Chapter 12 Drying and Control of Moisture Content and Dimensional Changes William T. Simpson n the living tree, wood contains large quantities of Contents water. As green wood dries, most of the water is removed. The moisture remaining in the wood Determination of Moisture Content 12–1 tends to come to equilibrium with the relative humidity of Oven-Drying Method 12–2 the surrounding air. Correct drying, handling, and storage of Electrical Method 12–2 wood will minimize moisture content changes that might Recommended Moisture Content 12–3 occur after drying when the wood is in service and such changes are undesirable. If moisture content is controlled Timbers 12–3 within reasonable limits by such methods, major problems Lumber 12–3 from dimensional changes can usually be avoided. Glued Wood Products 12–3 Drying of Wood 12–5 The discussion in this chapter is concerned with moisture Air Drying 12–6 content determination, recommended moisture content val- ues, drying methods, methods of calculating dimensional Accelerated Air Drying and Pre-Drying 12–6 changes, design factors affecting such changes in structures, Kiln Drying 12–6 and moisture content control during transit, storage, and Drying Mechanism 12–6 construction. Data on green moisture content, fiber saturation Drying Stresses 12–7 point, shrinkage, and equilibrium moisture content are given Dry Kilns 12–7 with information on other physical properties in Chapter 3. Kiln Schedules 12–8 Wood in service is virtually always undergoing at least Drying Defects 12–9 slight changes in moisture content. Changes in response to Moisture Content of Dried Lumber 12–10 daily humidity changes are small and usually of no conse- Moisture Control During Transit and Storage 12–14 quence. Changes that occur as a result of seasonal variation, Plywood and Structural Items 12–14 although gradual, tend to be of more concern. Protective coatings can retard dimensional changes in wood but do not Finish and Factory Lumber 12–15 prevent them. In general, no significant dimensional changes Dimensional Changes in Wood 12–15 will occur if wood is fabricated or installed at a moisture Estimation Using Dimensional Change Coefficient 12–15 content corresponding to the average atmospheric conditions Calculation Based on Green Dimensions 12–17 to which it will be exposed. When incompletely dried mate- Design Factors Affecting Dimensional Change 12–18 rial is used in construction, some minor dimensional Framing Lumber in House Construction 12–18 changes can be tolerated if the proper design is used. Heavy Timber Construction 12–18 Interior Finish 12–18 Determination of Flooring 12–18 Moisture Content Wood Care and Installation During Construction 12–18 Lumber and Trusses 12–18 The amount of moisture in wood is ordinarily expressed as a percentage of the weight of the wood when ovendry. Four Exterior Trim and Millwork 12–19 methods of determining moisture content are covered in Finished Flooring 12–19 ASTM D4442. Two of these—the oven-drying and the Interior Finish 12–19 electrical methods—are described in this chapter. Plastering 12–19 References 12–20 12–1 The oven-drying method has been the most universally property measured. Conductance-type (or resistance) meters accepted method for determining moisture content, but it is measure moisture content in terms of the direct current con- slow and necessitates cutting the wood. In addition, the ductance of the specimen. Dielectric-type meters are of two oven-drying method may give values slightly greater than types. Those based principally on dielectric constant are true moisture content with woods containing volatile extrac- called capacitance or capacitive admittance meters; those tives. The electrical method is rapid, does not require cutting based on loss factor are called power-loss meters. the wood, and can be used on wood in place in a structure. However, considerable care must be taken to use and inter- The principal advantages of the electrical method compared pret the results correctly. Use of the electrical method is with the oven-drying method are speed and convenience. generally limited to moisture content values less than 30%. Only a few seconds are required for the determination, and the piece of wood being tested is not cut or damaged, except for driving electrode needle points into the wood when using Oven-Drying Method conductance-type meters. Thus, the electrical method is In the oven-drying method, specimens are taken from repre- adaptable to rapid sorting of lumber on the basis of moisture sentative boards or pieces of a quantity of lumber. With content, measuring the moisture content of wood installed in lumber, the specimens should be obtained at least 500 mm a building, or establishing the moisture content of a quantity (20 in.) from the end of the pieces. They should be free from of lumber or other wood items, when used in accordance knots and other irregularities, such as bark and pitch pockets. with ASTM D4442. Specimens from lumber should be full cross sections and 25 mm (1 in.) long. Specimens from larger items may be For conductance meters, needle electrodes of various lengths representative sectors of such sections or subdivided incre- are driven into the wood. There are two general types of ment borer or auger chip samples. Convenient amounts of electrodes: insulated and uninsulated. Uninsulated electrodes chips and particles can be selected at random from larger will sense the highest moisture content along their length batches, with care taken to ensure that the sample is represen- (highest conductance). Moisture gradients between the surface tative of the batch. Veneer samples should be selected from and the interior can lead to confusion. If the wood is wetter four or five locations in a sheet to ensure that the sample near the center than the surface, which is typical for drying average will accurately indicate the average of the sheet. wood, the reading will correspond to the depth of the tip of the insulated electrodes. If a meter reading increases as the Each specimen should be weighed immediately, before any electrodes are being driven in, then the moisture gradient is drying or reabsorption of moisture has taken place. If the typical. In this case, the pins should be driven in about one- specimen cannot be weighed immediately, it should be fifth to one-fourth the thickness of the wood to reflect the placed in a plastic bag or tightly wrapped in metal foil to average moisture content of the entire piece. Dried or par- protect it from moisture change until it can be weighed. After tially dried wood sometimes regains moisture in the surface weighing, the specimen is placed in an oven heated to 101°C fibers, and the surface moisture content is greater than the to 105°C (214°F to 221°F) and kept there until no apprecia- interior. In this case, the meter with the uninsulated pins ble weight change occurs in 4-h weighing intervals. A lum- will read the higher moisture content surface, possibly caus- ber section 25 mm (1 in.) along the grain will reach a con- ing a significant deviation from the average moisture content. stant weight in 12 to 48 h. Smaller specimens will take less To guard against this problem, electrodes with insulated time. The constant or ovendry weight and the weight of the shanks have been developed. They measure moisture content specimen when cut are used to determine the percentage of of only the wood at the tips of the electrodes. moisture content using the formula Dielectric-type meters are fitted with surface contact elec- Moisture content (%) trodes designed for the type of specimen material being tested. The electric field from these electrodes penetrates well Weight when cut − Ovendry weight into the specimen, but with a strength that decreases rapidly = × 100 (12–1) Ovendry weight with depth of penetration. For this reason, the readings of dielectric meters are influenced predominantly by the surface layers of the specimen, and the material near midthickness Electrical Method may not be adequately represented in the meter reading if there is a moisture content gradient. The electrical method of determining the moisture content of wood uses the relationships between moisture content and To obtain accurate moisture content values, each instrument measurable electrical properties of wood, such as conductiv- should be used in accordance with its manufacturer’s instruc- ity (or its inverse, resistivity), dielectric constant, or power- tions. The electrodes should be appropriate for the material loss factor. These properties vary in a definite and predictable being tested and properly oriented according to meter manu- way with changing moisture content, but correlations are not facturer’s instructions. The readings should be carefully taken perfect. Therefore, moisture determinations using electrical as soon as possible after inserting the electrode. A species methods are always subject to some uncertainty. correction supplied with the instrument should be applied when appropriate. Temperature corrections should then be Electric moisture meters are available commercially and are made if the temperature of the wood differs considerably from based on each of these properties and identified by the the temperature of calibration used by the manufacturer. 12–2 Approximate corrections for conductance-type (resistance) necessary to tighten bolts or other fastenings occasionally to meters are made by adding or subtracting about 0.5% for maintain full bearing of the connectors as the members each 5.6°C (10°F) the wood temperature differs from the shrink. calibration temperature. The correction factors are added to the readings for temperatures less than the calibration tem- Lumber perature and subtracted from the readings for temperatures greater than the calibration temperature. Temperature correc- The recommended moisture content of wood should be tions for dielectric meters are rather complex and are best matched as closely as is practical to the equilibrium moisture made from published charts (James 1988).