The thermal conductivity of wood is relatively low because of the porosity of timber. Thermal conductivity declines as the density of the wood decreases. In the direction of the grain, the thermal conductivity of wood is about twice what it is perpendicular to the grain. For example, the thermal conductivity of pine in the direction of the grain is 0.22 W/moC, and perpendicular to the grain 0.14 W/moC. Increasing the moisture in the wood also increases its thermal conductivity. As the temperature of wood decreases, its strength usually increases. The thermal expansion of wood in the direction of the grain is very little. In the radial and tangential directions, temperature movements are much greater. The relationship between the thermal expansion coefficients and moisture contraction coefficients of wood in different directions relative to the grain is in the same class in terms of size. Repeated variation in temperature decreases the strength of wood. In a temperature less than 0 °C, wood may start to crack as water in the cell lumens expands as it freezes.
The heat storing capacity of wood depends on its density, moisture content, temperature and the direction of the grain. The average specific heat value of pine and spruce at +0 – 100 °C is 2,300 J/kgoC. An increase in moisture improves the specific heat of the wood, because the specific heat of the water is greater than that of the wood. The heat capacity of pine is almost the same as that of bricks, although the density of wood compared to bricks is only 1/3. Because of good heat capacity, a heavy-duty log wall is a relatively good external wall structure, although the heat insulation capacity of mineral wool, for example, compared to the thermal conductivity of wood is about three times as great.