Where 270mm comes from
The figure is the depth of mineral wool that brings a typical loft floor to a U-value of around 0.16 W/m²K, the target used in Building Regulations guidance for upgraded ceilings. It assumes a common conductivity for glass or rock wool. Materials with a lower conductivity — PIR, or high-performance wool — reach the same U-value at less depth; sheep's wool and some natural fibres need slightly more.
Equivalent depths
| Material | Depth for roughly the same U-value |
|---|---|
| Standard glass/rock mineral wool | 270mm |
| Higher-performance mineral wool | 200–250mm |
| Sheep's wool / natural fibre | 280–300mm |
| PIR board | 120–150mm |
| Blown cellulose | 250–300mm |
When the number changes
- Boarding. Joists are usually 100mm. Boards need raising on loft legs to keep 270mm beneath; otherwise the wool is crushed to 100mm and performs as such. Boarding.
- Funded work. Schemes specify the depth to be achieved; 270mm is typical, some specify 300mm.
- Beyond 270mm. Going to 350mm or more adds little; the money is better spent elsewhere.
Not sure how much is up there?
Check my homeTell us what you can see and we will tell you whether a top-up is worth it.
Questions homeowners ask
Is 300mm better than 270mm?
Marginally. If a scheme specifies 300mm, fine. If you are paying, 270mm is the sensible stop.
Does the depth include the joists?
The depth is measured from the ceiling plasterboard to the top of the insulation, so the joist depth is inside it. Typically 100mm between joists and 170mm across.
Sources and when they were last checked
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026