I have a vented attic space with poor insulation in the attic floor and rafters, so I'm going to button it up.
My energy auditor suggested I insulate the floor as follows: cut blocks of rigid foam for the ends of the joist bays, and tack them in place with spray foam. Then, fill the joist bays with loose cellulose, and top that with rigid foam (ISO) seam-sealed with spray foam. This should create the air and vapor barrier between the conditioned and unconditioned spaces. Finally, since I'd want to use it as storage space, I'd put plywood or OSB decking over the top.
The rafters will be fiberglass bats (there are some in place that were improperly installed, but in good shape, so I want to reuse them) with the foil side facing towards the roof about 1" off the sheathing. I'll then staple FSK across the rafters with the foil facing in.
I think I'm on board with everything, but I'm wondering about the decking and the FSK. Since the rigid will act as the vapor and air barrier between the conditioned and unconditioned space, do I have to worry about condensation forming on the decking? Is it really worth spending money on the FSK if the attic space is vented? I do have the ability to open and close the gable-end vents (the only active vents in the space).
This house is in Seattle, built in 1925.