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Best Option for Insulating Unsheathed Wall Without Demo

JoeBuild | Posted in Energy Efficiency and Durability on

Climate 4. 1930’s house with 2×4 (actual) studs, no exterior sheathing, + layer of tar paper (unsure if it is continuous), clapboard cladding. Walls are currently uninsulated. This is our primary home but we will likely turn it into a rental in the next 5 years so don’t want to do anything too novel or risky.

The attic/loft roof is insulated with a hodge-podge of fuzzy stuff and or mineral wool covered with cardboard and bits of luan. It was last re-roofed 3 years ago. HVAC is in one of the attic kneewall spaces. The rafters are 7 1/4” and the rafter bays of the attic are not consistently spaced so for most bays the insulation doesn’t extend fully to each side. It’s your standard roof  with a single shed dormer. It has a ridge vent but the soffit vent has been stuffed shut with fiberglass. There are kneewalls that have been dispassionately insulated with a mix of no-insulation, fiber, or luan/cardboard. My plan is to remove all prior insulation efforts and take the thermal layer to the roof. I’ll do a vented assembly. 1.75” vent space, polyiso baffles and blocking which will be sealed with great stuff. Headspace is at a premium so I’m thinking cut & cobble otherwise I’d furr out the rafters and use less expensive mineral wool or fuzzy stuff. Does the below approach make sense?

7.25” rafter 

1.75” air gap 

5.5” polyiso, 6.5 R per inch = 35.75R

+ 2” polyiso below R13 = 48.75 R

2” + 0.5” drywall ceiling penalty 2.5” headspace loss (ceiling is either painted paneling or drywall depending on location)

The attic/loft has some mineral wool and fuzzy stuff in the walls from someone’s prior insulating job. The wall covering is latex painted thin wood paneling. My plan is to remove the paneling, remove the old insulation and replace with Martin’s unsheathed interior rain screen detail. Furr out the 4” studs, use 1” gap, 1” polyiso or eps foam, followed by 3-3.5” of mineral wool, and 0.5” drywall. (Possibly add another 1-2” continuous foam before drywall to minimize thermal bridging)

The rest of the house has lath & plaster walls and the same unsheathed uninsulated assembly. The exterior sheathing is in pretty good shape so I’d prefer not to remove it until I have to. What are my insulation options without demo? From what I’ve read something with drying to both sides would be ideal since there’s guaranteed to be some water egress into the wall cavity. Cellulose sounds like an option but I’m unsure how it will perform in wet drying cycles. And I’ve read a few horror stories on this site.

 Is open cell foam an option? Could I pour a few inches of closed cell in my walls to protect the rim joists from a flood puddle, then fill the rest up with open cell, which could dry to both sides? Would the vapor perm rate be high enough to encourage drying to both exterior and interior? Or is what I’m proposing going to create a really big sponge?

Any insight you can provide would be immensely appreciated!

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