Insulating a flat roof?

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.
 
How is rockwool's R rating at that thickness? 45mm is R1.3, so dual layer would give 2.6.

I have considered rockwool as it is supposed to have good acoustic properties but it doesnt seem to be used for residential ceilings much so I cant find a lot of information about it, but I am still researching.
 
Anticon consists of Glasswool insulation faced with aluminium foil. The Glasswool acts as an insulator and noise barrier.

The foil aluminium prevents condensation. It comes in roll sizes of 10m x 1200mm R2.5, 10m x 120mm R2.0, & 15m x 120mm R 1.5.

Brian
 
I didn't know the anticon had ratings as high as R2.5.
I have put R3.5 batts in the part of the roof I can access on friday, man what a job that was, props to you guys that climb around in roofs all day, I couldn't do it for a living. My roof is pretty small, it only took me 4 bags to do this part, but it was the vacuuming the dust out first that was the hardest.

Decided to go with sarking on flat section with highest R rated fibreglass batts I can fit on teh ceiling while still leaving 25mm for the sarking to do its job. Wont be perfect but will be better than what I have now.
 
I would have suggested using fibreglass backed foil - same as you would use under a commercial metal deck roof. It won't sag, it is bonded to the foil and is installed in one action (by the roofer) and it comes in a roll.
 
Air Cell sarking.
Air Cell is a brand name, i think, but there would be other versions around.
Think of bubble wrap with a layer of thick foil on each side. Comes in a roll. Easy DIY job. You'd be mad to not use it. It's not all that expensive.
 
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