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Insulating an old wall with interior sheathing?

thearrow | Posted in General Questions on

I’m renovating a late-1800s house and I want to add some insulation to the walls (carefully, without rotting anything out). Climate zone 5A, southeast Michigan.

I’m starting in one room on the second floor. I’ve attached a diagram of the exterior wall I’m dealing with (accurate to the best of my knowledge after a little exploratory demolition). Balloon framed, no top plate, I can see all the way up and down the stud bays. Relatively-modern vinyl siding on the outside is in fine shape.

My plan was to go at it from the interior – remove all the trim, remove the drywall, remove the lath and plaster, remove the 1×10 sheathing. Add some blocking to simulate a bottom plate and then air seal the stud bays (tape some vapor-permeable air-sealing membrane against the old wood siding and to the studs? just caulk the hell out of the stud bays?). Then add Rockwool R15 batts into the stud bays, then a smart vapor retarder (taped Intello Plus?) across the studs, then put drywall up.

Does that plan make sense? Why do I have 1×10 sheathing on the interior of my studs? Can I remove it? Do I need to add some sheathing back for structural purposes? What are those strange layers outboard of the old wood siding?

How would you deal with these walls?


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  1. thearrow | | #1

    Additional ideas/approaches:

    1. Take down the top and bottom 1x10s, add blocking, air seal, reinstall 1x10s, drill holes and blow in cellulose. Still some potential for air movement through the insulation in this approach because I won't be able to air seal the entire cavity.

    2. Take down all of the 1x10s, add air sealing membrane and insulation, put the 1x10s back up.

    3. Give up on doing these walls from the interior and work from the exterior instead - removing current siding, two additional layers, and then old wood siding, then adding blocking and insulation from the exterior, new sheathing, WRB, siding, etc.

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