|Conceptual Reference Database for Building Envelope Research||
wood thermal conductivity vary with temperatureGood point. I wonder how many variables do control the insulative quality of any material --- and how many variables out there, that we don't know about.
Most materials exhibit lower thermal conductivity (higher thermal resistance) at lower temperatures than they do at higher temperatures. For example the R-value of extruded is typically R-5.4 per inch at 40 deg. F and R-5.0 per inch at 75 deg. F. The same holds true for woods, fiberglass, etc.. Other properities that greatly affect the thermal conductivity are the moisture content of the material, density and fiber (or grain) orientation. Higher moisture content levels contribute to higher conductivities. Geometry is another factor. Thermal conducitivity is typically tested in 1-dimension. Meaning that the other two dimensions are controlled for in the test. For wood, the conductivity is affected by whether the heat flow is parallel or perpendicular to the wood grain. Generally if the fiber orientation or grain is parallel to heat flow, one would expect the conductivity to be higher than in the perpendicular case. At densities higher than above a certain optimial density (optimal density for controlling air convection through and within the material) lead to increased conductivity as well. Variables difficult to control for might include a variation of density within a material such as core density versus perimeter density. I have no evidence but I would get an older log (bigger timber) may have a core density that is higher than the perimeter; this guess is based on the thought that over time the outer cellular rings (new growth) would tend to exert compaction forces on the older layers toward the center of the tree.
Source: Joe Charlson, USBT, United States Building Technology, Inc.
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