The Cordwood Sauna

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Cordwood Construction

Over the years, MOTHER has published several articles about cordwood masonry, but, for new readers, my standard definition will help: Cordwood masonry is a building technique by which walls are built of short logs (called log ends) laid up transversely in the wall, much as a rank of firewood is stacked. The walls derive exceptional thermal characteristics from the log ends (which have value as both insulation and mass) and the special mortar matrix, which features an insulated space between two separate mortar joints, one on the interior and one on the exterior of the wall.

A list of the best woods to use for a cordwood sauna would include white cedar, white or red pine, spruce, poplar or cottonwood (quaking aspen) and other light and airy woods. These woods should be debarked, cut to length and dried for a year outdoors in single covered ranks prior to use. Species to avoid would be aromatic red heart cedar (unbearable odor at high temperatures), very pitchy woods such as balsam fir, and dense hardwoods such as oak, beech, maple, elm and cherry. These woods will shrink substantially, even if you dry them for a long time, and if you get the log ends too dry, there is a danger of structural damage from swelling.

A good mortar mix for cordwood saunas is 9 parts sand, 3 parts soaked sawdust, 2 parts Type I Portland cement and 1 part lime. The sand should be clean, washed sand of fine gain, such as mason ry sand. The sawdust should be softwood sawdust from a sawmill, which has been passed through a half-inch screen and soaked in an open-topped vessel at least overnight. The sawdust retards the set of the mortar, greatly reducing mortar shrink age cracks between log ends. The lime to use is Type S or hydrated or builder's lime (it goes by many names). Do not use agricultural lime.

Always wear rubber gloves when mixing and working with mortar. The cement and lime can cause nasty "cement holes" in your skin, which take forever to heal. With an ordinary garden hoe, dry mix the ingredients in a wheelbarrow until the batch reaches a consistent color. Then add water and mix it in with the hoe. Keep adding water until the mortar passes the "snowball test." When you throw a "snowball" of mortar three feet in the air, you should be able to catch it without it going "sploot!" in your gloved hands. Nor should it crumble apart. Only with mortar of the right consistency can you juggle two or three snowballs of the stuff.

Building a cordwood wall is easy. Just follow the mantra: mud, wood, sawdust. First, with your rubber gloves - a trowel just slows you down - place the double mortar matrix down on the foundation. An 8"-thick wall is good for a sauna, so the mortar (mud) joints should each be about 2 1/2"-wide and about 1" thick. Next, the 2 1/2" cavity between the inner and outer mortar joints is filled with insulation; the best I've found is a sawdust/lime mixture, at a ratio of about 11:1. The insulation is poured into the cavity with a small spouted bucket or a tin can. Do not omit this step! A "dead air space" simply does not do the trick; the sauna will not perform well, if at all. Finally, set a log end into place, spanning from the inner to the outer mortar joints. Place the log end into the mud with a slight vibrating motion, creating what I call a "suction bond."

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