Cordwood - Thermal Mass and Insulation

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 denser 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, but is used in conjunction with it.

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 (150 mm) of perlite or vermiculite insulation between mortar joints.

However, wood is an anisotropic material with respect to heat flow. That means its thermal resistance depends on the direction of heat flow relative to the wood grain. While wood has a commonly quoted R-value of about 1.25 per inch (depending on the species and moisture content), that only applies if the heat flow is perpendicular to the grain, such as occurs in common wood frame construction. With cordwood/stackwall construction, the direction of heat flow is parallel to the grain. For this configuration, the R-value is only about 40% of that perpendicular to the grain. Thus, the actual R-value of wood, when used in cordwood/stackwall construction is closer to about 0.50 per inch.

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 usually provide sufficient insulation for comfortable living in snowy regions, unless it is of the highly insulative Throughwall type.

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.