Is Cordwood Green?
By Rob Roy
I write a Q and A column about cordwood for Kelly Hart’s excellent Green Home Building website, greenhomebuilding.com. The following question inspired me to reply in detail.
Question from Anonymous: How can you consider cordwood as “green” when it uses so much cement in the mortar?
Great question, one I’m glad to have the opportunity to answer. First, I guess we have to come up with an understanding about what “green” means, with regard to building. My own view is that a green building must have a significant degree of the following elements: sustainability, leaving little impact on the planet, energy- efficiency in the making (often referred to as “embodied energy”) and energy- efficiency in performance (fuel efficiency for heating and cooling.) A closely related element would be that the building is healthy, particularly not chemically toxic.
Cordwood masonry “stacks up” very well here. With hybrid poplars, and other fast- growing woods, you can grow your own house in five to seven years. And these lightweight woods are good with respect to both their insulation value and their volumetric stability (expansion and shrinkage). A related consideration is that cord-wood masonry can make use of scrap wood which is unsuitable as firewood or for taking to the sawmill: curved logs, hollow logs, shorts, driftwood, re-killed wood, logging slash and ends and pieces from the sawmill. With 8-inch to 16-inch pieces, you can get a lot of log-ends from “waste” woods. I clean up the yard at a local log home manufacturer every now and again, and get nice dry white cedar pieces – 12 inches to 60 inches long – for free. In fact, I haven’t paid for cordwood in many years. Try any company that makes wood products: log cabin builders, sawmills, furniture makers, fence post makers. Ditto landscapers or tree clearing companies. The amount of wood that gets tipped into the land ll is both a shame and a minor environmental disaster. Surely it is green to make good use of this waste. Extend yourself when it comes to creative cordwood procurement. Nothing ventured, nothing gained. You’ll be amazed with the results.
Leaving Little Impact on the Planet
Eventually, all our structures must return to the earth from which they came. When its useful life is done, a cordwood building will biodegrade better than most, particularly when one of the greener binder options is used, as discussed below. But, given protection and good build quality, a cordwood home will last a long time indeed—a hundred years or forever, whichever comes first. “Protection,” incidentally, comes from keeping the cordwood masonry o the ground and guarding the walls against constant dampness by employing a decent roof overhang.
Low Embodied Energy
Classic cordwood masonry walls have three material components: the log-ends, the mortar matrix and the insulated cavity within the mortar. Let’s see how these components measure up on embodied energy.
The cordwood measures up very well indeed if it is local. If you haul it in on a flatbed truck from 1,500 miles away, well, there’s a lot of fossil fuel that goes into haulage, so a good part of cordwood’s advantage in this category would be lost. Use local woods, maybe even trees that you need to clear for the building site. The other embodied energy in the cordwood itself is the energy required to cut the trees into short log-ends. Typically, this is done with a chainsaw, so gas and oil are consumed, but not a great amount for the quantity of building material you get out of the process. Cordwood can also be cut with a large diameter crosscut saw (buzzsaw) powered by the power takeoff (PTO) from a tractor, or by a gas or electric motor. This type of saw can cut a lot of wood quickly—and accurately—with a lesser amount of fossil fuel.
The mortar matrix is what binds the wall together and gives cordwood masonry its pleasing textural appearance. It is also a key element in energy efficiency, discussed in the next section. Your question implies that we are using a lot of Portland cement in the mortar, and fair enough. The manufacturing process with Portland makes up something like ten percent of man’s energy use on this planet, a shocker.
I do not claim to be a purist with regard to green building, although green and natural building are near and dear to my heart. I also drive a car to get from A to B, and even get on a plane if I have to go as far as C. In 69 years on this planet, I have learned very little, if anything, that I would categorize as “absolute truth.” But these come close: (1) Everybody’s different. (2) Be wary of dogmatism, because truth— thanks to (1)—is both personal and transient. (3) Embrace tolerance, a logical corollary, perhaps, of (1) and (2). And, finally, I like this one from my father: (4) Exercise moderation in all things. You can have too much of almost anything: water, food, money, purity of ideals . . .
