Some weeks ago, I shared my notes on a building project I’ve undertaken — a sauna. At that point my team and I had leveled the building area, poured the footer and erected the frame.
Since then we’ve finished the roof and started erecting the walls. The roof is a typical metal deal. We opted for this with the intent of collecting rain water from the roof’s surface. I’ve helped on this type of roofing before, and don’t have much of interest to note.
The walls, however, are something to write home about. We chose to build with cordwood. If you aren’t familiar with this building technique, keep reading. If you are familiar, also keep reading.
Cordwood is a style of using log ends instead — as you would see in a log cabin — of log sides. Between the log ends is a mortar mixture and insulation.
Our sauna walls are eight inches thick, which means eight inch long logs with a few inches of mortar surrounding each log, and a few inches of insulation in between every two mortared logs.
Logs for cordwood must be dry, not rotten and debarked. Through extensive book reading I learned that hardwood is not ideal for cordwood — softwood is much better for a couple reasons.
The major reason is that hardwood swells more if it gets wet. Swelling in a cordwood wall can result in structural problems. The cell structure of softwood means that swelling tends not to result in a problem. If cordwood logs shrink, gaps can be filled.
The cell structure of softwood also makes it a better insulator with a higher R-value (a measure of how a barrier blocks the flow of heat).
So, I’ve stuck mostly to pine for my cordwood walls, although I’ve learned that although poplar is a hardwood, its cell structure is more like that of a softwood, rendering it suitable for cordwood. Purportedly, cedar is best for cordwood, but I haven’t gotten my hands on any cedar logs yet.
The mortar mix for cordwood can vary. The recipe I’ve used so far is nine parts mason’s sand, three parts soaked softwood chips, three parts hydrated lime, and two parts cement. The unusual part of this mix, the soaked wood chips, slows the setting of the mortar. Fast setting mortar cracks and pulls away from the wood.
I’ve used dry wood chips for the insulation, mixing with lime at a ratio of 12:1. The lime calcifies if the insulation gets wet, keeping the wood chips from rotting. It also keeps bugs from making a home in the walls.
To keep the wood walls out of the rain we made a two foot overhand on all sides of the roof.
So far (two and a part wall into the technique) I like the results. I particularly like that it builds the finished inside wall, outside wall and insulation all in one go.
I also like the aesthetic, and the flexibility in creating patterns with the cordwood, although doing so is very challenging.
I like that in learning how to build cordwood I learned the why’s of it, rather than following a set of rules I didn’t understand.
I also like that it has given me the opportunity to learn to use a chainsaw and to learn to drive my father’s truck.
I also can’t wait until all the hard work is done and I can take a nice steam in the sauna I built with my own two hands (and help from family and friends).
