Which? updates loft insulation costs and savings guidance

Which? published updated guidance on loft insulation costs, savings and payback on 5 May 2026, drawing its savings figures from the Energy Saving Trust and its cost data from the Royal Institution of Chartered Surveyors¹. The consumer body said loft insulation could reduce energy bills by up to £340 per year, depending on the home¹.

The savings figures cover a typical gas-heated home in England, Scotland and Wales and are based on fuel prices as of July 2024¹. They are split between insulating a loft from scratch and topping up existing insulation to the recommended 270mm depth¹.

Which? said professionally installed loft insulation will typically take a couple of years to pay for itself through heating bill savings when a home goes from no insulation to the recommended amount¹. The recommended thickness is 270mm¹. The cost tables, based on glass mineral wool quilt laid over and between the joists at 270mm thick, are behind a Which? membership login and the prices are not shown publicly; the underlying RICS data is current as of November 2024¹. Which? also said loft insulation typically costs around £20 for a 100mm-thick roll designed to cover 8.3m², and that some energy suppliers still offer free insulation to households meeting certain criteria, such as being in a vulnerable situation or receiving certain benefits¹.

"Professionally installed loft insulation will typically take a couple of years to pay for itself through the savings you’ll make on your heating bills if you go from no insulation to the recommended amount."

The guidance notes that where a loft is used for storage, the required 270mm of insulation over the joists would prevent boards being laid on top, and sets out two approaches: insulating between the joists with mineral wool and laying rigid insulation boards and wooden floor boarding on top, or raising the floor level to fit enough mineral wool beneath it¹. It warns that squashing the mineral wool when fitting boards on top will reduce its insulation properties¹. Where a loft has been converted into living space, the roof itself must be insulated instead¹.

On energy performance, Which? said an Energy Performance Certificate rating runs from A, the most efficient, to G, the least, and that a seller in England or Wales must obtain an EPC, as must a landlord marketing a property for rent¹. It said insulation is a good way of improving a home's EPC rating¹.

Why it matters for households

Loft insulation is one of the few measures where the payback arithmetic is short enough to matter within a typical household's planning horizon: Which? puts it at a couple of years when a home starts with nothing¹. The gap between the two savings columns is the practical point. A home with no loft insulation at all stands to gain several hundred pounds a year, while a home already at 120mm gains £17 to £30 a year by topping up to 270mm¹. The same physical work produces very different returns depending on what is already in the loft, which is why the guidance begins by asking households to check whether insulation is present and how much¹.

For a home's energy independence, loft insulation reduces the heat that has to be bought in the first place, so a given level of warmth depends on less purchased gas or electricity. The figures carry two caveats: they assume a gas-heated home and July 2024 fuel prices, so the cash amounts will move with the price cap and with how a home is heated¹. The cost side is also incomplete in the public version, since the RICS-based price tables sit behind a membership login¹.

What happens next

No dated next steps are set out in the guidance. Which? directs households to its separate pages on insulation grants and on types of loft insulation, and notes that some suppliers still offer free insulation to households meeting certain criteria¹.

Sources1 cited

1. Loft Insulation Costs And Savings - Which?, which.co.uk