Which type of wood expands the least?

The moisture-related properties of wood

Wood is a hygroscopic material, i.e. it binds moisture. Water penetrates the wood in three ways: as a capillary liquid through the cell lumen, as vapor through the cell lumen and through molecular diffusion through the cell walls. The moisture of wood is given as the ratio between the mass of the water in the wood and the mass of the waterless wood. (If a piece of wood weighing 100 kg contains 50 kg of water, the moisture content of the wood is 100%). Freshly sawn wood usually has a moisture content of 40–200%. In normal use, the moisture content of wood varies between 8-25 percent by weight, depending on the relative humidity.

The equilibrium moisture content of wood is the constant moisture content of the wood at a certain air temperature and relative humidity. The equilibrium humidity is based on the relative and not on the absolute humidity. The relative humidity is the ratio between the instantaneous vapor pressure and the saturation vapor pressure of the water at the prevailing temperature. Predried wood reaches its equilibrium moisture content within a few weeks. The fiber saturation range is the moisture level at which the cell walls are saturated with water, but the cell lumens do not contain free water. Wood shrinks when it dries and its humidity is below the saturation range. Accordingly, when the wood becomes more humid, it no longer expands after reaching the saturation range. The saturation range of the most common Finnish tree species is around 30% at + 20 ° C. The property of wood to bind and release moisture (moisture capacity) can be used structurally. For example, wood thermal insulation can be used in buildings to compensate for the passage of moisture in structures.

The shrinkage and expansion of wood is different in the radial and tangential as well as in the grain direction. This phenomenon is called anisotropy. When wood is brought from the absolutely wet to the absolutely dry state, it shrinks by an average of 8% in the tangential direction, 4% in the radial direction and only 0.2-0.4% in the grain direction. Heartwood is always drier than sapwood, which makes it difficult to dry the wood. The anisotropy and internal stresses of wood can cause twisting in the wood during drying. The change in shape due to moisture must always be taken into account when building. For example, it causes compression in building frames. In addition, excessive shrinkage in the tangential direction causes cracks in large pieces of wood. Wood usually cracks where the distance between the surface and the core is shortest.

The changes in shape usually increase with the density of the wood. When wood dries, its strength properties improve. The compressive and flexural strength of fresh wood, for example, doubles when its moisture content drops to 12–15%. The tensile strength of wood is highest at a moisture content of 6–12%. The strength properties of wood are significantly better when the humidity is below the fiber saturation range. Wood moisture must also be taken into account when dimensioning wooden structures because it affects the strength of the wood.

Wood will be damaged if its moisture content is over 20% over the long term. In such cases, the relative humidity is usually over 80%. Wood begins to rot after a few months at a relative humidity of over 80%. A relative humidity of 70% can already be considered a critical value, and when it exceeds 90% the wood begins to rot. However, wood only begins to rot and rot at a temperature of + 0–40 ° C. Although the relative humidity in frost can be over 85% in the long term, wood will not be damaged because the temperature is not high enough. Mold spores and putrefactive fungi also need oxygen and nutrients, which are usually available in sufficient quantities in the wood and in the air.

Mold cannot penetrate the wood surface and so does not damage the strength of the wood. Mold spores, on the other hand, are hazardous to health because they can cause various allergic reactions and mild symptoms of poisoning, such as persistent runny nose, dizziness and headaches. That is why mold must always be taken seriously. Blue discoloration of wood is often mistakenly equated with mold. The blue discoloration of wood is caused by blue stain fungus and the color penetrates deep into the wood. Blue fungi multiply through spores or the spread of their mycelia and mainly attack stored coniferous wood. Blue fungi do not develop at temperatures below + 5 ° C. Blue does not significantly affect the strength of wood.

Dependence of the wood moisture content on the temperature and the relative humidity

Example of the application of the picture (red dashed line)

Initial data:

- indoor air temperature + 22 C

- Relative humidity of the indoor air RH 50%

The table shows that the moisture content of the wood in the specified conditions is about 9.5%