How can I insulate Florida room ceiling of plywood + shingles?
You can go with rigid insulation, a typically sheet is 4′x8′ and you can get it in varying thickness from 1/2″ up to 2″ (a 2″ pink rigid insulation sheet adds about an R-10 to the insulation value.) SO it would only lower the ceiling height by two inches.
There are specialty screw and cap pieces that are made so that this material can be lightly screwed into the existing roof, you could then either cover it or leave it bare. For a more finished look you could drywall it in, or you could cover it with ceiling tiles or paneling, or fabric or whatever suits your fancy.
It is also mold and water resistant and because it is rigid it does not break apart and won’t fall in pieces if you left it uncovered
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You may try lowering the ceiling.
Your roof should be shingles, tar paper and decking (usually plywood.) The plywood has to set on joists. The joists should be at a very minimum three and a half inches. You may need to take the sheet rock off of the inside, if you have any there, and install insulation, then re-sheet rock. You may also be able to pay some one to blow in insulation.
Without seeing exactly what you have, it’s hard to do anything but guess.
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Fiberglass insulation and drywall. Any General Contractor could easily knock this out in one work day. They will have to come back and mud the seams a couple times tho.
Try calling a roofer and ask him to install a can vent or turbine vent in that area. It will help substantially.
There is also blown in insulation. Nothing wrong with that as long as the roof can still breath in a majority of the decking.
Edit - ROFL Sarge, we answered at the same time