The Importance Of The Right Humidity In Window Frame Construction

August 7th, 2018

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 wood 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 kinds of wood are classified as large, 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.

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