Greetings,

I’m currently remodeling a single family house in Miami Beach.  The house has a low pitch roof – 3/12.  All the drywall has been removed from the ceiling, which contained old blown-in insulation.  The wood underside of the roof and rafters is in very good condition.

I removed the old rigid metal ductwork and replaced it with new, flexible ductwork that’s tightly sealed, which will sit above the  new drywall ceiling (to be installed).  The AC air handler is in the house, in a closet to one side of the house (not above the ceiling with the ductwork).

I’m debating between open cell spray foam sprayed on the underside of the roof, between the rafters vs. blown-in insulation.  My big concern with open cell spray foam is that it will seal all the soffits, which allow the air from the attic (or space above the ceiling) to vent to the outside.  If the soffits are sealed, will humidity build up in the space above the ceiling and cause the wood to rot and/or mold to grow?   Because of the high humidity in Miami Beach year round, this concerns me.

I spoke with the spray foam company regarding this concern.  I was told that, while the soffits will be sealed, the openings in the ceiling drywall – (ex: high hats and/or return registers (grills) will allow sufficient air flow to prevent an excess building up of humidity in the attic.

I’ve read about the possible rotting caused to the underside roof wood &/or rafters by open cell spray foam.  My bigger concern is whether using open cell spray foam in a VERY HUMID climate like Miami Beach is appropriate.  Logically, it would seem that the tight seal created by the spray foam could capture the humidity within the space above the ceiling and cause problems to the wood over time.

I’d like to hear from those far more knowledgeable about insulation, particularly the use of open cell spray foam in humid south Florida.

Thank you in advance for any thoughts and/or advice.