How much loft insulation do I need?

Why 270mm?

Insulation gets better the thicker it is, but each extra centimetre adds a little less than the one before. Around 270mm of mineral wool is the point commonly recommended in the UK, for example by the Energy Saving Trust, as a good balance for a loft floor. Building regulations set performance targets as a U-value rather than a depth, and the depth needed to hit them depends on the material.

Different materials, different depths

What matters is how well the material resists heat. A material with a lower thermal conductivity (lambda) needs less thickness for the same effect. Rolls of mineral wool, sheep’s wool and recycled polyester all perform in a similar range, so they need similar depths. Rigid PIR boards insulate roughly twice as well per millimetre, which is why they are used where space is tight. See insulation types compared.

What most lofts actually have

Many homes were insulated years ago to 25, 50 or 100mm, the standard of the time. Insulation also settles and gets flattened by boxes and feet. If the top of your insulation is level with the joists, or you can see the joists sticking up through it, it is almost certainly well below 270mm.

Is more always better?

Going well beyond the recommended depth gives diminishing returns, and very deep insulation makes it harder to keep the eaves ventilated and to fit storage above it. The aim is an even layer at the right depth, with no gaps, no squashed patches and the hatch insulated too.

Next: how to check your loft insulation.