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Managing Humidity In Timber

Rob Winstone on Equilibrium Moisture Content — Part 2

LAST month I explained how and why timber moves, and the importance of Equilibrium Moisture Content (EMC). Now we can look at the effect on timber of relative humidity and temperature.

The humidity has the largest effect on timber and is a product of temperature, relative humidity, humidity relative to temperature. Temperature has only a small effect on the movement of timber.

As discussed last month, timber will absorb or lose moisture to be in equilibrium with the environment around it. As the pressure is affected by the relative humidity, this allows the wood to absorb or lose that moisture.
Therefore the timber will increase or decrease in moisture content and as it increases in moisture content it increases in dimension and as it loses moisture it reduces in dimension.

There are some published charts that will give you the projected moisture content of a piece of timber at a given humidity at a given temperature. Forest Products Laboratory in America has produced a chart but the temperatures are listed in degrees fahrenheit. Peter Kazmar’s book. Wood Flooring, a Professional’s guide to installation (published by TRADA Technology) has a chart in the appendices.

From these charts and understanding of the range of humidity that can occur within a building we can understand how the moisture content of the timber will change throughout the year. Most timber flooring suppliers anticipate that the relative humidity will range between 40 and 60% at a temperature in the region of 20degC.

Using the TRADA EMC chart this will give us a timber moisture content of 9.5% to13%. Most timber flooring merchants supply their timber at about 9% ±1%.moisture content. Using the rule of thumb guide for medium movement timber of an increase in dimension of 2-3mm per meter width for every 1% change in moisture content we can see that the timber could increase in the region of 12mm for every 1m width of flooring.

According to the TRADA chart the moisture content increases by 1% for every 5% increase in relative humidity at 20degC above 45% relative humidity.

Timber is slow to react to changes in environment, so often the relative humidity has started to fall or rise before the extreme peaks and troughs have any major effect on the timber. However, as the last couple of years have shown us the summer and early autumn relative humidity has been very high for extended periods of time and floors that have been flat for many years have started to lift.

The early part of this year has had extended periods of very low relative humidity; throw underfloor heating into the equation and we have seen large gaps between boards and even boards splitting.

Rob Winstone is a flooring consultant, an expert witness for timber flooring and timber issues and an accredited civil and commercial mediator.
T: 07831 443088

This article has been reproduced from the Contract Flooring Journal website. You can find them at