Insulation / air barrier strategy for combined kneewall, sloped ceiling, and attic
I am rehabbing an old farm house, and I am somewhat stumped on this issue. The 1900 house is balloon-framed with the exterior wall studs continuing past the upper level about 4 feet, creating a 4-foot knee wall. Then 2×4 rough-sawn 12:12 pitch rafters create a sloped ceiling for another 4 feet, and finally ceiling joists create a flat ceiling. There is a shallow triangular attic space above the ceiling joists with about 4 feet of height between the ceiling joists and the ridge.
To complicate matters, the house has a 2-story addition that is perpendicular to the original house. The addition has full height walls with no knee walls or sloped ceilings. The ceiling levels are the same in both the original house and the addition, and the attic spaces are connected.
The attic in the addition is vented with cellulose on the attic floor. The attic in the original house is also vented with some cellulose on the attic floor. The sloped ceiling and walls of the original house are not insulated.
The 2×4 rafters leave very little room for adding insulation to the sloped ceiling, especially if I create a vent space under the sheathing.
For the exterior walls, I plan to add an air barrier and rigid insulation to the exterior side. It would make sense to continue the air barrier and rigid insulation on top of the roof sheathing. But once I get to beyond the ceiling level, I wanted the air barrier and insulation to follow the ceiling, not continue all the way to the ridge. It just seems to be a lot of work and an unnecessary expense.
And if I did run the insulation all the way to the ridge, what about the adjoining unconditioned attic? Would I have to convert that to a conditioned attic too?
Here is a simplified question: Can I have rigid insulation only at the sloped ceiling part of the roof? In other words, the lower half of the roof will become a hot roof and the upper half will become a cold roof.
Related question: does the sloped ceiling need to be R-49 in zone 5?
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