ENVIRONMENT—CONDITIONS FOR MOLD AND DECAY
8.4. WATER AND WOOD—RELATIVE HUMIDITY VERSUS MOISTURE CONTENT
With regard to wood and decay or mold conditions, there are two ways to express moisture as it exists in buildings: relative humidity (RH) and moisture content (MC). Relative humidity is an expression of the amount of water vapor in air, while moisture content is the weight percent of water within a material, in this case wood. Relative humidity is the percent of moisture in air, expressed as RH¼100(p/p0), where pequals the actual amount of water present in air andp0 equals the maximum water vapor that the air could hold at the same temperature.
Wood moisture content is the percent of water in wood expressed as a percent of the oven-dry weight of wood or [(weight of wood in the wet condition2weight of oven dry wood)/weight of oven-dry wood] in percentage.9 In normal indoor ambient conditions, wood moisture content is low, generally below 20%
(Table 8.3) and is constantly striving toward equilibrium with the surrounding air.
At wood moisture content below the fiber saturation point (FSP), the relative humid- ity of the surrounding air controls wood moisture. Below FSP (below 20 – 30% MC) TABLE 8.2. Common Building Materials
Wood-based building materials Wood lumber
Surface-treated wood Preservative-treated wood Laminated wood Parquet
Cork
Plywood boards Fiberboard Sawdust insulation Cellulose insulation Paper
Wallpaper
Non-wood-based building materials
Gypsum board [CaSO4– 2(H2O), hydrated calcium sulfate]
Ceramics Paints and glues Plastics Wood adhesives Concrete
TABLE 8.3. Calculated Moisture Contentaof Wood at Various Temperature and Relative Humidity Levels
Relative Humidity (%)
Ambient Air Temperature (8F)
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
aActual moisture content (mc) varies according to drying history of the wood and the wood species.
Source: After Table 3.4 in Ref. 10.
water exists in wood in a chemically bound state; water molecules are hydrogen- bonded to the cellulose and hemicelluloses in the wood. As the relative humidity of the indoor air increases, as is typical in summer months, the number of bound water molecules in wood increases, and wood swells, while the opposite occurs in winter months, as heating systems generally lower the relative humidity of indoor air. At the same relative humidity and temperature conditions, wood will attain different equilibrium moisture content (EMC) levels depending on its drying history (Fig. 8.4).10 For example, wood equilibrating from the green condition attains an EMC of 16% at conditions of 708F and 80% RH, while wood that was pre- viously dried to 6% EMC, when exposed to the same conditions, (708F and 80% RH) will be unable to attain EMC of 16%, but instead may reach EMC of only 12 – 14%.
This hysteresis effect occurs when, on drying, bound water molecules leave the wood cell wall and hydrogen bonds form between adjacent cellulose molecules, rendering those sites inaccessible to water.
8.4.1. Wood Moisture Content and the Fiber Saturation Point
The fiber saturation point (FSP) is the moisture content of wood where bound water is at a maximum and any additional water must exist as free water. The wood cell wall is highly porous; this porosity allows absorption of water vapor into the cell wall. The amount of water molecules that can chemically react with the cell wall is limited by the cell wall structure. Somewhere between 20% and 30% MC, the cell wall becomes saturated with bound water. At this point, as wood is exposed to water, the cell lumens begin to fill with “free water” (Fig. 8.5). Below the fiber saturation point wood is striving for equilibrium with the surrounding air and only Fig. 8.4. Theoretical desorption and adsorption curves for the moisture content of wood.
Equilibrium moisture content (EMC) is based on percent moisture based on the oven-dry weight of wood. Initial desorption from the green condition yields higher moisture content than does subsequent absorption. (AfterThe Wood Handbook.10)
bound water exists. The FSP occurs somewhere in the range of 20 – 30% MC and corresponds to a relative humidity that is approaching 100%. Below the FSP the relative humidity and temperature conditions determine the wood moisture content. For example, at 708F, as RH increases from 30% to 80%, the wood moisture content will increase from 6% to 16% (Table 8.3).
Fig. 8.5. Scanning electron micrographs of Douglas fir showing the porous nature of wood:
(a) the largest pores are the cell lumens and are readily visible in the cross section and longitudinal sections through the tracheids; (b) cell lumens are devoid of free water; (c) the white color in the cell lumens indicates that the lumens are completely saturated with free water, or wood at its maximum moisture content (50 magnification). (Courtesy of the N. C. Brown Center for Ultrastructure Studies, SUNY College of Environmental Science and Forestry, Syracuse, NY.)
Within a building, for wood to exist in a state above fiber saturation, there must be a source of liquid water other than water vapor in the surrounding air, or changing conditions may cause condensation of water vapor on surfaces that are cooler than the surrounding air.
The porosity of wood is defined at three levels, and these levels help define bound versus free water.11The largest openings are the cell lumens with size ranging from 5 to 200mm (Fig. 8.5). The second level includes the pit apertures and openings in pit membranes that range in size from 11022to 5mm, and the smallest are the voids in the cell wall, which are less than 11022mm.11 Bound water occurs in the smallest cell wall voids. According to Griffin,11 corresponding relative vapor pressure (RH/100) for voids to begin to fill with water are 0.9998 or greater for the smallest voids, 0.9997 – 0.90 for openings in pits and pit membranes, and ,0.90 for cell lumens.
For lumber or furniture, or any wood destined for indoor use, the wood is typi- cally dried to a moisture content that will match the EMC range that the wood will attain in service. For this reason, recommended moisture content values for framing lumber are 15% to19%.10For furniture and millwork, the target moisture content is close to that found under typical indoor conditions (relative humidity 50%, temperature 708F) or a moisture content of 6 – 9%.
The fiber saturation point is also the defining point for the wood properties of strength and dimensional stability. Above FSP, strength and wood dimension do not change. Below FSP, as bound water leaves the wood cell wall, wood properties, especially strength, generally improve (Fig. 8.6). As RH decreases, wood MC decreases, and most strength properties increase gradually below FSP. At FSP and higher, shown in Figure 8.6 as 22% MC, strength properties are at the minimum and stay the same up to full saturation. Also, wood reaches its maximum dimension at this point.
Fig. 8.6. The effect of wood moisture content on wood strength properties at and below the fiber saturation point:A—tension parallel to grain;B—bending;C— compression parallel to grain;D—compression perpendicular to grain;E—tension perpendicular grain. (Reprinted from theWood Handbook.10)