GUIDANCE

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08

Relative Humidity

In the UK, getting a fully trained and experienced fitter to fit your wood flooring is essential. Put simply, the
cold climate means that the relative humidity of air indoors fluctuates and this has adverse effects on the
behaviour of any wood in your house.


Relative Humidity

Wood is known as a hygroscopic material, this means that it is constantly exchanging water vapour with the air in a room, picking it up when atmospheric relative humidity is high, and giving it off when relative humidity is low. Unsurprisingly wood swells as it absorbs water, and shrinks as it releases water, hence both its moisture content and its dimensions are controlled by the Relative humidity is the air humidity expressed as a percentage of the maximum humidity of air at saturation point. In other words it is the amount of moisture in the air as a percentage of what the air can hold at a given temperature. When the air can’t hold all of the moisture, then it condenses as dew. The relative humidity is closely linked to the temperature of the air; hot air can hold much more moisture than cold. Relative humidity does not vary much outdoors, the problems occur when air from outside comes into the house and is either warmed or cooled without any change in hydration. In this case the air’s relative. For example, if in the winter the relative humidity of air outside at 0degrees centigrade is 50%, then if that air is drawn into a house and warmed to 20degrees centigrade without any additional moisture added its relative humidity falls to about 10%. If in summer outside air at 20degrees centigrade and 50% relative humidity passes inside into a cooled room at 15o, the relative humidity shoots up to over 80%. These erratic swings in humidity make wood change in size and if not properly installed can easily buckle. Turgon have the skill and expertise to ensure you are protected against this as far as possible, and in any eventuality your wood floor is guaranteed for two full years from date of installation. Short of installing a humidifier/dehumidifier to control relative humidity indoors, the best way to minimize changes in wood moisture content and dimensions is to build with wood conditioned to the average equilibrium moisture content that it will see in service, and finish wood with low­permeability coatings.

Turgon recently commissioned some quantitative research into this, and the results are illuminating. By examining empirical data it was possible to model the absolute saturated vapour density for water in air with a polynomial fit to temperature. It was then possible to show the relative humidity inside a home during the winter and summer given the temperature assuming average temperature and humidity statistics for London in Summer and Winter. From the Met Office weather records for London the winter average outdoor Temperature = 4 degrees centigrade, and the average relative humidity = 80%. In the summer the average outdoor Temperature = 17degrees centigrade, and average relative humidity = 60%. Indoor air in winter often feels dry because it has been taken in from the outside and then heated up without any additional moisture being added and its relative humidity drops to 30%. A room held at 20 degrees centigrade all year would swing from 30% relative humidity in winter to 50% relative humidity in summer. Such swings play havoc on the wooden floors. This may be all well and good, but what effect does being in an increased relative humidity actually have on the wood? The parlance calls for an exposition on equilibrium water content, again wood is hygroscopic, taking up or releasing water depending on the amount in the atmosphere. The change in width of a piece of wood is directly proportional to the change in equilibrium moisture content. The more moisture the wood has, the bigger it gets. This change can be anything up to half an inch, depending on the type of wood, for a 10% change in moisture content. Wood floors must be installed with an allowance for this and it is particularly important that the site is thoroughly checked for moisture levels before installation. Take our advice, get it done properly. British Standard 8201:2011 gives plenty of guidance for site conditions and moisture content of wooden floors, if these are followed most problems can be avoided.