Loft Insulation Depth: Why 270mm Is the UK Building Regs Benchmark
UK building regs expect roughly 270mm of mineral wool loft insulation for a 0.16 U-value. Here is what the depth means, what your quote should specify, and how to check it.
Loft insulation is the highest-return energy upgrade most homes can make, and it is also one of the most common things a builder's quote gets vague about. "Insulate loft" tells you nothing. The number that matters is the depth, because depth is what turns into a U-value, and the U-value is what the building regulations actually care about. Here is the benchmark, why 270mm is the figure you keep seeing, and how to check your quote meets it.
The benchmark: 270mm of mineral wool
The current target for an insulated loft in an existing home is a U-value of 0.16 W/m²K. With standard mineral wool (glass or rock wool), reaching 0.16 takes roughly 270mm of insulation: about 100mm laid between the ceiling joists and a further 170mm laid across them. New-build roofs are built to a tighter 0.11 W/m²K, which needs more depth or higher-performance board.
The 270mm figure is not a law in itself. It is simply the thickness of common mineral wool that hits the 0.16 U-value the regulations expect. If your installer uses a higher-performance material such as rigid PIR board, they can reach the same U-value in less depth (around 120mm of PIR is roughly equivalent), which matters where headroom is tight, like a room-in-roof.
This is the loft equivalent of the cavity wall insulation thickness question, and the two together cover most of the heat a typical house loses.
Why depth beats "insulated"
A ceiling with 100mm of old insulation loses far more heat than one with 270mm. The first 100mm does a lot of work, but the regulations target assumes the full modern depth. Two practical points:
- A cross-layer beats a single deep layer. Laying the top 170mm across the joists (rather than all of it between them) covers the timber, which itself conducts heat. This is why the standard spec is "100mm between, 170mm over".
- Compression kills performance. Mineral wool only works at its rated thickness. If it is squashed under boarding or storage, it loses much of its value. Raised loft boarding on legs is the correct fix if you need storage up there.
The ventilation gap people forget
Insulation and ventilation have to coexist. Push insulation hard into the eaves and you block the airflow that keeps the loft dry, which leads to condensation and, over time, damp timbers.
Check that the quote keeps a clear ventilation gap at the eaves (commonly 25-50mm) and fits eaves baffles or vents where needed. "Insulate to 270mm" with no mention of eaves ventilation is an incomplete spec, especially on an older roof.
When the regs actually bite
For a simple top-up of an open loft, you are improving the home rather than triggering a formal building control sign-off. The regs become directly relevant when insulation is part of a bigger job:
- A loft conversion must insulate the new roof slopes and walls to current standards. See loft conversion costs for the full picture.
- A re-roof is the moment to upgrade insulation, and many quotes quietly leave it out to look cheaper.
- Any work where you claim a grant or an EPC improvement needs the spec and paperwork to match, or it will not count.
If your quote covers a loft conversion or re-roof and says nothing about insulation depth or U-value, that is a missing building regs item, not a saving.
What your quote should specify
A clear loft insulation quote names:
- The material (mineral wool, sheep's wool, PIR board)
- The finished depth or the target U-value (270mm / 0.16, or better)
- Whether it is a top-up or a full replacement
- Eaves ventilation and baffles
- Loft boarding on raised legs if storage is needed
- Loft hatch insulation and draughtproofing (a cold spot people forget)
Grants in 2026
Loft insulation is one of the measures supported under the government's Warm Homes Plan, which is replacing the older ECO and GBIS schemes through 2026. Eligibility depends on your income, benefits, and property, so check the current scheme before assuming you qualify, and make sure any grant-funded work still specifies the depth and ventilation above.
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