Sawdust insulation is probably the most cost effective insulating material avaliable.
Our family is planning to build a two story cordwood house in the Northeast, and we're considering adding insulation to the air spaces between the log-and-mortar walls (as mentioned in “The Thermal Efficiency of Cordwood Walls” and to the mortar mix itself. Is this a common practice? Also, what proportion of insulation (and what sort of material) should be added to the mortar?
Sawdust insulation is probably the most cost effective insulating material available. When used to fill the space within the wall, it should be buffered with hydrated lime (three shovelfuls of lime to a wheel barrowful of sawdust). Lime acts as a preservative, an insecticide, and a dehydrator.
Any sawdust added to the basic 2 1/2 to 1 mortar mix (the ratio of aggregate to cementing material) will lower the compression strength of the wall. However, I feel you can safely add 50 percent of the total volume in sawdust.
Using shovelfuls for measures, the following recipe will make an average mixer load: 1 part portland cement, 3 parts hydrated lime, 10 parts sandy gravel, and 7 parts sawdust. Although adding insulation to the lime mortar mix will have only a minimal effect on the R-value of the wall, the added sawdust will have two other important effects. First, by retaining moisture within the mixture, the tiny wood particles help the matrix cure more slowly, resulting in a stronger wall. Second, they build up the basic batch, thus allowing the mix to go a little further, which is a significant consideration in these times of tight money.
— Jack Henstridge, author of several books on stack wood construction