Insulating (and ventilating?) dropped ceilings
Looking for advice on how to insulate (and if to ventilate) sections of house under what is nominally an unvented cathedral ceiling, but has a section of flat lid. Details attached.
First section is above the new addition part of the living room, the roof has 2×6 framing and is unvented below the roof (Delta Trella underlay provides a bit of ventilation for the standing seam metal roof). There is no insulation on top of the roof deck here, so I plan to start with 2″ of closed cell spray foam below the deck. With the flat sheetrock lid (supported by 2×6 spanning ceiling), there is a triangle that I’m not quite sure what to do with (highlighted in blue on cross section). I was considering furring down the 2×6 to by 4″, and adding 7.5″ Rockwool batts. But where the roof tapers, this will be difficult. Could I use blown-in cellulose or fiberglass in this space, maybe held in place with netting before I install sheetrock? If I spray foam under the deck and in the rafter tail blocking, can I close it up with no ventilation?
The second section in question is on second floor above the master bedroom. Roof here has 3″ polyiso above the deck, and still no venting. In the rest of the upstairs, the ceiling is vaulted, furred down by 2″, and 7.5 Rockwool batts. In this room, there is a dropped ceiling with 2×6 spanning the room at 8′ height. There will be closed cell spray foam in the rafter blocking areas. The area in question is highlighted in blue again (lower image in detail). Do I need to ventilate this space? How can I insulate it? How can I soundproof it from the loft? Could I use blown-in fiberglass/celulose over the flat ceiling, then something in the batts between studs on small vertical wall between blue area and loft?
2400 sq ft 2 story house in CZ03 coastal, zip 95060. HRV ventilation. Mini (multi) split for heat/cool (18,000 Btu/h each upstairs & downstairs).
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