Exterior insulation mistakes
lets suppose you screw up insulating a flat roof from the exterior in climate zone 4A. Let’s suppose 1.5” of polyiso is installed to the exterior. For a non derated value of R-9.75. Let’s further suppose it’s very costly to fix this problem (because lots of solar panels are installed on top). What is the maximum amount of fluffy stuff you can install to the interior? Further the joists are 2×6.