Can you board over loft insulation?

You can, but not by laying boards straight onto the joists. At full depth the insulation stands well above them, and squashing it under boards takes away much of its effect.

The problem with boarding on the joists

Ceiling joists in most lofts are around 100 to 150mm deep. The recommended depth of mineral wool is 270mm. Boards laid straight onto the joists leave room for the first layer only, so either the insulation is left thin under the boards, or it is crushed down to fit. Crushed insulation has had the air squeezed out of it, and the air is what does the insulating.

Raised boarding

The answer is to lift the boards. Raised loft boarding uses legs or stilts fixed to the joists to hold a floor above the full depth of insulation, with an air gap under the boards. You keep both: full insulation and a storage deck. See raised loft boarding.

What to check first

The joists. Ceiling joists were designed to hold up a ceiling, not a store room. We check their size, spacing and span against what you plan to store.
Trussed rafters. In many homes built from the 1960s onwards, the timbers can’t be cut or moved and storage is limited. See 1960s to 1980s houses.
Ventilation. Boards shouldn’t block the eaves or seal the insulation in completely.
Access. A ladder and a decent hatch make storage usable.

Already boarded?

See insulating a loft that’s already boarded.

Common questions

Can I use thicker joists to raise the boards?

Adding timbers across the joists is another way to raise the floor. It needs to be done properly so the loads go into the joists safely. Purpose-made raised boarding systems are usually simpler.

How much of the loft should I board?

Only what you need for storage. Boarding the whole loft adds cost and weight, and leaves the eaves harder to reach.

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