The strength of plywood and OSB (oriented strand board) structural panels is less at elevated temperatures than at normal temperatures. In the range of 0° F to 200° F, the strength of the panel at 12 percent moisture content or more will increase or decrease approximately 1/2 percent for each one degree increase or decrease in temperature from 70° F. However, such panels exposed to temperatures up to 200° F for a year or more may not experience any significant or permanent loss in strength. If drying occurs, the increase in strength due to drying may offset the loss in strength due to elevated temperature.
The thermal expansion of wood is much smaller than expansion due to absorption of water. Because of this, thermal expansion can be neglected in cases where wood is subject to considerable swelling and shrinking. Thermal expansion may be of importance only in assemblies with other materials where the moisture content is maintained at a relatively constant level. Plywood and wood expand upon heating as do practically all known solids. The thermal expansion of wood, however, is quite small and requires exacting techniques for its measurement.
The effect of temperature on plywood dimensions is related to the percentage of panel thickness in plies having grain perpendicular to the direction of expansion or contraction. The average coefficient of linear thermal expansion is about 3.4 x 10-6 inch/inch per degree F for a plywood panel with 60 percent of the plies or less running perpendicular to the face. The coefficient of thermal expansion for panel thickness is approximately 16 x 10-6 inch/inch per degree F.
Conductivity
The ability of a material to conduct heat is measured by the thermal conductivity, k. This term is typically expressed in units of Btu per hour per sq ft per degree Fahrenheit per inch of thickness. The higher the k value, the greater the ability of the material to conduct heat; the lower the k, the higher the insulation value. Examples of k are 2,700 for copper (a heat conductor), 427 for window glass and 0.27 for glass wool (a heat insulator).
The table below lists representative values for thermal conductivity, k, for plywood species groups as defined in PS 1. The values presented in the table represent volume-weighted averages of the wood species included in each species group. Note that these values would be accurate only if all veneers in each panel were of the group listed. In practice, plywood either carries no group designation at all, or is described by the species group of the face plies, with species of other groups allowed in inner plies.
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Average Thermal Conductivity
(k, for Plywood Species Groups at 12% Moisture Content)
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Species Group
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k (Btu/hour/sq ft/degree
Fahrenheit/inch thickness)
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1
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1.02
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2
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0.89
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3
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0.86
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4
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0.76
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Resistance
For most practical purposes it is neither necessary nor feasible to determine the actual species makeup of the plywood panel. For determining the overall coefficient of heat transmission (U value) of a construction assembly, APA publications use k = 0.80 for softwood, as listed by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Use of this single value simplifies computations, and produces only insignificant differences in resulting design heat losses. The table below shows thermal resistance, R, for several plywood panel thicknesses, based on k = 0.80. Thermal resistance represents the ability of the material to retard heat flow and is the reciprocal of k, adjusted for actual material thickness.
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Thermal Resistance
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Panel Thickness
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Thermal Resistance R
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1/4"
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0.31
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5/16"
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0.39
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3/8"
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0.47
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7/16"
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0.55
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15/32"
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0.59
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1/2"
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0.62
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19/32"
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0.74
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5/8"
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0.78
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23/32"
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0.90
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3/4"
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0.94
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7/8"
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1.09
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1"
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1.25
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1-1/8"
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1.41
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Exposure to Extreme Heat
From an appearance standpoint, unprotected plywood should not be used when temperatures exceed 200° F (93° C). At temperatures above 200° F, plywood undergoes slow thermal decomposition that permanently reduces its strength. Between 70° F and 200° F strength loss is recovered when temperature is reduced. Between 70° F and 200° F, the need for design adjustment depends upon whether or not the plywood moisture content is reduced by the elevated temperature. Exposure to sustained temperatures higher than 200° F (93° C) will result in charring and weight loss. Using plywood in applications involving periodic exposure to temperatures from 200° F to 302° F (93° to 150° C) should be based on the amount of exposure and the amount of decomposition that can be tolerated without impairing the serviceability of the panel.
One example of plywood use in extreme conditions involves plywood pallets used in an annealing oven. Although the temperatures reach 350° F, the plywood performs well, despite the slight charring and discoloration.
Thermal Degradation and Ignition Point
When the temperature of dry wood is raised above 212° F (100° C) a slow exothermic decomposition takes place. This degradation involves the loss of carbon dioxide and volatile materials such as extractives, in the form of gases or vapors. The rate depends upon temperature and air circulation.
The thermal degradation and ignition point of wood and plywood may be generalized by the following:
- 230° to 302° F (110° C to 150° C): The wood will char over time with the formation of charcoal. If the heat is not dissipated there is some possibility of spontaneous combustion. Examples of the thermal degradation of maple blocks are:
- 1050 days at 225° F (107° C): 10 percent loss in weight and slight discoloration.
- 1235 days at 248° F (120° C): 30 percent weight loss and a chocolate color.
- 320 days at 284° F (140° C): 60 percent weight loss and charcoal appearance.
- 302° to 392° F (150° to 200° C): Charring takes place at a somewhat greater rate. If the heat source is close to the wood, the surface temperature may be higher than the temperature of the surrounding air due to radiant heating. Gases released at these temperatures are not readily ignited by an outside flame source. A greater chance for spontaneous combustion is present if the heat is not dissipated.
- In tests, after 165 days at 302° F (150° C) maple blocks showed a 60 percent weight loss, and the samples had the appearance of charcoal.
- 392° to 536° F (200° to 280° C): The formation of charcoal takes place at a rapid rate. Spontaneous combustion is probable.
- 536° F (280° C) and greater: Spontaneous combustion will occur in a short period of time.
A number of attempts have been made to measure a definite ignition temperature of wood, with little success. A specific temperature is hard to define because there are so many contributing factors, such as size and shape of the material, air circulation, rate of heating, moisture content of the wood and so on. Estimates range from 510° to 932° F (270° to 500° C), but no value should be accepted as an absolute.
Cryogenic Temperatures
Investigations of wood in low temperatures, down to -300° F (-184° C), have shown mechanical strength increases. The increase is up to three times the property measured at room temperature, depending on the strength property and moisture content. This increase is consistent with other materials that exhibit increased resistance to changes in form as the temperature drops. Cycling of freezing and thawing do not seem to affect the properties of the wood itself, but may reduce the strength of some fastenings by as much as 10 percent.
In practical applications of wood products, the increase in strength due to exposure to subnormal temperature will tend to offset strength losses caused by other factors. With regard to glue performance, studies have shown that the joint strength of plywood made with phenolic, urea and casein glues is not affected by a temperature of -68° F (-56° C).
On the basis of available test information, published stresses for plywood are considered applicable at temperatures down to -300° F (-184° C).
Plywood has been successfully used as part of an insulation jacket for ship hulls transporting liquid natural gas (LNG). This gas is maintained in the liquid state at approximately -250° F (-157° C). The plywood is used in conjunction with insulating foam, and it reaches a service temperature of approximately -150° F (-101° C). Design engineers are well satisfied with the plywood performance for this purpose.
For more information on wood structural panels' thermal properties, consult ICC Evaluation Service ICC-ES Evaluation Report ESR-2586, and APA Performance-Rated Wood Structural Panels as Thermal Barriers, Form TT-060.