Un-vented roofs are now widely accepted in humid climates like NC, SC, Georgia and Florida.
>> Why Spray Foam Does Not Require Ventilation: Air Leakage Control
Spray foam has a low air permeance – low enough to be classed as an air barrier. Therefore, moisture movement through foam by air transfer is virtually nil.
>> Why Spray Foam Does Not Require Ventilation: Vapor Diffusion Permeance
The 1% of moisture that is conveyed by diffusion is usually not a problem, because the amount is so small that it is measured in nanograms (one billionth of a gram). Its effect is easily overcome by normal drying cycles inherent in wood materials.
Five inches (25 mm) of open cell spray foam has a vapor permeance of 10 perms (565 ng/m2/s). This property allows low rates of moisture diffusion to occur, just enough to allow breathing of adjacent framing and decking materials to prevent moisture entrapment. The minimal diffusion that does occur through foam will pass through the material without condensing, provided that the substrate to which it is attached is equally (or more) vapor permeable