Insulating a Log Cabin Kit
Hi,
I’m going to be assembling a 24’x32’ log cabin kit with 6” thick white pine logs this summer in Zone 5, Pennsylvania. It will mostly be three season use with an occasional winter weekend here and there. There will be a full basement, superior walls pre-insulated beneath it.
The way the kit is designed you have to drill and chisel out the log walls for the electrical outlets in the exterior walls. This slows construction and I’ll be rushing to get it under roof. I’ve been toying with the idea of framing out the interior walls. The two options I’ve come up with are:
Continuous R-13 2” Polyiso attached to the walls with furring strips. This option will cost about $3500. Should I be concerned about any moisture issues at the interface of the foil and the logs? While this would be easier than wiring in the logs, I would still have to route the polyiso to run the romex and use old work boxes.
Next option would be to frame out with standard studs and use R-15 rockwool insulation. This would run about $3700 and allow for conventional wiring. This is my preferred method.
Anything else I should be considering?
The ceiling is essentially a cathedral ceiling over the entire cabin framed with 2×10 rafters on a 8:12 pitch.
My current plan is to make site made baffles continuous from the sofit to the ridge vent with ½” polyiso, then insulate with 2×8” R30 rockwool, then cover the underside of the rafters with continous 2” polyiso and strap it with furring strips. This should give me a R-46, which I’m not sure if that’s even required or not. It seems that cathedral ceilings are generally R-30 unless they’re over 500 sqft, which mine is. If anyone has any comments or cheaper suggestions I’m all ears.
Thanks,
Travis
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If this is not a full time house, there is not much point of insulating.
I'm not familiar enough with log homes, I do know they have a lot of issues with air sealing, you loose a lot more heat through these than insulation.
Overall, the logs are quite permeable and provide some real R value. The 6" log (around R7.5) is good enough for condensation control of up to R20 of fluffy insulation. You can definitely frame the walls and fill it with batts without issues.
The detail that would need to be figured out is how to create a solid air barrier in this wall and tie it to the ceiling air barrier. A sheet good on the outside of the stud wall (heavy duty house wrap is probably the lowest cost option) that you can tape to the ceiling foam should work.
Log home generally means woods, so make sure you take care of your critter barrier as well. This has to be continuous along any cross section of the house the same as your air barrier.
If you're building full stud walls and rafters, what's the point of building a log home? Most poeple build these for the aesthetics both inside and out. If you don't need the look inside, build a standard wall system and use partial logs for the cladding on the exterior. Way simpler build.
As far as your specific questions, there are several challenges with your proposal. Furring out the logs for installation of polyiso would be very finicky, as each log is a different size and has an uneven surface. You would end up shimming at every connection. Can be done, but not much fun. The bigger challenge is accommodating shrinkage of the logs. Most log home shrink by an inch or more as the logs dry out and take a set. Log home kits generally have special connection details to handle this shrinkage where stick-framed interior walls connect to the log structure. If you install full studs and rafters directly against the log structure, they will be crushed, or the logs will end up hanging on the studs with open joints. Neither one is a good thing.
Your roof insulation plan is fine. Foil-faced polyiso on the interior acts as an interior vapor barrier and breaks thermal bridging. You don't specifically need the strapping - you can use long drywall screws and fasten through the polyiso if you want.
But, as discussed above, why insulate at all? If it is occasional winter use, a big wood stove typically provides plenty of heat for comfort on the occasional winter weekend or chilly night in spring and fall. Do you plan to air condition in summer? If you've got no major heating or A/C uses, then insulation becomes far less important. Pay careful attention to chinking/caulking for air sealing. You may have to do it again after the house settles in. You can't get a very airtight shell with logs, but if you're diligent, you can make it not too bad.
If you are a basement, why not running the electrical underneath and install a floor outlet where you need it or code requires?
Depending on moisture content, a log wall can settle several inches due to shrinkage and compression. Slip joints are required at all framing intersections and above all openings. Care will need to be taken for kitchen cabinet installation etc. Anything that is scribed to fit the log wall profile will need to be re-done after the settling is complete.
From what you describe I think it is a milled kit with the logs all shaped to the same profile?
I personally would feel no need to insulate the walls, and would build an insulated roof much as you describe. Chinking will provide a fine air barrier and should be continued around your window framing. With a properly sized wood stove you will be super comfortable 3 seasons, and the odd winter weekend just keep the fire up. Floor plugs for elec as mentioned above, they can come with decorative flush mount plates. The only chiselling needed will be for light switches near exterior doors.
Cheers
Thanks for the replies, I'm glad I posted here. The plan all along has been to just build it with uninsulated walls, it just popped into my head last week to frame it out for insulation and wiring. Part of the reason is we may consider retiring to the cabin, but that would be 25 years from now. A lot can happen in between now and then. I think I'll save the money and worry about it in 25 years if I don't have good performance.
Yes it is a log kit with the logs milled uniformly and kiln dried to 16-19%. The interior walls will be flat and there is a foam gasket placed on top of the tongue in between each log. There is also a bead of caulk in between each log. That being said I was still worried about settlement as far as framing out a stud wall.
I plan on heating with a pellet stove when necessary and installing a 12k BTU mini split mostly to pull humidity out when necessary.
Floor outlets are an interesting idea. I'll have to look into those more, they seem pricey but might be worth it in certain areas. One area where it still might be worth framing out is in the kitchen as counter top outlets seem to be even more of a pain.
The costs included in my original post actually included enough 1x8" tongue and groove to put over the insulation and studs to make it feel like a log cabin again. Significant amount of labor though.
I was looking at premade baffles such as accuvent to speed things along rather than site made out of polyiso. I'd be at R-43 then, I'm not sure how picky they're going to be when I go for my permit.
I wouldn't go with floor outlets, these are problematic with dirt and are always in the way.
Install the outlets in the lowest log closest to the floor. I would also install them horizontally to be less visually obtrusive. You can make a quick template for a router jig and router all of them in no time. You can then drill up from the basement into this pocket for wiring. I have done something close to this with all brick houses and even there with chipping brick, not quick but not all that much work either.
Travis,
They sound similar to Panabodes. I've built seven of these and renovated a dozen more. https://panabode.com
Not only do they settle, but they move seasonally. The big challenges accommodating this are around window and door detailing, chimneys, and plumbing waste lines which need compressible collars on vertical runs. Even with gaskets and sealants between the logs they leak air like a sieve creating pressure differentials that can make fireplaces and wood stoves a challenge.
That said they are very pleasant t0 be in and make good seasonal cottages. I agree with the other posters who responded: You just have to bite the bullet and give up on any effective insulation and air-sealing to accommodate the aesthetic. Covering up the logs on the interior defeats the whole purpose of building with them in the first place.
I've been busy building the log cabin all summer. The roof is on and I'm going to start making the insulation baffles out of 1/2" polyiso this weekend. One basic question I have is the best way to attach ceiling light boxes through the 2" polyiso strapped to the bottom of the ceiling? My assembly below the rafters is 2" Polyiso 1x4" strapping and then pine tounge and groove board.
There will be a ceiling fan at the peak which I can box out for, but there will be two hanging chandeliers part way up the cathedral ceiling. Nothing too heavy but I'd still like them secure. Maybe I could attach the boxes to the strapping?
Thanks,
Travis