A. Paul Eldrenkamp, owner of Byggmeister, a custom remodeling firm in Newton, Mass., responds: Eaves areas behind knee walls are notoriously leaky. If you run hvac ducts in that space, you complicate things even more — especially if the ducts are also leaky.
It would indeed be a good idea to bring that eaves area — and particularly those ducts — into the conditioned part of the house. This means that the insulation and the air barrier need to be in the rafter plane rather than in the knee wall. Stuffing fiberglass batts in the rafters will provide the insulation but not the continuous air barrier....
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