Computing Moisture Content of Wood
The moisture content of wood is directly related to the humidity and
temperature of the surrounding air. The equilibrium moisture content
(EMC) occurs when the wood has reached an equilibrium with its environment
and is no longer gaining or losing moisture. Here is a calculator for
computing the EMC of wood at or below the fiber saturation point (about 30%
moisture content) given the temperature and relative humidity:
The table below provides EMC values for a fairly representative range of
atmospheric conditions that wood is likely to be exposed to. Values in
this table are applicable to wood of any species for most practical
purposes.
| Relative Humidity % |
Ambient Air Temperature – degrees Celsius and Fahrenheit
(Celsius rounded to nearest degree) |
|
C: |
-1 |
4 |
10 |
16 |
21 |
27 |
32 |
38 |
43 |
49 |
54 |
|
F: |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
130 |
|
5 |
1.4 |
1.4 |
1.4 |
1.3 |
1.3 |
1.3 |
1.2 |
1.2 |
1.1 |
1.1 |
1.0 |
|
10 |
2.6 |
2.6 |
2.6 |
2.5 |
2.5 |
2.4 |
2.3 |
2.3 |
2.2 |
2.1 |
2.0 |
|
15 |
3.7 |
3.7 |
3.6 |
3.6 |
3.5 |
3.5 |
3.4 |
3.3 |
3.2 |
3.0 |
2.9 |
|
20 |
4.6 |
4.6 |
4.6 |
4.6 |
4.5 |
4.4 |
4.3 |
4.2 |
3.0 |
3.9 |
3.7 |
|
25 |
5.5 |
5.5 |
5.5 |
5.4 |
5.4 |
5.3 |
5.1 |
5.0 |
4.9 |
4.7 |
4.5 |
|
30 |
6.3 |
6.3 |
6.3 |
6.2 |
6.2 |
6.1 |
5.9 |
5.8 |
5.6 |
5.4 |
5.2 |
|
35 |
7.1 |
7.1 |
7.1 |
7.0 |
6.9 |
6.8 |
6.7 |
6.5 |
6.3 |
6.1 |
5.9 |
|
40 |
7.9 |
7.9 |
7.9 |
7.8 |
7.7 |
7.6 |
7.4 |
7.2 |
7.0 |
6.8 |
6.6 |
|
45 |
8.7 |
8.7 |
8.7 |
8.6 |
8.5 |
8.3 |
8.1 |
7.9 |
7.7 |
7.5 |
7.2 |
|
50 |
9.5 |
9.5 |
9.5 |
9.4 |
9.2 |
9.1 |
8.9 |
8.7 |
8.4 |
8.2 |
7.9 |
|
55 |
10.4 |
10.4 |
10.3 |
10.2 |
10.1 |
9.9 |
9.7 |
9.5 |
9.2 |
8.9 |
8.7 |
|
60 |
11.3 |
11.3 |
11.2 |
11.1 |
11.0 |
10.8 |
10.5 |
10.3 |
10.0 |
9.7 |
9.4 |
|
65 |
12.4 |
12.3 |
12.3 |
12.1 |
12.0 |
11.7 |
11.5 |
11.2 |
11.0 |
10.6 |
10.3 |
|
70 |
13.5 |
13.5 |
13.4 |
13.3 |
13.1 |
12.9 |
12.6 |
12.3 |
12.0 |
11.7 |
11.3 |
|
75 |
14.9 |
14.9 |
14.8 |
14.6 |
14.4 |
14.2 |
13.9 |
13.6 |
13.2 |
12.9 |
12.5 |
|
80 |
16.5 |
16.5 |
16.4 |
16.2 |
16.0 |
15.7 |
15.4 |
15.1 |
14.7 |
14.4 |
14.0 |
|
85 |
18.5 |
18.5 |
18.4 |
18.2 |
17.9 |
17.7 |
17.3 |
17.0 |
16.6 |
16.2 |
15.8 |
|
90 |
21.0 |
21.0 |
20.9 |
20.7 |
20.5 |
20.2 |
19.8 |
19.5 |
19.1 |
18.6 |
18.2 |
|
95 |
24.3 |
24.3 |
24.3 |
24.1 |
23.9 |
23.6 |
23.3 |
22.9 |
22.4 |
22.0 |
21.5 |
|
98 |
26.9 |
26.9 |
26.9 |
26.8 |
26.6 |
26.3 |
26.0 |
25.6 |
25.2 |
24.7 |
24.2 |
EMC Equation
The EMC calculator and table employ the following equation to derive
moisture content:
M = 1800/W [ KH/(1-KH) + (K1KH + 2K1K2K2H2)
/ (1 + K1KH + K1K2K2H2)
]
where
M = moisture content (%)
T = temperature (oF)
H = relative humidity/100
W = 330 + 0.452T + 0.00415T2
K = 0.791 + 0.000463T – 0.000000844T2
K1 = 6.34 + 0.000775T – 0.0000935T2
K2 = 1.09 + 0.0284T – 0.0000904T2
Source: U.S. Forest Products Laboratory
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