Loft boarding and insulation: keeping both

The problem with boards on joists

In a typical loft, the joists are much shallower than the 270mm of mineral wool the Energy Saving Trust (EST) recommends. If you fill the gaps between the joists and screw boards straight onto them, you can only have as much insulation as the joists are deep. The EST says that insulation between the joists alone may not meet modern recommended levels. Add a second layer and then press boards down on it, and you squash it, and the EST is clear that compressing mineral wool reduces its effectiveness.

A loft boarded straight onto the joists has exactly this problem: a thin layer under the boards, and a thicker layer elsewhere.

The EST’s method: raise the floor

The EST describes the fix in four steps:

1. Insulate between the joists as normal.
2. Raise the floor slightly, using supports fixed to the joists, so the new floor sits above the height of the full insulation.
3. Add a second layer across the tops of the joists, underneath where the raised floor will go.
4. Fit the boards on the supports, leaving a small gap between the insulation and the underside of the boards for ventilation, to prevent condensation.

The result is a storage platform with full-depth insulation beneath it, and an air gap that lets moisture escape.

Planning the storage area

Board only what you need. Most people use the area within reach of the hatch, and the rest of the loft is rarely visited. A smaller platform costs less, keeps more of the loft at full depth, and leaves the eaves clear. Keep heavy items near the walls or over supports, and do not store anything on the unboarded insulation, which flattens it just as boards do.

Check the loft can take the weight before you add storage. If in doubt, ask a builder or surveyor.

Boards, schemes and funded installers

If your loft is being insulated through a scheme, boarded areas need a conversation before the visit. Ofgem’s ECO4 guidance records the existing insulation in a pre-existing loft declaration, and insulation hidden under old boards may not be visible to the assessor. Decide whether the old boards will be lifted, whether the installer will relay them, and whether raising the floor is included. A funded job may insulate the open areas and leave boarded areas as they are, which may be fine for a small platform and less fine for a fully boarded loft.

Doing it yourself

A raised storage platform is within reach of a competent DIYer in a dry loft. Loft-leg supports and boards are widely sold, and mineral wool rolls are standard DIY stock; bought materials carry standard-rate VAT, while insulation installed by a contractor is zero-rated until 31 March 2027. Our DIY guide covers laying the layers, and loft ventilation explains why the air gap and the eaves matter.