Cordwood construction

Cordwood construction (also called "cordwood masonry," "stackwall construction" or "stackwood construction") is a term used for a natural building method in which "cordwood" or short lengths pieces of debarked tree are laid up crosswise with masonry or cob mixtures to build a wall.

Construction

Walls are usually constructed such that the pieces of wood are "proud" of (protrude from) the mortar by a small amount (an inch or less). Walls typically range between 12 and 24 inches thick.

Cordwood homes are attractive for their visual appeal, maximization of interior space (with a rounded plan), economy of resources, and ease of construction. Wood usually accounts for about 40- 60% of the wall system, the remaining portion consisting of a mortar mix and insulating fill. Cordwood construction can be sustainable depending on design and process. Unlike brick masonry, the mortar does not continue throughout the wall. Instead, three or four inch beads of mortar on each side of the wall provide stability and support. Cordwood walls can be load-bearing (using built-up corners, or curved wall designed) or laid within a post and beam framework which provides structural reinforcement and is suitable for earthquake-prone areas. As a load-bearing wall, the compressive strength of wood and mortar allows for roofing to be tied directly into the wall. Different mortar mixtures and insulation fill material both have an impact on the wall's overall R value, or resistance to heat flow.

History

Remains of cordwood structures still standing date back as far as one thousand years in northern Greece and Siberia. More contemporary versions can be found in Europe, Asia, the Americas. The exact origins of cordwood construction are unknown. It is, however, plausible that forest dwellers eventually erected a basic shelter between a fire and a stacked wood pile. Perhaps a better historical persepctive is found in the work of William Tischler of the University of Wisconsin. After studying the technique and writing a research paper on it, he states that "current" cordwood probably started in the late 1800s in Quebec, Wisconsin and Sweden. He believes that the technique started in these areas at the same time.

Wood

Cordwood construction is an economical use for log ends or fallen trees in heavily timbered areas. Other common sources for wood include sawmills, split firewood, utility poles (without creosote), split rail fence posts, and logging slash. It is more sustainable and often economical to use recycled materials for cordwood walls. Regardless of the source, all wood must be debarked before construction begins. While over 30 different types of wood can be used, the most desirable rot resistant woods are Pacific yew, bald cypress (new growth), cedars, and juniper. Acceptable woods also include Douglas fir, western larch, Eastern White Pine, Spruce Pine, Poplar, Tamarack, and Monterey pine.

Less dense, airy woods are superior because they shrink and expand in lower proportions than dense hardwoods like elm, maple, oak, and beech. Most wood can be used in a wall if it is dried properly and stabilized to the external climate’s relative humidity. Furthermore, logs of identical species and source are preferred because they limit expansion/ contraction variables.

Mortar

Rob Roy, an experienced cordwood builder, recommends a mortar mix by volume of 9 parts sand: 3 sawdust: 3 builder's lime (not agricultural): 2 Portland cement. The sawdust should be from light, airy softwood and passed through a ½ inch screen. Saw mills and chainsaw dust are great sources. Saw dust, presoaked in water before use, acts like a sponge from which the mortar draws moisture, drying slowly and reducing cracks. A commercial cement retarder can be substituted for sawdust, but has a larger environmental impact. Builder’s lime makes the wall more flexible, breathable, and self healing because it takes longer to completely set than cement. Portland cement chemically binds the mortar and should be either type I or II.

Richard Flatau, in his book Cordwood Construction: A Log End View (2007) suggests using a mortar mix of 3 sand, 2 soaked sawdust, 1 Portland Cement and 1 Hydrated Lime. This mix is for non-load bearing cordwood (i.e. post and beam framework) and has the advantage of curing slower and displaying less cracking than mortars that are "light" on sawdust. Flatau also recommends shading the masonry work from the full sun and covering it at the end of the day.

Cob mortars are more environmentally green and use on site soil, but require further steps for weatherproofing and maintenance. External waterproofing finishes for cob cordwood walls include lime-sand plaster, linseed oil coatings over earthen plaster, and Earthbind 100 compound (a waterproofing additive). Long overhanging eaves and a high foundation also help reduce weathering.

