Insulation is one of the cheapest upgrades for energy savings, and one of the easiest to under-order.
Why 270 mm?
For mineral wool lofts, 270 mm is the widely recommended depth and gives an insulation U-value of around 0.16 W/m²K. Many older lofts have 100 mm or less, which loses far more heat.
Two layers, crossed
The first layer goes between the joists, usually 100 mm to match joist depth. The top-up layer, often 170 mm, goes across the joists at right angles. This covers the joists themselves, which are a cold bridge.
Counting rolls
Rolls = layers × loft area ÷ coverage per roll. Coverage depends on thickness: roughly 8.3 m² for 100 mm, about 6 m² for 150–170 mm and about 5.6 m² for 200 mm. Check the pack.
Things that go wrong
- Blocking eaves ventilation. Fit rafter trays or eaves carriers first.
- Squashing insulation under boards. Compressed insulation loses much of its value.
- Covering recessed lights not rated for contact with insulation.
- Forgetting the loft hatch: insulate and draught-seal it too.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.