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.