Preserving the vapor profile of an old Maine Cabin
A I have a little bit of a conundrum here in climate zone 6. I live in a log cabin that was built as a kit in the 80s. There is NO insulation just 6 inch logs and tongue and groove paneling attached to the flat interior face of the logs. I’ve been wanting to rip of the paneling add some continuous insulation to the interior log faces, strapping, and then drywall. Its very important that I maintain the walls ability to dry in both directions since the logs do take on some moisture etc. A couple of questions have come up:
What type of insulation and at what thickness should I get to maintain a breathable/ durable wall?
Can the strapping screwed through 3 inches or more of insulation properly hold the weight of drywall?
How do I attached a smart vapor membrane to the comfortboard if there are no studs? Do I tack it to the strapping?
I was also thinking of trying out product like this GPS
https://www.progressivefoam.com/product/halfbackh20/
However I was concerned that at 3 inches or more it would limit the walls ability to dry to the interior. Thanks!
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Replies
If you're looking at adding 3 inches of rigid insulation, I would suggest constructing a 2x4 wall in front of the exterior log walls and fill that with the fibrous insulation of your choice, then add a responsive or smart vapor retarder like Certainteed's Membrane. This will allow an inward drying potential plus the framed wall has the added benefit of a service cavity for any electrical or plumbing. This also takes care of your new interior finishing fastening.
I agree with Randy, and recommend Maine-based TimberHP's wood fiber insulation for the fluffy part, or dense-packed cellulose. The borates in both help protect the wood they touch from microbial action, even when moisture levels are elevated.
Even better would be to encapsulate the exterior of the logs. With insulation on the interior, they will be colder and thus wetter in cooler months than they are now--i.e., they won't get as much heat energy to push moisture through to the exterior. My mom has a 1995 manufactured log home in western Maine and I've seen first-hand how much they leak air and water. But I realize that's a project not everyone can take on.
I never considered the beneficial properties of borate treated products so thanks for that extra info. Would a moody wall type assembly be a good option? Maybe a 2x4 service cavity strapped over with 2x stock to add some continuous insulation? Ironically I'm trying not too add too much insulation so I don't get into trouble if there is any wetting of the logs. I run a wood stove all winter so that probably would push some heat and drying through the assembly.
I'm a fan of super-insulation but in a case like this I think it's smart to keep it minimal, to allow some heat to help push moisture to the exterior in cold weather. I find that service cavities are rarely used enough to justify them; a little air-sealing around electrical boxes is usually adequate, but others prefer service cavities. If you want continuous insulation, you could simply hold the interior wall 1 1/2" off the logs and insulate behind the studs. (Blown insulation will fill an 1 1/2" space well enough.) If you prefer the Mooney wall approach, that also works, though I have found that I need to add so much vertical blocking for finishes that it negates part of the thermal break.
Logs may continue settling over time and possibly growing with humidity so it's a good idea to not make the interior wall inadvertently structural. A typical approach on new log home construction is to create a channel at the top of the interior wall that holds it in place while the logs move up and down.
I should also add--to be clear, evidence that borates actually protect the wood the insulation touches is mostly anecdotal, but from highly respected building scientists and builders. If the insulation is continually saturated, some or all of the borates can leach out. I'll attach a scientific study that supports the anecdotal evidence.