the assessor.

What else moves the rating

Loft insulation, heating controls, the boiler or heat pump, glazing and lighting all count. A cavity fill and loft top-up together are often the cheapest pair of measures.

Related services

Sources: GOV.UK: Warm Homes Plan; Energy Saving Trust: cavity wall insulation.

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