Moisture Management for a Log House Covered in Siding
Hi Green Builders,
I have an 1872 pioneer log house in Maberly, Ontario, Canada. I have gutted out the 3 layers of interior siding which were applied over the years and am preparing to re-apply mortar chinking over the spray-foam that I put between the logs.
Now, I am realizing I have potentially a serious problem because there are also 3 layers of Exterior siding which have been applied over the old logs. At some point, maybe in the 1920s, someone strapped the logs and put on nice pine cove siding. Then someone put insulbrick over that. And most recently, perhaps in the ’80s or so, someone put on a layer of white compressed wood siding.
My question/conundrum is twofold: first, what is going to happen in that space between the logs and the siding? Should it be vented? It used to be vented. OK, this is an important detail. On top of the log house, which had a 3 foot kneewall upstairs, someone, probably in the 1920s, took off the roof and made a full second story with an uninsulated 2×8 framed wall. That wall vented up to the attic, and the space between the logs vented into that framed wall. This resulted in good things on three walls, but on the north wall, which gets no sun, it resulted in the sheathing and the pine cove siding completely rotting away, as well as significant damage to the top logs on that side.
OK, so the upper part, where there is a 2×8 wall; that is now packed full of roxul and sealed from the indoors with a good poly vapour barrier. Also, it is now separated from the log area; I put a hardware cloth barrier (for mice) and spray-foamed the entire gap. I now think the upper wall is reasonably safe from rot.
But its that lower wall. What is happening in that space between the logs and the siding?
Add to that another important detail: one wall of the house has had an addition built onto it: a 2×4 framed kitchen and washroom. This was done in the era of insulbrick. That means that THIS log wall is now an interior wall. But there is still the old exterior siding on top of the logs (as part of this now interior wall), and the space between the logs and the siding is continuous around the corners of the building. So, presumably, warm moist indoor air is moving around the corners of the building, in between the logs and the old siding, and getting into the very cold spaces between the logs and siding on the three sides of the building which have no addition built against them.
The basic question here is how to manage moisture and condensation. Or, put a simple way, How do I keep this house from rotting inside the walls?
Its a big set of questions. Any answers which might give me some ideas would be much appreciated.
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Since the logs have real R value, they will act as insulation. This unfortunately creates an in-between air cavity that is neither inside the house or outside the house. These are very problematic and need to be eliminated. At best it creates a mold issue, at worse, it will rot the structure.
You might be able to dense pack this cavity, but I doubt it is big enough.
So you have three choices, none will be pretty.
Remove the SPF you installed between the logs and allow enough airflow behind the insulbrick to make the gap part of the interior space. Might need to drill holes at the bottom and at the top to improve air flow.
Strip the siding, add to the existing strapping as needed to have an insulation cavity and insulate with either batts, dense pack or spray foam against the logs. Install sheathing, WRB, flash your windows/doors and re-install siding.
Strip the siding completely, expose the logs and refinish. Frame stud wall on the interior and insulate with batts.
Unfortunately when dealing with old houses, there is never an easy answer.
Couple of comments:
DO NOT use spray foam and mortar. Neither of these materials allow for movement of the logs, and moving is what the logs are guaranteed to do. Knock out anything that is currently between the logs and use a log chinking product, like Permachink. A foam backer rod is usually used as part of the system.
Could you detail the exterior siding much as you would a rain screen, and have a vent channel installed at top and bottom?
For the interior log wall I would strip off the siding and re-finish the logs. I have used an angle grinder with an abrasive sanding disc on many an old log house, works good, shitty job. Then air seal the transition to outside with chinking.
Logs work differently than a standard wall assembly, instead of straight R-value you need to account for thermal mass. This means that as the structure warms, the mass of the logs act as a heat sink and will feed this heat back into the structure. Things working against you are air sealing (this is where the chinking comes in) and logs of that vintage are probably not that large in diameter as modern, crane built log homes have.
Got any pictures of the project?
Hello Akos and Plum Bob, and the rest of you Green Builders, too:
Thank you both very much for your replies. They give good ideas and food for thought. I hope more people will comment, as this is a bit of a thorny problem, and I would like to find the smoothest way through it.
I will attach some pictures here. They show:
1) an interior view of the logs with the kind of spacing between them, and through the spacing the "inside view" of the old exterior sheathing. This view where you can see through the log gaps is from the wall which abuts the kitchen addition; thus it is now an interior wall.
2) There will also be picture of logs with the gaps foamed and with metal lath applied, ready for chinking to be applied. This is the present condition of the three exterior walls.
3) And lastly some pictures of the exterior of the house. The logs go up about as far as the bottom of the upstairs windows. After that, the 2x8 frame wall begins. The kitchen is the part to the right, with the patio doors.
I've been working on this house for 7 years, and I have corrected a lot of problems in the original construction. It was a horrendous affair when I bought it - full of hotel board and pink, green, and purple shag carpet, and all the accoutrements that go with such decor. It had no insulation to speak of - a drafty freezing old boat. Now it is cozy and warm, with new windows, lots of attic and wall insulation, and all sorts of other things. I am now just trying to make sure that my "corrections" do not cause a new set of problems.
Thanks again for your time and thoughtful replies.
I do not have much else to add, except to reiterate that spray foam and masonry mortar are not the correct solution for logs as the inherent movement will make air sealing impossible.
The logs themselves look really nice and could be sanded and finished to give a very attractive wall, especially the interior wall that does not need to be insulated. I see a draw knife in one picture, brings me back. What type of wood are the logs?
Looks like you have a complex project, and as far as moisture management I think the the best bang for your buck is to do a good job at capturing the moisture at its source by the use of good bath and kitchen fans, and whole house ventilation. It seems hard to predict how your wall assemblies will fair in the long term.