Thermal mass and insulation

Depending on a variety of factors (wall thickness, type of wood, particular mortar recipe), the insulative value of a cordwood wall, as expressed in R-value is generally less than that of a high-efficiency stud wall. Cordwood walls have greater thermal mass than stud frame but less than common brick and mortar. This is because the specific heat capacity of clay brick is higher (0.84 versus wood's 0.42), and is more dense than airy woods like cedar, cypress, or pine. Thermal mass makes it easier for a building to maintain median interior temperatures while going through daily hot and cold phases. In climates like the desert with broad daily temperature swings thermal mass will absorb and then slowly release the midday heat and nighttime cool in sequence, moderating temperature fluctuations. Thermal mass does not replace the function of insulation material.

A Western red cedar log has an R value of 1.25 per inch. In comparison, concrete is .13 per inch, stone masonry .08 per inch, common brick .20 per inch, and fiberglass insulation 3.16 per inch. The longer the logs (and thicker the wall), the better the insulation qualities. A common 16” cordwood wall for moderate climates comprises 6 inches of perlite or vermiculite insulation between mortar joints.

Batt fiberglass, blown in cellulose, or an insulated sawdust mixture are also insulators in cordwood walls. Rob Roy has found that a mixture of only vermiculite and sawdust, soaked in water overnight, results in an R value of 2.1- 2.5 per in. In a 16” wall this translates to an R value of 11-16. The use of cellulose (with an R value up to 3.7 per in.) can equalize the overall R value to that of a 2 x 6 stud frame with fiberglas insulation. Other materials like polystyrene or packaging peanuts have been economically used, and provide a purposeful use of recycled materials. A wall consisting of merely cordwood and mortar does not provide sufficient insulation for comfortable living in snowy regions.

A thermal performance analysis in 1998 using “HOT 2000” computer software showed the relationship of domestic wall types and their insulating values. The simulation revealed an R value of 20.5 for the sample cordwood wall. Compare this to the basic 2 x 4 wooden stud wall, and 2 x 6 foam insulated and sheathed wall with R values of 15.8 and 25.7, respectively. Cordwood walls are not the best natural insulators but can be built to thermal efficient standards. The R value of a cordwood wall is directly related to its ratio of wood to mortar and insulation medium. However, R value in cordwood construction is not as significant as it is in stick-frame building due to the high thermal mass which increases a significantly higher "effective R-value.” Builders tailor their design and ratios to the existing climate.

R-value testing was completed at the University of Manitoba in the winter of 2005. The findings compiled by the Engineering Department, found that each inch of cordwood wall (mortar, log end and sawdust/lime insulation yielded an r-value of 1.47.

Costs

A cordwood home can be constructed for significantly less initial cost than a standard stick frame house of comparable size if labor is done primarily by the owner or volunteers. If a cordwood wall is built competently by its owner, it will not only require less long- term maintenance but also be cheaper to construct. Likewise, if a cordwood house is poorly built with insufficient insulation, heating costs will be higher than a traditional stud frame house.

In a 1998 comparative economic analysis of stud frame, cordwood, straw bale, and cob, cordwood appears to be an economically viable alternative. A two story 2, 512 sq. ft. cordwood house in Cherokee, North Carolina outfitted with “high quality tile, tongue and groove pine, Russian woodstove, live earth roof, hand shaped cedar trim, raised panel cabinets, and a handmade pine door,” cost the owner an estimated $52,000. With the owner providing 99% of the labor, the house cost him $20.70 per sq. ft. A comparably sized and furnished stick frame house in 1998 would cost between $75,000- $120,000 with zero owner labor. The 1997 residential cost data shows an "average" trim level 1000- 2000 sq. ft. house costing $64.48- $81. 76 per sq. ft. Both the acquisition of materials and source of labor play major roles in the initial cost of building a cordwood house.

Process

In certain jurisdictions construction plans are subject to the building inspector's approval. Before building, soil conditions on the site must be verified to support heavy cordwood masonry walls.

