The longer the logs (and thicker the wall), the better the insulation qualities. A common 16” cordwood wall for moderate climates comprises of perlite or vermiculite insulation between mortar joints. Another insulation option, used for over 40 years by Rob Roy and other cordwood builders is dry sawdust, passed through a half-inch screen, and treated with builder's (Type S) lime at the ratio of 12 parts sawdust to 1 part lime. With light airy sawdusts, this insulation is similar in its R-value to manufactured loose-fill insulations, at a fraction of the cost.