Products & Applications

Between Rafters

Insulating a pitched roof between the rafters creates a warm roof space in a new or existing building where the ceiling is to be attached directly to the underside of the rafters.

This warm roof space can be used as additional living or storage space, adding extra value to the property. To avoid condensation occurring a 50mm ventilated air space between the insulation and traditional sarking felt is required by the Building Regulations/Standards for 'cold roof' construction.
For this application we would recommend the use of Seconds Sarking Board.

Fixing details

To maintain a 50mm ventilated void above the insulation, side nail battens to the rafters to provide a 'stop'. This will prevent the insulation boards moving into the void (see fig. 1 & 2).
Boards that have been cut individually to fit the rafter spacings may be temporarily fixed to the battens using large headed clout nails.
Ensure accurate trimming to achieve close butting joints and continuity of insulation. It is advisable to measure the space between the rafters before cutting the boards as spacings can vary.
An additional restraint to the boards will by a plasterboard lining fixed to the underside of rafters.
The use of polythene vapour control layer on vapour check plasterboard should be considered to minimise the risk of condensation.
If the rafter depth cannot accommodate the thickness of insulation required and maintain a 50mm ventilation void, timber battens can be fixed to the inner face of the rafters to increase their depth.
To avoid a thermal bridge through the rafter, an insulated plasterboard can be adopted beneath the rafters.
Ventilation should be provided in accordance with Approved Document F, F2 (Condensation in Roofs) of the current Building Regulations or Technical Standard K (Ventilation of Buildings, Regulation 23) of the Building Standards (Scotland).
Boards should be cut using a sharp knife or a fine toothed saw.
To prevent a cold bridge between the cavity closer and the rafters, a vertical section of insulation should be installed between the rafters above the cavity closer.