If you're considering turning your attic into conditioned storage or living space, it's worth changing tactics to insulate between the rafters and wall joists instead of the floor. You can use rigid foam panels, or do as Tom Silva does with nearly all houses these days: Hire a pro to install spray foam insulation (see Upgrades That Slash Energy Costs
). Foam blocks airflow, needs no vapor barrier, and has a higher R-value per inch than loose fill or batts, so you'll get more protection with less depth. With spray foam, unlike with loose fill or batts, you must cover the soffit vents (they aren't needed to keep the roof cool). Then cover the foam with drywall as required for fire safety.