Loft Insulation Between Roof Joists

A cold deck flat roof will typically require insulation to be installed between the roof joists to comply with building regulation requirements.

Loft insulation between roof joists.

But if you plan to use your loft for storage only then a conventional cold roof option where the mineral wool insulation rolls are placed in between the ceiling joists above the top floor. The simplest and cheapest loft insulation solution is the classic cold loft option. You could even buy insulation and floor boards that are already bonded together which makes the job much faster and easier. Lay mineral wool between the joists and then fit rigid insulation boards across the joists with floorboards on top.

If the loft is going to be used as living space then the insulation should be placed at rafter level to create a warm roof. To increase the thickness of your roof insulation layer you can. Rolls of mineral wool can be used if the joists in your loft are regular. Most insulation you buy will be partially perforated allowing you to cut the roll of insulation easily to produce either 2 rolls that are 570mm wide or 3 rolls that our 380mm wide.

But if you simply fill the spaces between the joists with insulation and then board over the top you won t achieve an adequate depth of loft insulation. Unless the joists are exceptionally deep there is also likely to be a requirement to install insulation to the underside of the joists as the upper 50mm of joist space is usually required to be left clear for ventilation purposes. The first layer of wool is placed between the joists and then a second layer is placed at right angles which covers the joists and also makes the insulation up to the correct depth of 270mm. Joists are the horizontal beams which make up the floor.