I'm remodeling the master bathroom, and at the moment its ceiling has been completely removed, exposing the steel pan deck of the concrete suspended slab that forms the roof and temporarily giving me full access to the void between the ceiling drywall and pan deck for nearly the entire second floor. The old ceiling drywall in the bathroom had foil on the underside to form a radiant barrier. The only other insulation is minimally-useful insulation in the built-up roof above. I won't be putting in new drywall with foil on the back, because I refuse to use paper-backed drywall *anywhere* in the house. That leaves me with three real options for replacement of the insulation: * radiant-barrier insulation from Lowe's, placed between the new DensArmor drywall and the roof deck. * Blown-in cellulose from Home Depot or Lowe's * Nothing at all While I have my doubts about how useful the radiant barrier really is, or whether the radiant barrier purchased from Lowe's would be appreciably better than gluing aluminum foil to the underside of the new DensArmor drywall, I'm unlikely to skip it entirely unless someone manages to convince me that it's totally worthless. That leaves blown-in cellulose, and the big question: if there's only 3/4" between the ceiling drywall and metal pan deck above, will blowing cellulose insulation into that 3/4" gap above the rooms shaded pink in the pic below really make enough of a difference to be worth bothering with? I'm guessing that it will provide something like "R2". Likewise, if it would be largely useless and add minimal value over the radiant barrier already in place over the bedroom and study, would it still be worth blowing cellulose into the areas shaded other colors (where there's 3", 4", or even a foot or more of void to fill)? Realistically, I'd end up with the middle third of the roof shielded by 4 or more inches of insulation, with the two ends shielded by the radiant barrier drywall and possibly 3/4" of blown-in cellulose. Put another way, is it worth even screwing around with 3/4" of blown-in insulation? And if it's not, is it still worth blowing it into the middle third of the house, even if it would be a complete waste to blow it into the ceiling above the study and master bedroom? It's not shown in the pic below, but the stairs are in the area labeled "Open Below", and the master bedroom has a window like the one in the Study. The two long sides are both in direct contact with the neighbors' houses. The second pic shows the void above the right green area from inside the master bath facing towards the stairs.