Understanding How Moisture Causes Dimensional Changes In Wood

Dimensional Changes in Wood
Importance of Kiln Dried Wood
Equilibrium Moisture Content (EMC) of Wood in Outdoor Locations in the UK

A Hygroscopic Material

Wood is a hygroscopic material which means that it takes up and releases moisture according to the temperature and the relative humidity (RH) of the ambient air.

In any given environment wood will stabilise at what is known as the equilibrium moisture content (EMC). This is the moisture content at which the wood is neither gaining nor losing moisture and is a function of both the relative humidity and the temperature of the surrounding air. The EMC - also known as the hygroscopic equilibrium – is dynamic in nature because it changes with the relative humidity and temperature.

In other words when a piece of wood is placed in a certain environment, over a period of time it tries to achieve equilibrium with this environment, therefore the moisture content of the piece of wood changes to adjust to the relative humidity and temperature of its surroundings. After a certain period of time, the moisture content stops changing when the wood reaches its EMC. It remains at this equilibrium as long as the relative humidity and temperature of the surroundings are not changed.

Water and Wood

Before reading on it is worth gaining a little deeper knowledge about wood’s relationship with water.

The liquid content of a living tree is called ‘sap’ and is primarily made up of water. This water is made up of ‘free water’ and ‘bound water’.

The water in wood’s cell cavities is known as ‘free water’. If that water were to be squeezed out the wood would then be at its ‘fibre saturation point’. In this state the cell cavities are emptied of free water, but the cell walls are still saturated and still in their weakest condition. Only when the water leaves the cell walls does the wood shrink and increase in strength.

The water remaining in the cell walls is known as ‘bound water’. It is held in molecular form and is removed by placing the wood in a relatively dry atmosphere or a drying kiln. How much water is then removed from the wood depends on the (RH) of the atmosphere. It will either dry naturally over time to meet the RH of the surrounding atmosphere, or is dried to meet the approximate RH of the atmosphere. It is the RH of the surrounding atmosphere that determines wood’s equilibrium moisture content (EMC).

Dimensional changes in wood

Changes in the moisture content of wood will lead to variations in the dimensions of wood. It will shrink in size as it releases moisture and expand as it takes up moisture. Regardless of the wood species dimensional variations may occur across the width and thickness of the board, however variation in the length of the boards is considered to be virtually non-existent.

As a general guide the thickness or width of the wood may increase or decrease in increments of 0.25% for every 1.0% increase or decrease in moisture content (MC).

Variation in the moisture content of exterior wood in the UK

In the UK external wood may vary between a maximum of 22% MC in winter (e.g. cladding installed on north facing walls) and a minimum of 10% MC in summer (e.g. cladding installed on south facing walls exposed to summer sun), although higher or lower percentages than these are occasionally recorded. The seasonal variation on any one face of the building does not normally vary by more than 6 – 8%; however this change in moisture content is sufficient to cause swelling or shrinkage of the wood. According to BS1186-3 the normal variation of moisture content of timber cladding in use is 13 – 19%, suggesting a mean of 16%.

Take for example a cladding board delivered during the summer months measuring 146mm in width with an 18% MC. In extreme circumstances (i.e. installed on a south facing wall exposed to summer sun) this board could lose 8% MC. Using the calculation of 0.25% for every 1% MC the potential shrinkage can be calculated as follows: 8 x (146mm x 0.25%) = 2.92mm, therefore the board could potentially shrink to 143.1mm in width during the summer months if it is exposed to a lot of sun.

Let’s now look at the same example but with the board delivered during the winter months. In extreme circumstances (i.e. installed on a north facing wall receiving little sun) this board could gain 4% MC. Using the same calculation of 0.25% for every 1% MC the potential swelling can be calculated as follows: 4 x (146mm x 0.25%) = 1.5mm, therefore the board could potentially swell to 147.5mm in width during the winter months if it receives very little sun.

Allowing for moisture movement

Dimensional changes due to moisture exchange in wood can lead to defects such as warping, checking or splitting that may compromise the performance of the wood piece or product. Hence, most of the challenges of using wood in architecture or engineering involve understanding the relationship between wood and moisture and the influence of moisture in wood properties.

Use kiln dried wood rather than air dried or ‘green’ wood

For most applications kiln dried wood is preferable to green or air dried wood. Any wood with a moisture content above the fibre saturation point is considered ‘green’. For successful air drying, a continuous and uniform flow of air throughout the stack of the timber needs to be arranged. The main disadvantage of air drying is that all pieces of a given stack of timber will not necessarily dry to the same moisture content. Boards on the inside of the stack may have completely different moisture content to those on the outside. In other words, when air drying timber you don’t have the same control that kiln drying offers.

Modern drying kilns allow tight control of air flow, temperature and humidity so that all the timber dries uniformly and to the precise moisture content required. Kiln drying often means a more uniform product and the problems often associated with poorly air-dried timber are avoided.

Kiln dried wood also requires minimal acclimatisation as the overall moisture content of the entire shipment is usually much closer to the required equilibrium moisture on site.

