Water in the wood
A living tree is protected from the elements by its bark, but unfortunately when it’s been felled, debarked, and cut into timber, the wood is no longer waterproof and the timber is susceptible to the forces of nature. Water from the soil containing nutrients moves upwards through the elongated wood cells of the living tree to the leaves, and much of this water evaporates through the surface of the leaves helping to cool the leaves. During photosynthesis a basic sugar is produced for the trees own use. Sap carries this sugar back down the tree to be used to produce new cells close to the inner surface of the bark known as the Cambium Layer. These cells, which are instrumental to the life of the tree, are also the reason why timber expands and contracts.
Free water & bound water
After the tree is first felled the timber will contain water within the cell walls, known as bound water, depending on the species of wood this can be as much as 30% of the total water contained within the tree. The water within the cell cavities is known as free water and amounts to roughly 70% of the water.
The best way to illustrate this situation is with the use of a sponge and a bucket of water.
Imagine that a sponge is a piece of timber (bear with me). Immerse the sponge in the water and this is a good representation of the cells within that piece of timber at full moisture content. When you pull the sponge out of the bucket it immediately loses water, as timber loses free water via evaporation, squeeze the remaining water out of the sponge and all the free water is expelled.
You are now left with a sponge that is the same size and shape as before and is damp to the touch. This is the equivalent to the cells of timber when they are at fibre saturation point (FSP). They can take on no more water, the cell walls are saturated. In this state the cell walls are in their weakest state, only when water leaves the cell walls will they shrink and become stronger.
The effects of atmospheric moisture on wood
Timber is hygroscopic which means it will constantly absorb and desorb water, either from direct contact with rainwater, which can quickly take the cells back up to saturation point if the rain is prolonged, or via atmospheric humidity, known as relative humidity. When relative humidity levels fall the timber loses bound water and when relative humidity levels rise the timber regains bound water. When the balance of moisture exchange is established, the amount of bound water contained in a piece of timber is called the equilibrium moisture content (EMC) of the wood. For a more detailed look at moisture and wood click here. The graph below illustrates the relationship between the EMC of timber and relative humidity levels.
How moisture causes timber to expand & contract
Lets use a log cabin as an example. Your log cabin timbers will usually have been kiln dried to anywhere between 14% and 19% moisture content. By looking at the graph, timber dried to 14% will reach its EMC when relative humidity levels are at 75% (which is the UK average). This is an important point to consider when purchasing your log cabin, timber that is kiln dried higher than 14% will take longer to stabilise.
When relative humidity is at 100%, timber will reach an EMC of about 30% and for approximately every 5% drop in relative humidity, timber will lose about 1% of its moisture content. The timber will expand or contract by roughly one quarter of a percent for every 1% increase or decrease in moisture content. It’s a little complicated to get your head around at first but let’s look at an example.
our cabin has been kiln dried to 14% moisture content and relative humidity is around 75% so there will be no movement in our cabin as the timber has reached its equilibrium moisture content, but if relative humidity drops as low as 30%, which it did in the long dry summer of 2018, then the log cabin timbers will dry out to 6% moisture content. With Norwegian Spruce shrinking by 0.26% for every 1% reduction in moisture content that’s a total timber shrinkage of 2.08%. A log cabin that is 18 logs high and has a log face width of 130mm will have shrunk by just under 50mm. A 4 metre width log cabin with 45 roof boards measuring 100mm across their width will want to shrink by about 95mm and this will put considerable strain on the nails or screws holding the roof boards in place. To see some examples of the problems associated with moisture and log cabin timber movement click here.
Different species of timber absorb and desorb moisture at different speeds, Norwegian Spruce absorbs and desorbs at a much slower rate than pine making it a more suitable choice for your log cabin. These figures are based on a hypothetical situation; the truth is that despite being kiln dried to 14%, the moisture content of timber will change as soon as it leaves the kiln, and will continue to change along its journey from the factory to your garden. In most instances expansion and contraction will be much more severe than the percentages stated in the above example.
Stabilising the movement of timber
Now that we have a basic understanding of why timber expands and contracts it’s clear that despite the difference in durability between different species of timber and the kiln drying process providing an element of stability, the timber alone is not stable enough to prevent this movement. Stabilisation can only be achieved through the correct application of an exterior weatherproofing wood finish. There isn’t currently a finish manufactured that can totally repel moisture so the state we are trying to achieve is to reduce the movement of the timber throughout the seasons. The graph below highlights this beautifully.
Our aim is to stabilise these wild fluctuations throughout the year and in turn ease the side effects of timber movement such as warping, cupping, splitting, and gapping of the cabin. The whole process starts and ends with a good quality stabilisation finish.
A good quality stabilisation finish will contain wax, oil, or a combination of the two, and it should be solvent based as wax and oil are suspended in the solvent. Oil and wax finishes are known as penetrating finishes because they penetrate the surface of the timber blocking the pores and repelling moisture. They have great elasticity and will expand and contract easily as the timber expands and contracts. Solvent is the carrier of choice for oil and wax as it takes about 12 hours to evaporate, this gives the oil and wax plenty of time to penetrate deep into the timber.
Modern finishes contain polymers such as alkyds or acrylic and they are classed as a film finish as they sit on top of the timber surface forming a coating to protect against moisture. They can be predominantly solvent based or sometimes water based, making them more environmentally friendly. Due to tighter regulation by the EU around the use of solvents, more manufacturers have moved to water based polymer finishes but there is still a lot of controversy surrounding polymer finishes as their elasticity isn’t as good as a penetrating finish and they have a tendency to crack and peel which completely negates the protective element that they offer. If a film finish fails then it all has to be stripped back using a heat gun and scraper, whereas if a penetrating finish shows signs of wear then it can simply be overpainted.
Final thoughts
At Timmersol we’ve researched extensively the pros and cons of using a film finish over a penetrating finish, but our research showed us that a penetrating finish has stood the test of time and in our opinion offers better protection, and therefore better value to the customer, and we didn’t want to mess with a winning formula just because regulations have become tighter. We understand timber, we understand log cabins, and we’re very proud of our products and the high levels of protection they provide. To get a more detailed explanation about the benefits of usingTimmersol Exterior Timber Stabiliser click here.