I have an unconditioned knee wall space. I wish to condition it but am faced with a small dilemma. My concern is: can I install fiberglass against the rafters (assuming I have baffles), if the vaulted ceiling leading up to the ridge vent is obstructed by foam insulation? Picture to illustrate.

Where is says "Install blocking and rafter baffle...", the vaulted ceiling in the conditioned space adjacent to the knee wall, is open-cell foam. Therefore, I'm concerned that not providing a clear air path from eave to ridge vent would cause moisture buildup even if I had baffles along the rafter. Right now supposedly it's not a problem because part of the knee wall attic does have airflow (it's not regularly shaped), but if I restrict the channels to between the rafters only, about half of them are blocked by the vaulted ceiling's foam barrier.

  • If I understand this, there is no air movement between the knee wall and the attic space above the main living area, and thus the air in the knee wall is never exhausted out of the ridge vent on the top of the house. If so, I had this issue. I ended up making the plenum above my stairs a route for the air to get to the ridge vent. Worked wonderfully. – Evil Elf May 31 '16 at 17:37
  • Since the knee wall space connects to half foam, half fiberglass/baffles ("L" shaped space between two different rooms), I suppose I could leave a small air gap under the rafters between the part I insulate in the knee wall and the baffle in the vaulted ceiling it would normally connect to. – saigafreak May 31 '16 at 18:12
  • Some people use PVC pipe to create air flow channels but that normally requires installation from the outside because of the lengths of pipe. This in turn requires removing the fascia. – Evil Elf May 31 '16 at 19:08
  • Do you mean boring a relatively small hole in the studs to run PVC through, so air can circulate between cavities? – saigafreak Jun 1 '16 at 15:45
  • The roof rafters form a space between the roof sheathing and the drywall. Some people run pipe from the knee wall attic to the main attic through this space. I created a plenum by insulating and creating a tunnel above my staircase. – Evil Elf Jun 1 '16 at 17:02

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