I own a 34 year old single story ranch with gable roof (just shy of 4 in 12 pitch). Location is in a hot & humid zone where it doesn't snow. The original attic insulation was 3.5" FG batts laid between ceiling joists, now compressed to about 2". Attic venting is accomplished by soffit vents (24" overhang) and near ridge ventilator caps.
Having lived through this summer's searing heat with the A/C running most of the time, energy costs are literally through the roof. It's my intention to do this job right the first time so I don't have to do again, ever. But I have a few questions.
The current plan is to remove and discard the FG batts from the accessible attic spaces (3), then: seal all air leaks from the ceiling & walls below, insulate both hot & cold water pipes, and re- tape all A/C ducts and insulate them since they run in the uninsulated area.
When those tasks are completed, the re-insulation will take place in 2 phases. Phase 1 will be to blow in FG loose insulation.
Phase 2 will occur during a later re-roof and will add both new decking that has radiant barrier and at least 2 if not 3 inches of rigid foam sheeting above the new decking.
My questions are:
1) If I make a 2 inch roof vent channel (probably 1/4" sheet plywood) underneath the rafters that are holding up the new radiant barrier decking, will it be a problem if I fill the entire remaining attic space up to that vent channel barrier? (It also means burying the A/C ducts & water pipes and capping any lighting protruding through the ceiling.)
2) If I'm going to use polyiso rigid foam sheets(in order to get the highest R value) above the new decking should I also use a vapor barrier on top of the polyiso sheets to avoid a water problem with the sheets? if so, could anyone recommend which one? I would, of course, still use roofing felt directly on top of new decking. My guess is that I will be using either a 30 or 50 year composition shingle during the re-roof and hurricane nailing will be used too.
Thanks for any help,