Flat roof insulation detail
Good evening all,
I want to double-check my plan for my new flat roof rebuild. The flat roof will be unvented.
The plan is to have 8” of EPS slope insulation on the outside, and the membrane will be a polyurea coating by Huntsman (Demilec), so I can use the roof as a terrace.
I have 16” trusses on the interior that I plan to fill up with blown-in cellulose with Siga Majrex as a water vapour control membrane. Siga Majrex is an “intelligent” membrane, where water vapour trap in the roof and trusses to dry out in the interior but block the vapour from the inside to enter in.
I’m in zone 6, and my understanding is that I need roughly 50% exterior insulation to 50% inside insulation.
The current plan put me roughly to R30 exterior EPS foam and R60 interior if I fill out the 16” trusses with blown-in cellulose.
So, I don’t respect the 5o/50 recommendation, more like 33% outside and 66% inside.
The fact that I use thick outside insulation, an “intelligent” water vapour control membrane, and a non-vented flat roof means it can safely use the current plan?
Many thx in advance for your inputs and advice.
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