We are getting the roof on our PPOR replaced in the next few weeks, we finally settled on a roofer today and have decided that this would be the best time to insulate the roof since a large portion of our roof is a flat roof and we can only get access when the tin is off.
The flat section covers the lounge room and two bedrooms, I have pulled a sheet of tin up to do some measuing and the trusses are only 150mm high. Since I need a minimum 25mm gap for the sarking to do its job I am left with about 100-125mm of space, I was thinking of using polystyrene as it seems to have an R value of between 3 and 3.5 for 100mm thickness, fibreglass batts of 115mm only have and R value of 2.
Can anyone see any issues I haven't considered by doing it this way? we had originally planned to use Anticon but I thought this way would be a better (even though a bit more expensive) result for the flat section, the rest of the roof will be fairly standard with Anticon and fibreglass batts between the joists.
The flat section covers the lounge room and two bedrooms, I have pulled a sheet of tin up to do some measuing and the trusses are only 150mm high. Since I need a minimum 25mm gap for the sarking to do its job I am left with about 100-125mm of space, I was thinking of using polystyrene as it seems to have an R value of between 3 and 3.5 for 100mm thickness, fibreglass batts of 115mm only have and R value of 2.
Can anyone see any issues I haven't considered by doing it this way? we had originally planned to use Anticon but I thought this way would be a better (even though a bit more expensive) result for the flat section, the rest of the roof will be fairly standard with Anticon and fibreglass batts between the joists.