How we get our loft insulation figures
Every insulation figure on this site comes from one public dataset, the Energy Performance of Buildings register, and we worked it out ourselves rather than repeating somebody else. This page is the method, so you can check it instead of taking our word for it. It is written for anyone who wants to quote a number from here and needs to know what it actually counts.
We read the register on 23 September 2026. The certificates in it were issued between 14 October 2004 and 9 September 2026.
Source: Energy Performance of Buildings Register, bulk domestic download, analysed September 2026. Depths are as recorded by the assessor, not as measured by us. Licensing: the register's own licensing restrictions, read 22 September 2026.How a figure on this site is made
Six steps, in this order. The order matters: doing the third one after the fourth changes the answer by thousands of homes.
- 1
Download the whole register
The government publishes every domestic Energy Performance Certificate as a bulk download. The extract we hold carries 3,896,340 certificates across 54 councils.
- 2
Keep the six councils we work in
Cambridge, South Cambridgeshire, East Cambridgeshire, Huntingdonshire, Fenland and Peterborough. That leaves 451,464 certificates. Everything on this site describes those six and nowhere else.
- 3
Turn certificates into homes
A house that has been sold three times has three certificates. Counting certificates would count that house three times, so we keep one row per address and take its most recent certificate. We match on the property's UPRN where the register has one, and otherwise on postcode, floor area and roof description together.
- 4
Keep houses and bungalows
Flats and maisonettes are dropped: the loft above a flat is not the resident's to insulate. That leaves 248,651 homes, and that is the number every share on this site is worked out from.
- 5
Read the depth out of the assessor's own words
The register does not hold a tidy number. It holds the assessor's sentence, such as Pitched, 270 mm loft insulation, and we take the figure out of it. Where the sentence says there is no insulation we record nought. Where it does not give a depth at all, the home has no depth, and we say so rather than guessing one.
- 6
Only then work out the share
Every percentage on this site says how many homes it was worked out from. We will not publish a share from fewer than 30 homes. Below that you get the counts and no percentage, because a percentage of a handful of homes reads as authoritative and is not.
What we leave out, and why
This is the part that decides whether a figure means anything. Every exclusion below makes our own numbers look no better, and several make them look worse.
- 70,311 homes with no depth written down. They are counted as homes and left out of every depth figure. They are not assumed to be badly insulated, and they are not assumed to be well insulated either.
- 5,920 homes with a room in the roof. There is no loft to measure and the insulation sits in the slopes.
- 2,057 homes with a flat roof. No loft at all.
- 570 thatched homes. A specialist trade, and not comparable to a tiled pitched roof.
- Any group of fewer than 30 homes. Counts only, never a share.
We also do not publish anything at postcode or street level. The address fields in this register are licensed separately from the rest of it, and a single home's energy data is personal data. Our area figures are aggregates of a town or a council, which is a different thing.
71.7% of the 248,651 homes we count have a loft depth written on the certificate by an assessor. Every share on this site is worked out from those 178,340 homes, never from all 248,651.
What this data cannot tell you
Four limits. We would rather name them than have somebody else find them.
A home only has a certificate if it has been sold, rented out or newly built since 2008. Homes that have stayed in the same hands for thirty years are missing, and those are not a random sample of the housing stock.
The depth is what an assessor wrote on the day, which may be years ago. On 27,797 of these homes the assessor's own wording says the roof was assumed rather than inspected.
Certificates issued in 2026 are not the same homes as certificates issued in 2014, so a change between two years is not the same homes improving. Where we wanted to ask whether homes improved, we compared homes against themselves.
A town figure describes a town. It says nothing about one address, and it is not a prediction about yours. The checker reads your own certificate instead.
All non-address fields are published under the Open Government Licence v3.0. The address fields are not, and carry Ordnance Survey and Royal Mail rights.
Energy Performance of Buildings data: licensing restrictions, read 22 September 2026
Questions people ask about these figures
Can I check any of this myself?
Yes, and that is the point of publishing the method. The register is a free public download from the service linked above. Take the six councils named in step two, keep one certificate per address, keep houses and bungalows, and read the depth out of the roof description. You should land on 248,651 homes.
One of your numbers is slightly different in two places. Why?
Because we will not quietly change a published figure to make it tidy. The count of homes formally told to increase their insulation to 270mm reads 29,189 on our pages. Re-running the same work from a later copy of the same data gives 29,207, which is 18 more, a difference of 0.06%. We publish the lower, original figure until we have established which copy is authoritative, and we would rather tell you the gap exists.
Do you keep my postcode when I use the loft checker?
No. The checker looks the address up and shows you the answer. The postcode and the address are not logged, not stored and not sent anywhere, and there is no analytics of any kind on that page.
How often is this updated?
The register is updated by the government through the year. We last read it on 23 September 2026 and every figure on the site carries that date in our own records. We are not promising a schedule we have not kept yet.