I have been trying to learn about cordwood construction. So far it seems plausible that I could use this method. However I am concerned about the mortar. What was traditionally used in the 1800s for cordwood cabins? Why can't clay soil be used instead of a mixture of concrete, lime, sand and sawdust? In some tutorials and videos I seed using just mortar to fill in between the logs, others I see mortar on the ends with sawdust and lime stuffing for insulation. Which method last the longest? Which insulates more? Any other links, blogs, pics etc a appreciated as so far this is my favourite potential building technique.
Proper Cordwood construction calls for two bands of mortar with insulation in the gap.
This is done for areas where the climate gets very cold, it also helps a great deal with the shrinkage that always occurs to a freshly built wall, the mortar cures and shrinks a bit, the logs (which swell a bit during the build also shrink back and leave a gap at the junction of log and mortar).
Good mortar for cordwood would contain; Portland cement, sawdust, lime and if you did testing first for bond and durability, clay.
It is always best to do a small test build when developing your mortar mix, keeping good records of these trials make it easier to duplicate the mix that was found to work best.
There have been many mixes tried but in the old days (1600's to 1900) the mortar was usually 1 part Portland, 3 parts sand, 1/8 part lime just like for bricks. They didn't use mortar in the middle just like you don't now.
Our modern mixes actually work better more because of the better quality of ingredients and more experimentation prior to actual building of a project.
There is a new method for insulation, which also bonds the cordwood and helps seal leaks, that is the use of expanding foam in-between the mortar runs.
To use this method you first set the log sections with mortar then fill the middle spaces with expanding foam.
The method takes a little care since you don't want to use so much foam that it expands enough to displace the mortar on either side.
There is also the double wall method, where there is an exterior cordwood wall, an insulation space filled appropriately, then an inner wall of cordwood, all tied together with either bonding strapping or occasional long pieces of cordwood that traverse the whole wall span (exterior wall to interior wall)
It is perfectly acceptable to use a cob like mortar for building a cordwood wall, this would need to be sealed so rain or high humidity didn't cause deterioration of the cob mix.
In either method you will be coming back in a year and resealing the wood to the mortar since there will be shrinkage to deal with for controlling air leaks created by the shrinkage.