Story | 10/09/2023 06:57:27 | 5 min Read time

Plywood moisture behaviour is similar to that of wood

Whilst the moisture behaviour of plywood is similar to that of wood, plywood also has its own set of characteristics. In addition to the dimensions of plywood, its strength and elasticity properties change in relation to moisture absorption. In this article, we will discuss the moisture absorption of plywood and the water-repellent WISA-SpruceWR plywood, as well as coated film plywood, which slows down the absorption of moisture into the plywood.

Wood is a renewable material that is processed into plywood products for various end-uses, including into the construction and transport industries. The popularity of wood-based plywood has grown even more with the attention paid to the climate impact of building materials. Plywood also has superior strength properties compared to wood.

Plywood is manufactured by gluing and pressing together wood veneers that have been rotary cut or sliced from wood. Because of its raw material, plywood behaves in a similar way to wood. Like wood, plywood swells when exposed to moisture, which also affects its other properties. It is important to be familiar with the effects of moisture in plywood to ensure the dimensional accuracy of the materials. This will contribute to the smoothness and quality of construction.

Untreated, uncoated plywood is both a hygroscopic and an anisotropic material, like wood. Hygroscopicity means that wood absorbs moisture from the air, making the dimensions of plywood change as moisture is transferred within the cellular structure of the wood. Anisotropy means that wood shrinks and expands in different ways in the direction of the radius and tangent of the annual rings and in the direction of the radius or length. These wood properties also affect the moisture content of plywood.

In this article, we will discuss the effect of moisture on the dimensional and strength properties of plywood and the various options for reducing the effects of moisture absorption.

  • Moisture in plywood
  • The effect of humidity on the dimensions and mechanical properties of plywood
  • Moisture permeability of plywood
  • Properties of water-repellent WISA-SpruceWR plywood slow down moisture absorption 

Moisture content of plywood after delivery from the factory

The moisture content of plywood is typically between 7 - 12% after delivery from the factory. After delivery, the moisture content usually increases during transport, storage or further processing. As with other wood-based products, the dimensions of plywood change with changes in humidity. For this reason, moisture conditions must be considered, especially when plywood is under load at the point of use.

The equilibrium moisture content of plywood depends on the relative humidity and temperature of the air. Equilibrium humidity is the state in which the moisture content of wood remains constant. Under basic conditions of 20 °C and 65 % relative humidity, the equilibrium moisture content is about 12 % for thin-veneer plywood (hardwood, softwood and combi) and about 10 % for thick-veneer softwood plywood.

Effect of humidity on the dimensions and mechanical properties of plywood

When the moisture content of plywood changes from dry to wet, its dimensions change by a maximum of 1 mm per metre, which is a good basic rule for simple home joinery. For more demanding applications, please refer to the Technical Product Information Document for detailed information and calculation formulas.

The mechanical properties of plywood are given in the Declaration of Performance (DOP) document. The values are valid for plywood with a moisture content of 10-12%. With increasing moisture content, the strength and elasticity properties of plywood, i.e., stiffness, decrease.

Unlike other wood-based panels, plywood regains its original strength and elasticity properties when the moisture content returns to normal. Any stabilisation of the panel due to changes in humidity is a normal and should be distinguished from wetting.

Moisture permeability of plywood

It is important to know the water vapour permeability of a sheet product when designing, for example, buildings, exterior walls, or roofs of buildings. The water vapour transmission coefficient of plywood indicates the amount of water vapour diffused through a sheet of plywood in a unit of time when the relative humidity of the air on different sides of the sheet is different and when there is a certain difference in the partial pressure of water vapour.

The moisture permeability of plywood is a function of the moisture content of the plywood. When the moisture content of plywood increases, the moisture permeability also increases.

Water-repellent WISA-SpruceWR plywood slows down moisture penetration into the board

The moisture resistance of uncoated and edge-coated plywood is limited, despite the weather-resistant adhesive. Therefore, for permanently damp conditions, coated and edge-protected plywood, properly installed and maintained, should be used to withstand the stresses of moisture and weathering.

UPM Plywood has developed a water-repellent WISA-SpruceWR plywood for the WISA plywood family, which is treated with a wood-based material to slow down the penetration of moisture into the plywood and thus the moisture release of the plywood.

The water-repellent treatment of the surfaces and edges allows WISA-SpruceWR plywood to retain its dimensional accuracy, shape, and other properties better than the base plywood. Construction is made easier and quicker as edge trims fit more securely when installed, seams remain tight, and the panel does not require a long time to stabilise before installation. WISA-SpruceWR's breathable water-repellent coating allows moisture to evaporate, allowing the board to dry freely.

Find out more about WISA-SpruceWR

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