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Any
pieces of wood will give off or take on moisture from the
surrounding |
|
atmosphere until the amount of
moisture in the wood balances with that in |
|
the atmosphere. The moisture
content of the wood at the point of balance |
|
is called the "equilibrium
moisture content" and is expressed as a percentage of the |
|
oven-dry weight (no moisture
content) of the wood. Assuming constant temperature, the |
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ultimate moisture content that a
given piece of wood will attain depends entirely upon the |
|
relative humidity of the atmosphere
surrounding it. The relationship between equilibrium |
|
moisture content and relative
humidity at different temperatures is shown below. |
|
Moisture content of wood in
equilibrium with stated dry-bulb |
|
temperature and relative humidity |
|
|
|
Relative humidity, percent |
|
|
|
- |
5 |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
55 |
60 |
65 |
70 |
75 |
80 |
85 |
90 |
95 |
98 |
 |
| 30... |
1.4 |
2.6 |
3.7 |
4.6 |
5.5 |
6.3 |
7.1 |
7.9 |
8.7 |
9.5 |
10.4 |
11.3 |
12.4 |
13.5 |
14.9 |
16.5 |
18.5 |
21.0 |
24.3 |
26.9 |
| 40... |
1.4 |
2.6 |
3.7 |
4.6 |
5.5 |
6.3 |
7.1 |
7.9 |
8.7 |
9.5 |
10.4 |
11.3 |
12.3 |
13.5 |
14.9 |
16.5 |
18.5 |
21.0 |
24.3 |
26.9 |
| 50... |
1.4 |
2.6 |
3.6 |
4.6 |
5.5 |
6.3 |
7.1 |
7.9 |
8.7 |
9.5 |
10.3 |
11.2 |
12.3 |
13.4 |
14.8 |
16.4 |
18.4 |
20.9 |
24.3 |
26.9 |
| 60... |
1.3 |
2.5 |
3.6 |
4.6 |
5.4 |
6.2 |
7.0 |
7.8 |
8.6 |
9.4 |
10.2 |
11.1 |
12.1 |
13.3 |
14.6 |
16.2 |
18.2 |
20.7 |
24.1 |
26.8 |
|
70... |
1.3 |
2.5 |
3.5 |
4.5 |
5.4 |
6.2 |
6.9 |
7.7 |
8.5 |
9.2 |
10.1 |
11.0 |
12.0 |
13.1 |
14.4 |
16.0 |
17.9 |
20.5 |
23.9 |
26.6 |
|
80... |
1.3 |
2.4 |
3.5 |
4.4 |
5.3 |
6.1 |
6.8 |
7.6 |
8.3 |
9.1 |
9.9 |
10.8 |
11.7 |
12.9 |
14.2 |
15.7 |
17.7 |
20.2 |
23.6 |
26.3 |
|
90... |
1.2 |
2.3 |
3.4 |
4.3 |
5.1 |
5.9 |
6.7 |
7.4 |
8.1 |
8.9 |
9.7 |
10.5 |
11.5 |
12.6 |
13.9 |
15.4 |
17.3 |
19.8 |
23.3 |
26.0 |
|
100... |
1.2 |
2.3 |
3.3 |
4.2 |
5.0 |
5.8 |
6.5 |
7.2 |
7.9 |
8.7 |
9.5 |
10.3 |
11.2 |
12.3 |
13.6 |
15.1 |
17.0 |
19.5 |
22.9 |
25.6 |
|
▲ |
Dry bulb temperature in degrees
Fahrenheit |
|
|
Examples of Moisture Equilibrium
(Chart Use) |
| The above chart
may be used as a guide in determination whether or not the
conditions in a construction area are suitable for receiving
woodwork. For example: If wood paneling with an 8%
average moisture content is to be installed and the average
temperature in the building will be maintained at 70° F, it
can be determined by following the 70° F column horizontally
to the right until the moisture content figures of 7.7 and 8.5
are reached. here the figures show that ideally a
relative humidity of 40% to 45% should be maintained in order
to achieve perfect dimensional equilibrium. After the
paneling is painted or finished, moisture changes in the wood
are retarded so that maintenance of humidity between the
practical limits shown on the table, i.e., 25% - 55%, is
usually satisfactory. |
|
|
Job
site conditions can be checked with a hygrometer |
|
SHRINKAGE |
EXPANSION |
|
Due to Drying |
Due to Moisture in the Air |
|
 |
 |
|
25%
relative humidity |
85%
relative humidity |
|
|
TS=2RS |
|
Tangential Shrinkage |
|
Approximates Twice |
|
the Radial Shrinkage |
|
|
Shrinkage of ... |
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1" x 8" x 10' |
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Dried from Green to Oven Dry
|
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1" thickness =3/64" |
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8" width = 3/4" |
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10' length = 1/8" |
|
|
Expansion of ... |
|
1" x 8" x 10' |
|
When relative humidity increases
from 25% to 85% |
|
|
