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Community and Q&A

Insulation and Moisture Control With Gapped Wood-Plank Sheathing

tognamakes | Posted in General Questions on

Hey all, I’m currently renovating a 1949 story and a half house in central Ohio and for the first the first time during the project I’m completely stumped. I feel way over and under informed a the same time after reading everything things I can find about insulation for the past three weeks.

After taking down the old sheet rock in the kitchen, I noticed that the plank sheathing of the house isn’t tongue and groove. Most of the planks have and eight of an inch gap between them, some a quarter of an inch. Behind the planks you can see a black felt house wrap, maybe tar paper.

Should I fill these gaps before adding insulation? Should I add insulation at all?

The outside of the house is cedar shake siding and from the interior there’s a couple of spots where it looks like water has gotten behind the sheathing, but really not that much for a seven decades old house.

My concern, spurred by dozens of internet articles, is that moisture will get behind the planks get trapped by the insulation and cause mold and damage.

My current plan is to put spacers between the rockwool and the sheathing (maybe like the air chutes you use in attics), leave the gaps open so that moisture can leave and cover the rockwool with aquabar Kraft paper as a vapor retarder.

Unfortunately the membrain product that is always recommended here appears to be unavailable to the point that I can’t tell if it’s produced any more.

My main question is whether it would be better for me to go through and fill the gaps to try to prevent moisture from getting in in the first place or to leave them open so that the moisture can escape.

OR, is it not even worth insulating? The house has stood for 70 years being able to dry itself out maybe it’s best not to mess with it.

Thanks in advance! There’s such an absurd amount of insulation information and opinions out there, it’s hard enough to figure out what’s suitable for your climate let alone a less typical building structure.

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  1. SierraWayfarer | | #1

    Sounds like an 'interior open plank cladding system' if there is such a thing. Probably not but I was listening to Coffee with Joe the other morning and he was talking about Open Plank Cladding and how it works. Pretty strange to me but maybe it works on the inside with sheetrock over it?

  2. Jon_R | | #2

    Whatever you do for insulation, make sure you have a good air sealing plan. And then test with a blower door to verify.

  3. Expert Member
    Akos | | #3

    Since it sound like you have a house wrap outside your board sheathing, you can safely insulate the wall with batts. You should still check that flashing details at your windows are done properly, bulk water leaks plus insulated walls means mold.

    The bigger issue is how to air seal this assembly properly. Usually the simplest is to dense pack the walls if you can find a local contractor.

    The only other way to air seal the board sheathing is with sprayfoam. You can fully fill the walls with open cell SPF or with about 1.5" of closed cell SPF. In case of the closed cell, the remaining cavity can be filled with squished R8 attic rolls.

    Sometimes either option above is cost prohibitive, in which case do your best to detail your drywall as an air barrier. This would mean caulking around the perimeter and replacing your device boxes with air tight ones (plastic ones with a flange). These device boxes might be special order but usually are only a couple of bucks at your local box store.

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