The science of cordwood walls hasn’t changed dramatically over the decades, but builders have added preservation methods and carefully tweaked building practices to squeeze the energy performance to up to R-35 from this system.
Cordwood
Cordwood walls tend to be 40% mortar. To the uninformed observer, they don’t seem very energy efficient.
“The mortar portion of the wall actually has superior thermal characteristics to the wood portion,” says Rob Roy, founder of Earthwood Building School in West Chazy, N.Y.
A 16-inch cordwood wall can have an insulating value of R-18 to R-24. Roy uses a mortar mix of Portland cement, sand, presoaked sawdust and builder’s lime. But the key feature, unseen in a finished home, is an air space between the inner and outer walls. That cavity can be filled with sawdust, or—for non-purists—blown cellulose or spray foam.
The thermal performance of cordwood walls was documented in 2005, at the University of Manitoba in Winnipeg. The temperature gradient through the logs was measured at different points for three winter months on test walls, 24 in. thick. Thermal resistance totaled R 1.47 per inch, or R-35 for the wall.
To achieve high performance, cordwood builders are assembling best practices. They have been passing along the information at annual cordwood conferences and through a 54-page building permit code. Richard Flatau, a cordwood expert in Merrill, Wis., has summarized some best practices:
> Frame with post and beam. That allows the roof to go on first and keep the cordwood masonry work out of the rain. The roof must overhang at least 24 in. to reduce splashback.
> Bathe logs in a borate solution to repel insects and fungus, and to preserve the wood.
> Air dry wood to 12% moisture so it won’t shrink and create gaps. Use softwood, which has a higher R-value than hardwood, and is less likely to swell and crack mortar joints.
> Fill any mortar gaps that do appear with Perma Chink or Log Jam.
These and other techniques, along with the Cordwood and the Code guidelines, reflect a goal of standardizing cordwood construction so it can be more widely used, Flatau says. “It may not work in the suburbs, but this is an attempt to make cordwood palatable,” he explains.
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