The majority of houses are insulated above the upper floor ceiling with a ventilated roof void over. Any habitable accommodation in this roof space will therefore need to be insulated between the accommodation and the roof covering.
The various elements of the new storey will need to achieve the following 'U' values:
- Roof with a flat ceiling under - no more than 0.16 W/m²K,
- Roof with a sloping ceiling under - 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, and
- Windows and rooflights - no more than 1.8W/m²K for new windows and rooflights.
- 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 (e.g. Celotex, Kingspan, etc.) at least 100mm thick between the rafters and 40mm thick under the rafters will be sufficient. The advice on providing ventilation to the roofspace above the insulation should be followed.
- 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 its thermal conductivity.
- To achieve a 'U' value of 1.8 W/m²K for the windows and rooflights, these will need to be low-E glass with 16mm gap between panes. The amount of windows and rooflights is limited to the equivalent of 25% of the new floor area.
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