It is extremely important for the timber used in window frame construction to have been acclimatised before the frames are put in place. Timber that is too wet or too dry can cause a range of problems, from minute aesthetic issues to important safety concerns.
The reason for this is that wood is a hygroscopic material. This means that, much like the potatoes you will have experimented on in school, the timber will try to absorb or release moisture until it is at equilibrium with the humidity of the environment.
This isn't because wood just likes to soak up water like a sponge, but rather because water is a large chemical constituent of a growing tree. When wood is wet and untreated its water content chemically-bound within its fibres is typically around 25-30% - the water fills up and bulks out the cavities between the cells. When the wood is dried, this water is lost and so these cavities close up, causing the fibres to contract and the timber to shrink.
Drying of timber for window frame construction is undertaken for two principal reasons:
Wet woood that is kept at a high humidity for extended periods of time without treatment is likely to suffer from mould, causing not only a health hazard but weakening structural integrity and causing decay. Generally the moisture content of wood is reduced to below 20% in order to avoid this issue, as well as the problem of staining.
Wood that has not been dried to the appropriate range fitting for its environment (typically 12-14% in the UK), will shrink after it has been put into service. As it does so this will cause what is known as 'movement' in the timber, which can cause issues if the grain of the wood is not dead straight.
Many woods are classified as lrage, medium or small 'movement' timber. Where small movement timber is used in more humid and wet conditions in order to minimise the issues that can arise with expansion. The most prevalent issue caused by movement is the warping of the wood, which distorts the aesthetic of the timber, causing your window frames to look ugly and out of shape. Expansion of the wood through soaking up too much moisture can also cause the windows to jam in place, rendering them useless.
If, however, the timber becomes too dry (a rarer issue), cracks can occur, causing not only visual deterioration but reducing the strength and load bearing capabilities of the window frame. This can mean that the wood becomes weakened, with the potential for the glass to fall out or be broken – a major concern for manufacturers. As such, we ensure that all of our wood has been dried to the appropriate range for British weather conditions, negating any risk of movement or the issues that movement can cause.
Timber products that face the outside world, i.e. windows and external doors, suffer the worst that the environment can offer. They have to be resilient to changes in temperature, weather conditions and infection. As such, it is very important to consider what products you use as an outer cover for your timber.
Externally exposed wooden window frames that are not protected with a finishing coat will weather badly inducing the timber's colour to fade, causing aesthetic damage, though the problems aren't just skin deep. Timber that isn't treated to face outdoors conditions is susceptible to damage from mildew and UV light amongst other things.
There are four main options to choose from when considering what product to treat your wooden window frames with. They are: preservatives, paints, varnishes and exterior wood stain. Depending on the product and the desired effect all these products have their pros and cons, though we strongly advocate the use of water-based paints in the final finish.
Most oil based paints and preservatives that you buy will be high in volatile organic compounds (VOCs), which are employed to keep the paint in a stable state whilst not in use. These VOCs generally evaporate at room temperature giving off a strong smell as it leaves the paint to dry. When airborne they can accelerate the rate at which nitrogen oxides in the atmosphere react with UV rays, catalysing the creation of photochemical smog and low-level ozone. Unlike many other outdoor coatings, water-based paints are virtually VOC free, thus minimising the effect of the industry upon the environment.
In addition to its environmental impact, water-based paint is also more beneficial to the timber you are covering. With varying humidity naturally occurring within wood, timber often expands and contracts day to day. Oil based paints are less malleable and so don't react too well to this movement.
Water-based varieties instead move more freely and also allow the surface beneath to 'breathe' more easily, preventing water retention that can harbour mildew which will cause the timber to rot from within. This is extremely beneficial in harsh weather conditions, and will ensure a lengthier protection on your timber window frames. Importantly, however, you must remember not to paint over oil finishes with a water-based alternative, because as the coat below expands and contracts, it will crack and blister under the new coat, rendering it useless.
Some may argue that you cannot get the same finish with a water-based paint as you can with oil, however as regulations have been tightened on the use of VOCs in the industry, a lot more time and money has gone into developing water-based alternatives which will provide the same required finishes as their more harmful predecessors.