Flat roof, no insulation and needs replacing
Just purchased a home, the living room addition in the 1980’s added a 20×30 flat roof that has ZERO insulation. The construction is large LVL type beams with 2.5″ tongue and groove decking spanning the beams. The roof is a old build up with tar and rock ,and it is in bad need of replacing. My father in law is a commercial roofing consulting and is offering to coat the roof with a commercial emulsion/silcone top coat. This is the option I am going to go for in making the repair on the roof (it will be free and we both want to explore with using this product in the residential market).
The plan for adding insulation is then to build down the roof from the interior 3.5-5.5″ with dimensional lumber and fill the cavity with spray foam. I am concerned since the new silicone coating will be vapor impermeable making sure I do not trap moist air on the 2.5″ decking causing rot.
If I use open cell spray foam, and tongue and groove wood on the interior, will this provide breathing into the house for any moisture that comes form the outside?
I am located in South Carolina, climate zone 3, so my understanding is I should design for air breathing to the inside of the home.
Should I use open cell spray foam in this case?
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