Celotex Isover

Insulation for Between and Under Joists

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

Either a rigid PIR board from Celotex can be cut to an exact fit and installed between the joists, or a more flexible glass mineral wool (GMW) roll or slab from ISOVER can be cut slightly oversize (i.e. 10mm wider then necessary) to ensure a friction fit when installed between the joists.

As for insulating below the joists, an insulating laminate (insulation bonded to plasterboard) such as Celotex PL4000 or a separate rigid PIR insulation board finished with a separate sheet of plasterboard can be securely fixed to the joists to provide an internal finish to the roof. Alternatively, battens can be secured to the underside of the joists and GMW fitted between these in much the same way as between the main roof joists. The underside of the rafters in this case will need to be finished with a suitable vapour control layer (VCL) and plasterboard to provide a suitable internal finish to the roof.

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.


Insulation products to consider
Product Name Thickness Range Sizes Lambda BBA
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex PL4000 25-65mm (+12.5mm Plasterboard) Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
ISOVER Spacesaver Plus 100-200mm Width 1160mm, Length 3500–7000mm 0.040 W/m.K Tick Icon
ISOVER Spacesaver 100-200mm Spacesaver: Width 1160mm, Length 5200-12180mm | Spacesaver Ready-Cut: Width 3x386 or 2x580mm, Length 5200-12180mm | Spacesaver Lite: Width 1160mm, Length: 3500-7000mm 0.044 W/m.K Tick Icon
ISOVER Timber Frame Batt 32 50-140mm Width 570mm, Length 1175mm 0.032 W/m.K Tick Icon
ISOVER Timber Frame Batt 40 90-140mm Width 570mm, Length 1175mm 0.040 W/m.K Tick Icon
ISOVER Timber Frame Batt 35 90-150mm Width 570mm, Length 1175mm 0.035 W/m.K Tick Icon
ISOVER Timber Frame Batt 43 90-140mm Width 570mm, Length 1175mm 0.043 W/m.K Tick Icon
ISOVER Timber Frame Roll 32 90-140mm Width 2x570mm, Length 2700-4200mm 0.032 W/m.K Tick Icon
ISOVER Timber Frame Roll 35 90-140mm Widths 2x570mm, 3x400mm (140mm only), Length 4000-5300mm 0.035 W/m.K Tick Icon
ISOVER Timber Frame Roll 40 90-140mm Width 2x570mm, Length 6500-10130mm 0.040 W/m.K Tick Icon
ISOVER Metac 50-200mm Width 1200mm, Length 3200-9300mm 0.034 W/m.K Tick Icon