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Best way to add insulation to log walls

knokej | Posted in Energy, Heating & Insulation on December 8, 2022 09:23pm

So I just bought a 40 year old log (kit) home in Wyoming.  I’d like to keep energy bills reasonable in the winter and there is some evidence of air leakage between some of the logs.  My two thoughts are to either: 1) build 2×3 walls on the interior, against the logs, and then spray foam; or 2) build an extra exterior wall, maybe with Larson trusses, and fill with foam or blown rock wool.  Main problem for me on #1 is that the rooms are not big, so I’d rather not lose another 3″ of floor/air space against the exterior walls.  Main problem with #2 is cost.  Foam seems like a good way to air seal at the same time and to fit against the curvy and irregular shapes of the logs, but I’d prefer something more eco.  Working on the interior allows me to do a room at a time, at my own pace.  Doing the work on the exterior would allow me to add more insulation, and to potentially create a more complete building envelope, but there are 2 porches in the way, and I’m not sure the roof overhangs are large enough.  Happy to hear ideas and let me know where you think the air barrier should be if I didn’t use closed cell foam.

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  1. User avater
    Tom_Massey | Dec 09, 2022 11:25am | #1

    Have you considered that sheet insulation? I saw some in the local big box store that could easily be covered with sheet rock. It was maybe an inch thick. Not sure how helpful that is, but it is an idea.

    1. knokej | Dec 09, 2022 09:30pm | #2

      That would be easy, but would it be OK that there would be air gaps against the irregularly shaped logs? And seems like I wouldn't get the air sealing that closed cell foam could provide.

  2. p_vilefort | Dec 10, 2022 11:21pm | #3

    Here is my take on your solution. You don't need any framing on the exterior. Seal the gaps in the logs with a liquid flashing and apply 3 inches of polyiso foam board in two layers to the exterior. The first layer has all the seams tapped and the second layer has the seams offset from the first layer and again tape all of the seams. Apply furring strips to the exterior insulation on 16 inch centers or farther apart but nor more than 24 inches. The furring strips are screwed to the logs through the insulation. Yes, you can add some screws while you are putting on the insulation. You use larger diameter plastic washers so the screws don't sink into the foam.
    Then I would cover the exterior with thick board siding that resembles cut and shaped logs with a tongue and groove connection. Another final covering option would be steel siding. There is a place up in that area called Bridger Steel. Check out some of their steel siding options. The furring creates an air gap so that any water getting between your foil covered insulation and your final siding can get out. I would also add a bug barrier layer to the top and bottom of the air gap. There are synthetic materials that would work as a bug shield but one that will last forever would be strips of Rockwool. The space is usually 3/8 to 3/4 inch so you would cut strips about two to three inches wide and the thickness of the space. Rockwool will allow water to pass through it as well as air and vapor. But if bugs try to bore or burrow into it, the rock fibers will destroy their exoskeletons and they will dry out.
    The great advantage to the exterior insulation is that warm wood means healthy wood. Any natural moisture in the logs can get out by moving into the rooms.
    If there is any chance of wildfire, then the exterior insulation should be Rockwool Comfort Board. Comfort Board will not burn or support combustion. Even with Rockwool you will still want to use an air gap behind any siding. The attached picture is metal siding.

    1. knokej | Dec 15, 2022 11:02pm | #4

      Thanks p_vilefort. I like most of what you suggest, but some questions:
      - since the existing, exterior log surface is irregular, wouldn't I need to shim out some leveling base, so the insulation panels would be flat and plumb (especially after the screws would pull it down tight)?
      - what would be the best way to detail the top and bottom of the insulation to prevent air from getting behind it (since there would be gaps against the irregular log surface)?

      1. p_vilefort | Dec 16, 2022 04:53am | #5

        It was my understanding that kit log homes had even surfaced logs or square face logs since many were built at the factory, marked for placement, and then disassembled and shipped to the site for erection. Many log kits also had fiber board that fit into slots between the logs to reduce or prevent air infiltration.

        When I wrote my previous suggestion I wasn't thinking so much about possible fire or wildfire exposure. In that case you would be better off using Rockwool comfort board. Since I don't know of an effective way of sealing the joints at the layers of the Comfort Board you will have to make sure that you offset any joints and butt the Comfort Board panels together and screw to the most outstanding log surfaces. Seal the log joints with a liquid flash or use the old method of wire mesh and mortar. Since the log surface is no longer going to see direct weather the only place you need to seal is at wall corners. The sealing is primarily to keep out critters so cut and trimmed Comfort Board will help to provide a flat face. If you have any logs whose outside surface sticks out beyond the other logs you could use a power plane and shave it down to match the other logs more closely A reminder; Rockwool is air and water vapor permeable. Any moisture caught back there (behind the insulation layer) will eventually come out as vapor or liquid if it can migrate to the bottom. Depending on the foundation to log interface there are different ways to ensure an air tight seal and all involve the application of a liquid flashing product. If it were me and the log over hung the foundation, after sealing I would put a metal flashing from the foundation wall log interface to the point where the insulation and siding stops with a downward facing lip. This serves two purposes; the metal reduces any bug or rodent exposure and if you have some water drainage, it hits the metal flashing and drains to an area not directly against the foundation. Adjust the metal flashing thickness based upon the strength of the material not to bend easily or attach a supporting wood strip that the flashing will rest on. That wood strip should be treated with a copper based anti-rot/anti-fungal of 0.40 or above. There are many excellent examples of bottom of the wall flashing applications that you should use. You definitely want some air gap there and it doesn't take much. In most cases 1/16 inch is sufficient to take care of any water drainage that might be from a hard rain getting behind the siding (big secret, there is no such thing as a dry siding. even masonry allows water through). The gap breaks any capillary attraction and the water can drain away.

        I suggested a thick board siding because the edges can be cut to create a log look or a nickel gap siding look. In the picture I attached of the metal siding, you should know that the siding above the window/roof overhang is metal too.
        If you are willing to cover the exterior log look, then consider using corrugated metal for siding. There are examples of corrugated metal in the vertical as well as horizontal positions and how it is applied around windows on the Bridger Steel website. The other thing that is nice is corrugated is very strong and stiff without being thick. It is also attached through the surface and that makes it easy for the DIY owner. Just use stainless steel screws. The corrugated is also available in many surface colors as well as galvanized or galvalume.

        Is the foundation poured concrete or concrete block or something else? What is the first bottom row of logs sitting on? If I left something out simply leave another message and I will get back to you.

        1. knokej | Dec 16, 2022 10:16pm | #6

          Thanks again. Unfortunately, I won't actually own this place for a month or so, and I am 2000 miles away, so can't take another look at the walls :). I know its a poured foundation and much of the concrete-to-log interface is covered by some clapboard the owners added below the logs. It is a log kit, so the (round) logs are pretty uniform, but I had been assuming they would not line up perfectly in the vertical plane. I don't know what original kit was used, nor what construction details were used (built 40 years ago). I know there are decent roof overhangs on the front and back, but its snow country and there are no gutters, so I am not even sure I can add 4" to the exterior walls (especially on the gable ends) and still feel good about the overhangs. By the way, if I add exterior depth, I'll either have to redo all the window and door frames, or add jamb extensions. But I don't know if I can just add extensions exposed to weather and prevent them from leaking.

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