Can insulation be placed between and over flat roof joists?
BS 5250:2011, the British Standard for the control of condensation in buildings, does not recommend installing insulation between timber joists as well as above the deck.
However, not all hybrid roofs will show condensation risk, our Technical Services team will be happy to assist with U-value calculations and condensation risk analysis where required, we will also discuss the potential risks involved, but please note all calculations will be marked “NOT RECOMMENDED” inline with the British Standard.