Insulation for Between and Under Joists

2020 SGIUK Flat roof between and under joists 24B_Flat-Roof_Between-_-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 (such as Celotex XR4000 or GA4000) 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 than 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 rigid PIR insulation board (such as Celotex TB4000 or GA4000) 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 (such as ISOVER Metac, ISOVER Timber Frame Roll/Batt) 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.

Key Considerations

When using Celotex or ISOVER products, you need to satisfy yourself that use of the product meets all relevant national Building Regulations and guidance as well as local, national and other applicable standards relevant for your construction or application, including requirements in relation to fire and applicable height restrictions. In addition to the product datasheet, please refer to the following product documents:

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

Insulation products
Product Name Thickness Range Sizes Lambda
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022
Celotex PL4000 25-65mm (+12.5mm Plasterboard) Width 1200mm, Length 2400mm 0.022
ISOVER Timber Frame Batt 32 50-140mm Width 570mm, Length 1175mm 0.032
ISOVER Timber Frame Batt 40 90-140mm Width 570mm, Length 1175mm 0.040
ISOVER Timber Frame Batt 35 90-150mm Width 570mm, Length 1175mm 0.035
ISOVER Timber Frame Roll 32 90-140mm Width 2 x 0.57m, Length 2.7 - 4.2m 0.032
ISOVER Timber Frame Roll 35 90-140mm Width 2 x 0.57m or 3 x 0.4m (140mm only), Length 4 - 5.3m 0.035
ISOVER Timber Frame Roll 40 90-140mm Width 2 x 0.57m, Length 6.5 - 10.13m 0.040
ISOVER Metac 50-200mm Width 1.2m, Length 3.2 - 9.3m 0.034
ISOVER Timber Frame Batt 43 90-140mm Width 570mm, Length 1175mm 0.043
ISOVER Spacesaver 100-200mm Width 1.16m, Length 5.2 - 12.18m 0.044
ISOVER Spacesaver Lite 100-200mm Width 1.16m, Length 3.5 - 7m 0.044
ISOVER Spacesaver Plus 100-200mm Width 1.16mm, Length 3.5 – 7m 0.040
ISOVER Spacesaver Ready-Cut 100-200mm Width 3 x 0.386m or 2 x 5.8m, Length 5.2 - 12.18m 0.044