Insulating the roof void is a key element of any energy efficient build. The minimum thickness of roof insulation we recommend in a non-habitable roof is 400mm laid in 3 layers, and it is important to ensure the job is well installed joining with wall insulation to form a 'complete' wrap.

Technical information

The total thickness of roof insulation we recommend is 400mm and laid in 3 layers. The first layer is laid between the ceiling joists and should be the same depth as those joists. The second layer should be laid on top and perpendicular to the first layer. The third layer will then be laid perpendicular to the second layer. The insulation should continue over the internal skin of the external walls and continue down to touch the cavity insulation, forming a complete insulated wrap of the house.

Ensure void between rafters and gable walls are filled with insulation to 500mm above the top of the main roof insulation to avoid heat loss.

The roof space above the insulation should be adequately ventilated using a universal roof vent system to allow for the removal of built up moist air. Breathable roof membrane should be used to further reduce the risk of condensation build up.

This is one solution for a cold roof but there are other solutions availiable. Please view our standard construction details section.

How to install

This animation illustrates the Travis Perkins plc recommended installation for a non-habitable roof insulation with a good balance between cost of the solution and thermal efficiency, using traditional building materials.

The roof construction consists of 400mm insulation, ceiling joists and 12.5mm plasterboard. This construction achieves a u-value of 0.11W/m²K. The construction has been accredited by LABC.

Watch video

Hints & Tips

Although not harmful, when dealing with fibre insulation, gloves and respiratory protection should be worn to avoid irritation.

Make sure insulation is fitted snuggly between joists and butted up over joist to avoid any air gaps that can contribute to unwanted heat loss.

Adequate ventilation space should be provided around recessed ceiling lights. Alternatively, proprietary recessed light protection boxes should be installed.

All electrical cables within the roof space should either be tacked to the sides of joists or laid on top of insulation. Do not encase cables within the insulation.

Ensure all pipework within the loft space is fully lagged with pipe insulation.

Ensure any loft hatches are well sealed and are themselves insulated.