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Insulation / air barrier strategy for combined kneewall, sloped ceiling, and attic

jimruss75 | Posted in Energy Efficiency and Durability on

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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Replies

  1. Expert Member
    ARMANDO COBO | | #1

    Your plan for the walls are solid. In CZ5 you are required to have R20 or R13+5 min., so installing 1"R5 rigid foam on the outside and 4" R15 dense pack cellulose on the wall will work well.
    In CZ5, all roof/ceilings are required to have R49. On the sloped section, you need to scab an dditional 2x8 on the bottom of your 2x4 rafters to create an 11.25" of space for R49 insulation. Once that is done, I would install an unvented cathedral ceiling approach by spraying 3" R20 ccSF under the roof decking and then filling the rest of the cavity with R28 with blown cellulose. The attic spaces above the flat ceilings, you can install 13" R49 blown cellulose on top of the ceiling. Make sure you have adequate ventilation in the attics.
    Make sure all those three sections are completely separated and air seal from each other.

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