How deep should loft insulation be? A plain answer
For mineral wool, around 270 mm is the depth commonly recommended. Here’s what that means in your loft, and how other materials compare.
| Recommended depth (mineral wool) | 270mm in total |
|---|---|
| Made up of | A layer between the joists plus a layer across them |
| Other materials | Depth depends on their thermal conductivity |
The short answer
Around 270 mm of mineral wool, laid in two layers: one between the joists and one across them. That’s the figure you’ll see quoted most often in the UK.
Why two layers
Joists are usually about 100 mm deep. A single layer between them leaves the timber exposed and well short of the depth you want. A second layer across the top covers the timber and brings the total up.
Other materials
Sheep’s wool and recycled polyester perform about the same as mineral wool for the same depth, so the answer is similar. Boards and foil work differently and are used where depth is limited.
Common questions
Can I have too much?
How do I measure mine?
Find out what your loft needs, in plain words
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