Hi

I'm insulating and residing my house in Wisconsin. It is a mishmash of construction techniques. It started as a 14'x14' log cabin, 100+ years old. The rest are additions added onto the original cabin, some old, some modern.

I'm wrapping the house in 3.5" of EPS and installing a rain screen with 3/4 plywood furring over the foam/tyvek. I have about 1/2 the house ready to accept the siding and working towards the only part of the house I don't have a strategy for: the log walls. One corner of the house has original oak log walls on the exterior. The original cabin has become the kitchen and the logs have been refinished and chinked on the inside. They look really nice. The exterior of the walls are rough. I stripped off 2 layers of old siding that was over the logs. The logs have been white washed several times, peeling, overall bad looking. Nothing is rotted though.

My plan is to keep the logs exposed only to the interior and cover/insulate the exterior side. I was thinking of furring out and anchoring a 2x6 wall to the logs so I have a flat surface for the siding. But what about insulation/moisture management? I have to assume there are cracks that allow small amounts of air exchange between inside and outside of these old logs, even with the chinking. Vapor barrier over the exterior logs before insulation? Cellulose? Has anyone dealt with this before?

Thanks for any advice,
Chris

You don't really need a vapor barrier if you're putting 3.5" of EPS insulation (~R15) on the exterior of the logs, but an air-barrier is a good idea. Even 10" thick logs don't have an R-value of more than 15, (10" of oak would be more like R7-R8), so even if the chinking leaks wintertime average temperature on the exterior side of the logs should average well above the average wintertime dew point of the conditioned space air.

The key is keeping it air-tight to the exterior so that the furring channels between the foam and logs doesn't become a thermal bypass. If it's possible it's a good idea to blow fiber insulation into that uneven cavity to provide some air retardency. That'll be easier if using 1x4 furring (shimmed for wall-flatness.) At 1.5" (the thickness of a 2x4) cellulose would add about R5 to the stackup, and would be fairly air retardent even at low density.

Logs undergo seasonal dimensional changes with temperature and humidity making it difficult to keep a log home really air tight. But once the logs are fully inside of air tight R15 of continuous insulation those seasonal shifts will become much smaller than previously. A self-healing flexible membrane such as Grace Ice & Water Shield over the transition point (such as the top of the wall) can probably help with the long term air tightness of the assembly, using plywood or OSB detailed as an air barrier (on either side of your foam layer.)

Thanks! My thought on putting a 2x6 (or even 2x4) wall up was that I could fill the whole cavity with cellulose tight against the logs, then OSB over the studs. Then, the the tyvek/furring for rain screen over the OSB without any EPS. These 2 walls butt into a pair of other walls, so the final furred out depth isn't important.

I could use EPS too though. I just thought it would be cheaper to make a deeper cavity and fill the whole thing with cellulose than deal with the EPS and long anchors for the rain screen furring. I have a bunch of OSB sitting in my barn already.

Would I be OK either way or would 2x4 furring+cellulose+EPS be the best route?

Thanks.

Edit: Saw your reply in another thread about exterior OSB.