TGD L 2011 – B.5.3 – ROOF CONSTRUCTIONS,Construction R3: Flat roof, timber joists, insulation below deck (Cold Deck)

B.5.3 R3: Insulation between and below joists, 50 mm air gap between insulation and roof decking Installation guidelines and precautions The insulation is installed in two layers, one between the joists, and the second below the joists. A ventilated air space as specified in Technical Guidance Document F should be provided above the insulation. Cross ventilation should be provided to each and every void. When installing the insulation, care is needed to ensure that it does not block the ventilation flow paths. The roof insulation should connect with the wall insulation so as to avoid a cold bridge at this point. The lower layer may be in the form of composite boards of plasterboard backed with insulation, with integral vapour barrier, fixed to the joists. The edges of boards should be sealed with vapour-resistant tape. Where the thermal conductivity of insulation between and below the joists is different, the material on the warm side (i.e. below the joists) should have a vapour resistance no lower than that on the cold side (i.e. between the joists). It is preferable if the insulation on the cold side is more permeable than that on the warm side (e.g. mineral wool outside with expanded polystyrene inside).

C:BCARTGDtgd LTGD%20L%20DWELLINGS%20-%202011TGD%20L%20-%20DWELLINGS%202011%20-%20APPENDIX%20B%20Diagram%20B5.pdf