How important is the thickness of loft insulation?

Insulating your loft is one of the quickest and easiest ways to reduce the amount of heat lost from your home.

If you have an empty loft, then the most common method of insulation is to use mineral wool (usually from glass or rock fibre) at floor level, between and above the joists.   The Energy Saving Trust recommends that this mineral wool should be at least 270mm deep in order to be effective.

If you plan to use the loft as living space, then you will want to install insulation directly under the roof tiles rather than at floor level.  In this case, you will use insulating boards fitted between the roof rafters – they can then be covered by plasterboard.

Rafters aren't usually very deep, so to get the best performance you may have to insulate over them as well, using insulated plasterboard.  If there isn't room to do this, make sure you use the highest performance insulation board available.