Are there any hard set rules when dealing with hybrid warm roofs? I'm looking at a future extension and need to keep the roof level as low as possible. Don't want a cold roof though as want to fit down lighters, roof light and also a terrace area on the top. Also got to really keep the roof level with the garage flat roof.
I've seen insulation values of 80mm on top and 40mm between the joist, but can't find a calculator that will deal with hybrids. Does the insulation on top have to be greater than below, or could I go with 70mm and 70mm for instance?
The other idea to reduce height would be to increase joist width which would allow me to drop from 170 to 145mm joists, but I'm guessing that then I will need to increase insulation to make up for the increased thermal bridge.
I've seen insulation values of 80mm on top and 40mm between the joist, but can't find a calculator that will deal with hybrids. Does the insulation on top have to be greater than below, or could I go with 70mm and 70mm for instance?
The other idea to reduce height would be to increase joist width which would allow me to drop from 170 to 145mm joists, but I'm guessing that then I will need to increase insulation to make up for the increased thermal bridge.
