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.
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.
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.
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.
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.
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.
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.
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.
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