We’re rebuilding a house that has brick sheathing in Zone 3 humid. Though we’re doing what’s practical to address air leakage and insulation in the walls, most of our effort will have to be focused on the roof.
On the roof, we’re adding 4 to 4.5 inches of (used) polyiso. This goes over the original 1 by decking and a new layer of taped plywood and a layer of tyvek protec. On top of that is another layer of tyvek, 2x lath, and a metal roof. No other insulation beneath the decking.
Inside the house, we’re using two ducted mini-splits (12000 an 9000 btu in an 1800 square foot house) with short runs through the attic space. The attic’s outside edges should be moderately well sealed. Some of the house will have cathedral ceilings with exposed rafters, but we’re leaving the old sheetrock ceilings in place in the bedroom areas — some ceiling areas even retain their old fiberglass insulation. The ceilings are by no means perfectly air-tight, but I would expect a temperature difference between the living area and the attic space.
I’ve seen articles suggesting deliberate conditioning of closed attics. My guess is that temperature difference is not enough to worry about during our shortish winters and long summers. Are there good reasons to worry — or to put a supply directly into the attic?