With felled timber, bark should be removed in the spring with either a shovel or chisel. The sap is still running in spring time and provides a lubricating layer of cambium between the bark and wood, making separation an easier task than if left until the fall when the two layers are well-bonded together. Once debarked, the logs should sit to dry for at least three summers to limit splitting and checking. It is important to cut the logs, once debarked to the chosen building length. Richard Flatau, Cordwood Construction: A Log End View (2007) suggest splitting 70% of the wood for better drying and seasoning.

After drying, the logs must be sawed to the desired length (usually 16, 18, or 24 in.). In this case a metal handsaw is preferable to a chainsaw because its finer cut helps to ward moisture and pest penetration. Actually a "cut off " saw or "buzz saw" will make quick work of cutting cordwood into chosen lengths. For especially furry ends like on cedar, rasps can be used for smoothing.

The wood then needs to be transported to the building site. It is convenient to have the source of cordwood and construction site nearby. Once a proper foundation has been poured which rises 12-24 inches above ground level with a splash guard, construction of the walls can begin. Temporary shelters can be used to cover the worksite and cordwood from rain. A post and beam frame supplies this shelter for subsequent cordwood mortaring.

Inexperienced homebuilders should experiment with a number of practice walls. This will ultimately expedite the building process and provide more satisfying results. First, two parallel 3 to 4 inch beads of mortar are laid down along the foundation, followed by a middle filling of insulation material. Then logs are laid on top with consistent mortar gaps, protruding no more than 1 inch on the inside and outside of the wall. Actual placement will depend on the size and shapes of the logs. Another layer of mortar is spread, then insulation poured in between, more logs follow and so on. The shape and exterior orientation of logs is important only for appearance. Pre-split “firewood style” logs check less when in the wall and are easier to point or smooth and press evenly around than round pieces because the mortar gaps are generally smaller. If constructing a house with corners, each course of cordwood should be cross hatched for strength. Near the end, small filler slats of wood may be required to finish the joining or tops of walls. Windows and doors are framed with standard window boxes and wooden lintels. Glass bottles can be inserted for a creative stained glass effect. (Plumbing and electrical wiring are issues to consider but will not be elaborated on in this article). A cordwood house should have deep overhanging eaves of at least 12- 16 inches to keep the log ends dry and prevent fungal growth. If the ends are maintained to be dry and well aerated, they will age without problem. Some owners have coated their ends with linseed oil, or set the outside log ends flush with the mortar for further weatherproofing. Over time, some checking is normal, and can be remedied with periodic mortar or caulking maintenance.

Sustainability

Although cordwood homes have been tested in -40F locations like New Brunswick, their thermal efficiency in any climate is below that of a purely cob house of comparable dimensions.. In frigid areas it is appropriate to either build a thicker 24 inch wall or two separate super insulated walls. In predominantly wet areas, the outside walls can be plastered, smothering the cordwood ends from air and moisture, but this hides cordwood's attractive log ends. The quantity of labor relative to gaining a specific R value for cordwood is higher when compared to straw bale and stick frame construction. Funds saved in construction may need to be allocated for heating costs or longterm exterior maintenance. An organic mortar like cob creates less of an environmental impact because of the use of readily available mud and straw, whereas toxins emitted during the production of Portland cement are very harmful, albeit less tangible in the final product. Like many alternative building styles, the sustainability of cordwood construction is dependent upon materials and construction variables.

Following the Cordwood Conference in 2005 at Merrill, Wisconsin, a document was published to address best practices in cordwood construction and building code compliance.The document entitled Cordwood and the Code: A Building Permit Guide assists cordwood builders get the necessary code permits.

Notes

References

Flatau, Richard (2007) Cordwood Construction: A Log End View
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Pierquet, P., Bowyer, J., Huelman, P. (1998). Thermal performance and embodied energy of cold climate wall systems. Forest Products Journal, 48, 53.
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Snell, C. & Callahan, T. (2005). Building green. Lark Books: NY.
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