So I use Portland cement in the mortar, a material that uses a lot of embodied energy in its manufacture and transport. But I use it in moderation. My Portland- based mortar recipe is 9 sand, 3 soaked sawdust, 2 Portland, and 3 hydrated lime, the amounts being equal parts (shovelfuls) by volume. With a standard pointy-ended spade, and consistent medium-sized shovelfuls, this recipe yields a wheelbarrow load of mortar, sufficient to lay up 2 square feet of a 16-inch-thick cordwood masonry wall. As I get 12 shovelfuls out of a standard 94-pound bag, I can do 12 square feet of 16-inch cordwood masonry from a bag. A house of 960 square feet of external wall, then, will require 32 bags of Portland cement, not a huge amount, and a whole lot less than the amount used in concrete foundations or concrete block buildings. Using smaller mortar joints and a narrower wall, I still marvel that Jaki and I did the cordwood masonry at Log End Cottage in 1975 with just six bags of Portland.
Lately, we have had good success with using lime putty mortar, with no Portland at all. This is very similar to mortar as it was made before 1843. Lime putty mortar has lots of advantages, not the least of which is that it contains very much less em- bodied energy than the cement variety. This use of lime putty mortar is the subject of Chapter 10.
But, green-wise, it can get even better. Quite a few cordwood builders in the past ve years have been doing cordwood masonry with cob as the binding matrix, instead of mortar. We even did some of this at Earthwood Building School when the famous cobbers Ianto Evans and Linda Smiley (Cob Cottage Company) came to visit. It worked well, but is not sustainable for us, as there is no ready source of clay nearby. But for anyone with accessible clay as an indigenous material—probably half the country, I’m guessing—then cob is a viable alternative. Cob’s constituent ingredients are: clay (about 25 percent by volume, if the clay is fairly pure), sand and (for use with cordwood) chopped straw as reinforcing binder. The clay is the cement of cob and the sand is where the strength and hardness come from, so coarse sand is okay. The straw ties the matrix together, much like the polypropylene fibers in reinforced concrete. This type of cordwood masonry is sometimes known as “cob- wood” and is very popular with natural building purists. Jaki and I demonstrated the construction technique at the Natural Building Colloquium in Kingston, New Mexico, in October of 2015. See Chapter 11.
For insulation, we use sawdust, a waste product from sawmills. It has an insulative value of about R-3 per inch. The insulation cavity of a 16-inch cordwood wall, then, has an R-value between R-18 and R-24. We treat the sawdust with hydrated lime as a preservative, one part of lime mixed in with 12 parts of sawdust. We once put a doorway through a 25-year-old exterior cordwood wall, and we were able to salvage and reuse both the cordwood and the insulation on a new cordwood wall. In short, sawdust insulation is the greenest kind I know.
It’s hard to call a building green which uses a lot of energy for heating and cooling. Thanks to the thick log-end walls and the wonderful juxtaposition of insulation and thermal mass (the mortar matrix) in cordwood masonry, cordwood homes are very energy efficient. Moreover, they maintain a steady and comfortable temperature, summer and winter. We use about 4 full cords of wood to heat the 2,400 square feet of living space at Earthwood. And that’s usable square feet. Using exterior dimensions, as an assessor might do, the place is over 2,800 square feet. We burn hard- wood slabs—a waste product from our local sawmill—through our masonry stove. For our other two woodstoves, we buy local firewood, and cut some deadwood to improve our five acres. All told, we spend an average of $600 a year on fuel, and nothing on air-conditioning. (In complete fairness, I have to say that the round shape and the earth-sheltered feature of the home contribute to the home’s energy efficiency, too, but the cordwood masonry is a big part of it.)
The Healthy Home
Cordwood masonry is inert. There is no o -gassing, outside of normal wood aroma, which is not unpleasant (with the exception of certain stinky elms, which I would not use anyway.) Lime and Portland mortars can cause skin damage when they are fresh, so we always use cloth-lined rubber gloves during the building process, and insist upon this with our students. But, once it has cured, the mortar presents no more of a health hazard than, say, limestone. We rarely use any coating on log-ends, and never any chemical or petrol-based preservative like, for example, Thompson’s Waterseal. I have occasionally used two or three coats of water-based urethane on certain special feature log-ends. And I have had good success with siliconized sealers on the exterior, such as Cabot’s Silicone-based Waterproofing, discussed in Chapters 1 and 21.
So, Is Cordwood Masonry Green?
Well, in this author’s admittedly biased opinion, it compares very favorably with any other building method.
Note: This is an excerpt from Rob Roy’s fully revised 2nd edition of the most widely read and used book on cordwood construction that presents the latest innovations and on-the-ground experience from four decades of cordwood building and research. You can buy it here.