The majority of houses are insulated above the ceiling of the upper floor with a ventilated roof void. Any habitable accommodation in this roof space will therefore need to be insulated between the accommodation and the roof covering.
'U' values required for thermal insulation of a new storey
- Roof with a flat ceiling underneath: no more than 0.16 W/m²K
- Roof with a sloping ceiling underneath: no more than 0.20 W/m²K
- External walls (including the side walls between the room and the unheated roof void): no more than 0.35 W/m²K
- New windows and roof lights: no more than 1.8W/m²K
Materials required to provide adequate thermal insulation
- To achieve a 'U' value of 0.16 W/m²K to the flat ceiling, mineral fibre insulation at least 150mm thick between the joists, with another 100mm thick layer covering the joists, will be sufficient
- To achieve a 'U' value of 0.20 W/m²K to the sloping ceiling, rigid urethane foam (Celotex, Kingspan, etc.) at least 100mm thick between the rafters, and 40mm thick under the rafters, will be sufficient.
Advice on providing ventilation to the roofspace above the insulation
- To achieve a 'U' value of 0.35W/m²K for the dormer cheeks and for the side walls between the room and the unheated roof void, mineral fibre insulation at least 125mm thick, or 90mm rigid urethane foam will be sufficient. Mineral fibre should not be compressed to fit into a frame, as this reduces its insulation.
- If a multifoil type insulation is used in the roof, additional insulation will be required to achieve the necessary 'U' value.
Other materials may be used, but the thickness needed depends on their thermal conductivity.
- To achieve a 'U' value of 1.8 W/m²K for the windows and roof lights, these will need to be low-E glass with a 16mm gap between panes. The area of windows and roof lights is limited to the equivalent of 25% of the new floor area.