Hi, hope someone can help.

I'm having a kitchen extension built with flat roof. Originally we planned to have just 100mm insulation as warm roof. Now we've been told it has to be 150mm, but we don't want to add another 50mm to the outside, so builder suggested we have the extra 50mm between rafters, eg a hybrid roof insulation system.

Does anyone know if this would be ok in relation to the dew point still being on the outside? We will be using 18mm plywood above and below the warm roof insulation and the joists will be eight inches. Can anyone help on this? Our builder looking to order insulation as soon as possible.