Wood Handbook, Wood as an Engineering Material

Methods for Calculating Density

The density of wood (including water) at a given moisture
content, ρx, may be determined by any of three methods:

Method 1—Equations Using Basic Specific Gravity
The specific gravity Gx based on volume at the moisture
content of interest may be calculated from Equation (4–9)
or (4–11) with basic specific gravity taken from Table 5–3,
5–4, or 5–5 (Chap. 5). Density is then calculated by

(4–12)

Method 2—Equations Using Ovendry Density
Density is given by

( ) 











-

-

= +

x

x S

S

x

100

100

ρ ρ 01 / 100 0 (4–13)

where Sx is calculated using Equation (4–7) and S 0 is taken
from Table 4–3 or 4–4. If S 0 is not known for the particular
species of interest, it can be estimated using the same rela-
tion as in Equation (4–10), which in terms of ovendry
density is

(4–14)

Method 3—Using Figure 4–6 and Table 4–6

Figure 4–6 depicts the relationship between specific grav-

ity Gx and moisture content for different values of basic

specific gravity. This figure adjusts for average dimensional

changes that occur below the fiber saturation point (as-

sumed to be 30% MC) and incorporates the assumptions in

Equations (4–7), (4–10), and (4–11). The specific gravity of

wood does not change at moisture content values above ap-

proximately 30% because the volume does not change. To

use Figure 4–6, locate the inclined line corresponding to the

known basic specific gravity (volume when green). From

this point, move left parallel to the inclined lines until verti-

cally above the target moisture content. Then read the spe-

cific gravity Gx corresponding to this point at the left-hand

side of the graph.

For example, to estimate the density of white ash at 12%

moisture content, consult Table 5–3a in Chapter 5. The aver-

age basic specific gravity Gb for this species is 0.55 (volume

when green). Using Figure 4–6, the dashed curve for

Gb = 0.55 is found to intersect with the vertical 12%

moisture content dashed line at a point corresponding to

G 12 = 0.605. The density of wood (including water) at this

moisture content can then be obtained from Table 4–6 (these

values are based on Eq. (4–12)). By interpolation, the spe-

cific gravity of 0.605 corresponds to a density at 12% MC

of 678 kg m–3 (42.2 lb ft–3).

Thermal Properties

Four important thermal properties of wood are thermal con-

ductivity, heat capacity, thermal diffusivity, and coefficient

of thermal expansion.

Thermal Conductivity

Thermal conductivity k is a measure of the rate of heat flow

(W m–2 or Btu h–1 ft–2) through a material subjected to unit

temperature difference (K or °F) across unit thickness

(m or in.). The thermal conductivity of common structural

woods is much less than the conductivity of metals with

which wood often is mated in construction. It is about two

to four times that of common insulating materials. For ex-

ample, the conductivity of structural softwood lumber

at 12% moisture content is in the range of 0.10 to

0.14 W m–1 K–1 (0.7 to 1.0 Btu in. h–1 ft–2 °F–1) compared

with 216 (1,500) for aluminum, 45 (310) for steel, 0.9 (6)

for concrete, 1 (7) for glass, 0.7 (5) for plaster, and 0.036

(0.25) for mineral wool. Thermal resistivity is simply the

reciprocal of the thermal conductivity. Insulating materials

of a given thickness are commonly compared by their “R-

value,” or thermal resistance, which is simply the thermal

resistivity times the thickness.

The thermal conductivity of wood is affected by a number

of basic factors: density, moisture content, extractive con-

tent, grain direction, structural irregularities such as checks

0.1 8

0.2 2

0.2 6

0.3 0

0.3 4

0.3 8

0.4 2

0.4 6

0.5 0

0.5 4

0.5 8

0.6 2

0.6 6

0.7 0

0.7 4

0.7 8

0.8 2

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0 .2 0

0 .2 4

0 .2 8

0 .3 2

0 .3 6

0 .4 0

0 .4 4

0 .4 8

0 .5 2

0 .5 6

0 .6 0

0 .6 4

(volume at current moisture content)

Specific gravity G

x

Moisture content (%)

Basic specific gravity Gb

(volume when green)

Figure 4–6. Relationship of specific gravity and
moisture content.

General Technical Report FPL–GTR– 190