by Kelly Hart
I have often admired the natural beauty of cordwood masonry construction, with its display of log ends stacked in random patterns visible on either side of the wall. It also appeals to my sense of appropriate technology for building in our region. So when I thought about how I might make some walls to enclose a pantry I was building on my homestead, my mind soon focused on cordwood.
The pantry itself is mostly underground, and for this portion I used earthbags filled with our native sand; the earthbag walls were inclined outward for stability. But since I wanted to be able to walk into the pantry from ground level, there would be three walls that extend above ground, and these walls needed to provide good insulation to keep the pantry from freezing. I had never tried my hand at cordwood construction before, so this seemed like a good opportunity to learn something new.
For over a decade I have been posting advice by cordwood guru Rob Roy on my website, www.greenhomebuilding.com, so I knew what the basics were. The walls are built upon a raised foundation that is wide enough to support the total width of the wall, which is usually between one and two feet. I cut my cordwood into 15 inch lengths to fit perfectly on the earthbag foundation.
The best wood to use for cordwood masonry is soft, lightweight, dry and without bark. The softer the wood, the better the insulation value and the less shrinkage or expansion can be expected. I made several trips into Forest Service land west of Saguache to collect wood with a firewood permit, looking for dead aspen or pine. Then I realized I could easily find dead cottonwood right here in the Crestone area. Not only is the cottonwood extremely light, but the rounds are often huge (which makes for faster wall making) and the bark falls off easily.
The process of assembling the wall is fairly simple. You first lay down two strips of mortar just a few inches thick on the outer sides of the foundation and then start laying the cut logs across them. This leaves a void in the middle that is filled with a mixture of sawdust and lime for insulation. As the wall goes up you use a pointing tool, like a bent table knife, to smooth out the mortar and press it into the wall a bit so that the wood rounds stand proud. Keep dong this and pretty soon you have a complete wall!
In order to keep the mortar from drying out too soon, damp sawdust is mixed with the standard cement/lime/sand mortar mix. This is done in a ratio of approximately 9 parts masonry sand, 3 parts soaked softwood sawdust (passed through a half-inch screen), 3 parts lime (hydrated, Type “S” or builder’s lime) and 2 parts Portland cement. Add enough water to make a fairly stiff mix that will support the wood without too much slumping. You can expect the dry cordwood to absorb some of the moisture, so the damp sawdust is important to balance this tendency. A more sustainable mortar can be made with cob (sand, clay and chopped straw), but this may not be quite as durable.
Cordwood can be laid between supporting vertical posts as an infill (as I did with my pantry), or it is possible to make cordwood corners with the just the short pieces of cordwood. This is done by laying alternate sections of cordwood transversely in a brick-like pattern to create a corner column that gets interlocked with the cordwood that crosses the wall in the ordinary pattern.
One of the beauties of cordwood masonry construction is that once the wall is laid up no further treatment is necessary. You don’t need to plaster it or even coat the wood with a sealer. It is advised that the end grain of the logs be left breathable to hasten drying out if moisture should enter the wall. This is a rather maintenance free system, where only occasional caulking may be necessary should any logs loosen in the mortar.
The thermal performance of cordwood masonry is quite good. The R-value depends on the thickness of the wall and the species of wood used, but you can generally expect about R-1 per inch. Combine this with the fact that all of that isolated masonry holding the wood in place on the inside acts as thermal mass that will stabilize interior temperatures, and you have an outstanding wall system.
Since only short pieces of wood are needed, and these can be of any diameter, or even split into smaller chunks, wood that might have no other value than for firewood can be used. It is also possible to incorporate other objects into a cordwood wall, like colored glass bottles connected together with a metal sleeve, to add color and light. I placed some surplus glass blocks into the wall that I built.
When I look at all of the dead standing Cottonwood trees we have in the Crestone Baca region I see a tremendous opportunity here for natural building projects that would utilize a resource that is otherwise going to waste. Cottonwood is not great firewood because it is too light and burns too fast to provide good BTU’s, but for cordwood masonry this is exactly what is desired. And the needed materials can be obtained for very little money.
I should tell you that the actual process of construction is not speedy. First you have to obtain and process all of the cordwood just to be ready to lay up the walls. Then you have mix the mortar and sawdust insulation, and then fashion the wall, complete with the pointing. All of this takes time, but once the wall is up you are done with it. I am quite pleased with how my walls turned out, and I would definitely consider doing more cordwood masonry in the future.