Firewood: Don’t Burn It, Build With It

posted in: Articles, Cordwood, Publications | 7

This article by Clarke Snell was originally published in the New Life Journal.

Western North Carolina is becoming an incubator for green and natural building. As far as green building goes, homes built to Healthy Built Home standards have skyrocketed. We’re also well stocked with the latest cutting edge technologies and building concepts. For example though they were rare only five or ten years ago, you pretty much can’t shake a stick without hitting a hydronic in-floor heating system these days. Solar hot water is WAY back in, too, I’m happy to say. On the natural building side, it’s a real smorgasbord around here. We’ve got a number of code approved straw bale houses in several area counties. I know of two Earthships (a housing system that uses old tires packed with dirt as the wall system) in good old Madison County for god’s sake. We’ve got some cob, adobe, clay-slip straw, and the other earth/straw permutations around, too, though they seem to be more often built below the code radar than not. Conspicuously absent from our collection has been cordwood construction….until now. Toby Crawley and Maria Muscarella are nearing completion on a code approved cordwood house in Leicester. Let’s check it out.

What is Cordwood Construction?

Cordwood is firewood: air-dried, unmilled wood cut to length. Cordwood construction (often called “cordwood masonry”) is a technique for building walls with firewood. In the most basic approach, sticks of wood are placed in two beds of mortar separated by a space, usually several inches wide, which is then filled with some kind of loose insulation such as sawdust or vermiculite. (PHOTO). There are many possible mortar mixes. One popular mix contains sand, wet sawdust, lime, and Portland cement. Another uses lime and sand. Another paper pulp. Yet another simply clay, sand, and straw (cob). Cordwood walls can be designed to carry roof loads or they can be installed in combination with some form of post and beam structure. Since wood can shrink or swell, species and drying time are variables that are often debated by cordwood enthusiasts.

Cordwood Pros, Cons, and Performance

Pros: If you live in the forest, then the main advantage of cordwood is obvious: it’s an abundant, locally available, affordable building material. If you choose to go with a cob mortar and sawdust insulation, you could collect almost all of your wall volume from your building site. That’s saying something these days! In addition, laying cordwood requires only basic tools and simple skills. Once laid, cordwood walls require no additional finish such as drywall or wallpaper with sea shells on it. (Note: I’m only talking about the cordwood portion of the construction here. You still need a foundation, window and door framing, a roof system, heating and cooling strategies and systems, and all the other things that make a house a complex animal.)

Cons: Laying cordwood is a lot of hard, physical work. It also takes a lot of forethought in terms of cutting and drying the wood. (It’s a good idea to let cut and split wood air-dry under cover for at least a year before using in a wall.) In addition, the exposed end-grain of each piece of wood facing toward the exterior is susceptible to water infiltration and therefore mold, insects, and other damaging forces. Good design such as a proper foundation and good roof overhangs can go a long way to solving this issue. For me, perhaps the main functional cordwood con is wood shrinkage which can cause gaps and cracks that lead to air infiltration and even separation of cordwood from the mortar.

Performance: Comparing cordwood’s thermal performance to a more conventional wall system is difficult to generalize and beyond the scope of this column. However, I will say that since cordwood is made on site, it’s thermal performance can be adjusted to suit the specifics of the house project it is serving. The thicker the wall, the better it will resist the flow of heat, so you can theoretically generate the performance you need by adjusting wall thickness. In colder climates, an option for increasing thermal performance is double wall cordwood masonry, a system employing two cordwood walls separated by a space filled with insulation. Wood is both a decent insulator and a good thermal mass, so it is competent at both resisting heat flow and holding heat. Another potential performance plus for cordwood is it’s hygroscopic nature…it’s ability to take on and give off water vapor in response to changes in humidity levels. This trait theoretically helps wood to balance indoor humidity levels and therefore potentially improve indoor air quality.

Toby and Maria’s House

As someone who has been hangin’ around the natural building water cooler for a number of years, I have to say that I’m always skeptical when I hear about the next wave of novice owner builders taking a shot at home construction. Sometimes it works out great and sometimes…well it’s a disaster. I’m happy to report that Toby and Maria are doing a good job and look like they are going to make it through intact. In my opinion, their secret to success has been (1) an initially somewhat realistic budget and (2) the financial flexibility to go well over their initially somewhat realistic budget.

After doing their research and checking out a variety of options, Toby and Maria chose cordwood over other “natural” building options because they thought they could muster the skills and reasoned that cordwood could pass code in the area. This turned out to be true probably mainly because they chose a post and beam structure with cordwood infill. The post and beam construction was stamped by a structural engineer leaving the cordwood infill with no official structural role. They cut cordwood from poplar harvested on their property and bought most of the rest of the framing lumber from a local mill. They salvaged hardwood floor from a dumpster (it looks great!) and bought most of their doors and windows from Habitat for Humanity.

The approximately 1,400 square foot building is 16-sided and roughly circular. It will have a living roof planted with sedums. The north section of the first floor and the small second floor are wood framed and insulated with Icynene spray foam leaving roughly the east, south, and west areas of the first floor in cordwood. Though they are hooked up to and existing well and septic system and have a flush toilet, Toby, Maria and family plan to continue using their sawdust toilet and composting their humanure for use in the garden. (Yeah, baby! See my rant against flush toilets in other of my writings or just stop me on the street to get an earful.) They have hydronic in-floor heating fed only by solar collectors, i.e. there is no boiler back-up and therefore no petroleum based fuel input. The back-up heat source is a high efficiency wood stove. Last, but not least, they are using Earthpaint finishes throughout the building. (If you don’t know about local paint and finish manufacturer, Earthpaint, get with it already!)

If you want more information, Toby and Maria have graciously agreed to supply a contact email address (tcrawley@gmail.com). They still have some cordwood to lay, so get in on the next cordwood party! As for me, I’m always looking to deepen my knowledge of the local natural and extreme green building scene, so don’t hesitate to send me leads and contact info for interesting projects at clarke@thinkgreenbuilding.com. Until next month, keep it green.

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