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Author Topic: Log insulation question ?  (Read 1645 times)

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Offline Jimbob

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Log insulation question ?
« on: July 12, 2008, 11:38:28 PM »
In building my log house, the plan is:
Using SYP from my own woodlot, I'll be milling 3 sides to 8"x8"x8" on my Logosol CSM, then removing the remaining outer side bark with a drawknife. Will be using butt and pass construction and connecting the logs approximately every 3 feet with rebar. After the logs dry and shrink on the rebar, will fill the flat gaps both inside and outside with strips of closed cell foam slit from lengths of pipe insulation. Nothing else will be added to the wall , neither on the outside nor the inside. Do not plan on chinking (other than the pipe insulation) because there will be a perimeter porch that will protect from rain and the closed cell foam will fit tight enough to keep out the bugs.
Question 1: Has anyone done anything similar to this and how did it work out ?
Question 2: Will this method possibly cause any moisture to be trapped between the inner and outer foam strips ?
Question 3: If yes to question 2, what would be a better substance to use for the insulation between the logs ?

Thanks for your answers,
Jim

Offline Don P

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Re: Log insulation question ?
« Reply #1 on: July 13, 2008, 07:52:18 AM »
 I'm in the middle of a discussion on something roughly similar on another forum that has become pretty much an emotional brawl. The problem there stems from the fact that people have invested alot of themselves in their projects and have a difficult time stepping back and looking at just the facts. Its tough to separate our irrational selves from our necessarily rational work sometimes  :-\. I'll try to hit some of the discussion points, my ideas, and we can go from there.

Rebar was the first sticking point. It is not a recognized fastener, as a dowel pin I don't really have a problem with it. In construction, nails, lags, bolts, etc each have a minimum steel bending spec depending on size and use. This is outside of rebar's intended use. I don't know that it makes it unusable, just things to think about. I have pinned things with rebar that are still there. That is anecdotal though. If it is inserted into a green timber that then dries, is it acting as a short column as well as a connector? Youbetcha. That might be worth considering. If the wall standing on slender pins with holes cut in it for windows and doors then meets a big storm, is it stable laterally? I can't say.

 If you install foam in the bearing area during the construction process you are building wall height on a compressible object. If you smash foam to practically no dimension it usually does not recover if needed later due to shrinkage.

Here's my feelings, they are worth what I'm selling them for. When you slab a log or timber flat and then it dries, the dried faces generally become convex. Imagine a box that has been inflated, tangential shrinkage is roughly double radial shrinkage. So we're back to standing on a single point this time a rounded surface. If you mill the timbers, sticker them and let them dry for awhile then pass a power planer down the middle of the bottom creating a slight recess then it will bear on a strip of wood on the inside and outside of the wall, we're stable. There is then a place to put foam that won't affect stack height nor will it crush the foam flat and ruin its recovery potential. The rebar pins, if the log is dry enough will not "hang up" the log. You should be bearing on the two outer bands of wood. If the logs gap later, backer rod is then forced into the gap and caulked or chinked over, it is then not a part of the bearing.

I drew this awhile ago for someone that was doing round inside and out but the concept is the same.


Think one step further, if you pre-drill the upper log with a slightly oversized hole insert a lag through the hole and screw into the lower log then it should be able to slide down freely if shrinkage still occurs keeping the bearing on the wood not on an undersized steel column.

If you rip a groove the depth of a skill saw into that underside of the cant immediately behind the sawmill, then checking will generally occur on that bottom face as it dries. Drying the cant before assembly lets you cull out logs that you probably would have regretted having in the wall later. It does risk some excess degrade that might have been restrained if the wall had been assembled green. I'm not really sure that we restrain anything when a bad timber wants to move though.

When you assemble, have the draw knife handy and make sure that all outside edges are under the log above. This involves setting the upper log, scribing any protruding ledges, removing the log, draw knifing the ledges out of the way and then fastening the log down. You want drip edges not water catching ledges.

A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart


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