Acclimatise the wood

An important step to reducing problems associated with moisture movement is to ensure that the moisture content of the boards when installed is as close as possible to the equilibrium moisture of the site using the information in the table below. To do this the installer should allow the boards to acclimatise before installation. The moisture content should be checked using a moisture meter.

Prior to installation the wood should be stored on site under a shelter stacked off the ground on bearers to allow air circulation. If stored in an open sided shelter it should be covered with a loose breathable sheet to protect from sun and rain.

Finish the wood on all sides prior to installation

Another crucial step that should always be taken to reduce problems caused by moisture movement is to apply a suitable finish to the wood. A breathable finish applied to all sides of the wood prior to installation will minimise and control moisture take up and release. This in turn will increase the woods dimensional stability thus reducing the extreme swelling and shrinkage that can cause warping, checking and splitting. Exterior finishes may take the form of natural wood toned penetrating stains that enhance the natural colour of the wood or semi-transparent and opaque stains in a wide range of colour tones. Even if a weathered grey appearance is desired, a product called Wood Sealer made by Sansin is designed to seal out excess moisture, enhance the wood’s dimensional stability and because it contains no pigments (and therefore offers little UV protection) will allow the wood to weather to a consistent silver grey.

The application of a finish can also prevent mould growth and staining in exterior wood by sealing out excess moisture. Wood that remains wet for long periods is susceptible to mould growth and water staining.Wood science experts recommend the application of a finish to all exterior wood to enhance both its performance and longevity.

Allow for swelling and shrinkage during installation

If you are installing wood (cladding for example) at the time of year when its EMC is at its lowest point, you will need to leave a slight gap between the boards to allow for the boards swelling as they absorb moisture during the winter months. In the extreme scenarios above the boards vary in size by 4.4mm between the seasons; however the reality is that the north facing boards are unlikely to ever shrink to the size of the south facing boards in summer and likewise the south facing boards are unlikely to expand to the width of the north facing boards in winter.

The information in the table below comes from a report by The United States Department of Agriculture - Forest Service entitled Equilibrium Moisture Content of Wood in Outdoor Locations in the United States and Worldwide. It shows EMC % for the different months of the year at various locations throughout the UK.

If you look at the table you will see that the greatest percentage change in equilibrium moisture content is in the London area where it rises from 13.1% in June and July to 19.4% in December. Therefore as a general guide you would expect a board delivered at 146mm wide with an 18% MC on delivery to shrink to 144.2mm in July and swell to 146.5mm in December - a difference in width of 2.3mm.

To allow for this dimensional change in the wood the installer should ensure that the wood is acclimatised to the appropriate EMC at the time of year the installation is taking place, by checking it with a moisture meter. There is no set acclimatisation period as it is dependent on the MC of the wood at the time of delivery and the EMC in the area at the time of year the installation takes place.

The installer also needs to take into account potential swelling or shrinkage at the time of installation. For example if cladding is being installed in London in June or July then as a general guide, once the wood has been acclimatised to an EMC of 13.1%, a 2.3mm gap needs to be allowed between the boards to allow for swelling as the wood takes on moisture. Conversely if the cladding is being installed in December it would be acclimatised to 19.4% and butted up tight to take into account that it will shrink as it releases moisture in the summer months.

Remember that this is a general guide and that some wood species experience greater dimensional variation than others, so it’s a good idea to read the technical data of the wood species you are installing. Also remember that as discussed above some extreme scenarios may fall outside these parameters.

Equilibrium Moisture Content (EMC) of Wood in Outdoor Locations in the UK

EMC (%)

Area Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Averages
Aberdeen 18.4 17 16.5 15.5 15.6 15.7 15.8 16.3 16.8 17.9 17.1 17.7 16.7
Birmingham 19.2 18.4 15.4 14.6 13.9 14.4 13.8 14.1 15.8 16.7 17.8 19.3 16.1
Cardiff 19.3 17.1 17.1 14.6 15.2 15.3 15.4 15.4 16.3 18.7 19.4 18.5 16.9
Edinburgh 18.4 15.8 15.4 14.6 14.3 14.4 14.9 15.3 16.8 17.3 17.8 19.3 16.2
Glasgow 19.2 17 16.5 14.2 13.6 14.8 14.5 15.3 16.8 18.7 17.8 19.3 16.5
Kirkwall 18.4 19.2 17.7 17.8 17.3 18.7 19.6 19.6 18.7 19.5 19.4 19.3 17.1
Leeds 16.5 15.9 14.1 13.1 12.4 13 12.5 13.1 13.7 16.7 16 16.5 14.5
Liverpool 18.5 17.1 15.5 14.6 14 14.1 14.1 14.9 16.3 17.3 17.9 18.5 16.1
London 19.3 16.5 15.5 13.5 13.3 13.1 13.1 13.1 14.5 17.4 17.9 19.4 15.6
Newcastle 17.6 17 15.4 14.1 13.9 14.8 14.9 14.1 15.2 16.1 17.1 19.3 15.8
Plymouth 17.8 17.1 16.6 15.6 15.2 15.3 15.8 15.8 17.5 16.8 18.7 18.6 16.7
UK Monthly Averages 18.4 17.1 16 14.7 14.4 14.9 14.9 15.2 16.2 17.6 17.9 18.7