WOOD - Materials for Civil and Construction Engineers - page 411

gravity, or density, is a commonly cited property and is an indicator of me-
chanical properties within a clear, straight-grained wood.
The dry density of wood ranges from (10 pcf) for balsa to
(65 pcf) for some species. The majority of wood types have densi-
ties in the range of 300 to

160 kg/m 3

1000 kg/m 3

700 kg/m 3 (20 to 45 pcf). Within common domestic

species, density may vary by ; 10%.

■ 10.7.2 Thermal Properties

Thermal conductivity, specific heat, thermal diffusivity, and coefficient of
thermal expansion are the four significant thermal properties of wood.

Thermal Conductivity Thermal conductivity is a measure of the rate at
which heat flows through a material. The reciprocal of thermal conductiv-
ity is the thermal resistance (insulating) value (R). Wood has a thermal con-
ductivity that is a fraction of that of most metals and three to four times
greater than common insulating materials. The thermal conductivity ranges
from 0.06 W/(m°K) [0.34 Btu/(h-ft-°F)] for balsa to 0.17 W/(m°K) [1.16
Btu/(h-ft-°F)] for rock elm. Structural woods average 0.12 W/(m°K) [0.07
Btu/(h-ft-°F)] as compared to 200 W/(m°K) [115 Btu/(h-ft-°F)] for aluminum
and 0.04 W/(m°K) [0.025 Btu/(h-ft-°F)] for wool. The thermal conductivity
of wood depends on several items including (1) grain orientation, (2) mois-
ture content, (3) specific gravity, (4) extractive content, and (5) structural ir-
regularities such as knots.
Heat flow in wood across the radial and tangential directions (with re-
spect to the growth rings) is nearly uniform. However, heat flow through
wood in the longitudinal direction (parallel to the grain) is 2.0 to 2.8 times
greater than in the radial direction.
Moisture content has a strong influence on thermal conductivity. When
the wood is dry, the cells are filled with air and the thermal conductivity is
very low. As the moisture content increases, thermal conductivity increases.
As the moisture content increases from 0% to 40%, the thermal conductiv-
ity increases by about 30%.
Because of the solid cell wall material in heavy woods, they conduct
heat faster than light woods. This relationship between specific gravity and
thermal conductivity for wood is linear. Also affecting the heat transfer in
wood are increases in extractive content and density (i.e., knots) which in-
crease thermal conductivity.

Specific Heat Specific heat of a material is the ratio of the quantity of heat re-
quired to raise the temperature of the material one degree to that required to
raise the temperature of an equal mass of water one degree. Temperature and
moisture content largely control the specific heat of wood, with species and
density having little to no effect. When wood contains water, the specific
heat is increased because the specific heat of water is higher than that of dry
wood. However, the value of specific heat for the wet wood is higher than just
the sum of the specific heats for the wood and water combined. This increase