Relative Humidity

In the UK, getting a fully trained and experienced fitter to fit your hardwood floor is essential. 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.

Wood is 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.

Wood swells as it absorbs water and shrinks as it releases water. Therefore both its moisture content and its dimensions are controlled by the relative humidity of the surrounding air.

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 will condense and form droplets. 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, making the air's relative humidity either fall or rise sharply.

Examples of Relative Humidity

Winter

If the relative humidity of air outside at 0° is 50%, and that air is drawn into a house and warmed to 20° without any additional moisture added, its relative humidity falls to about 10%.

Summer

If outside air at 20° and 50% relative humidity passes inside into a cooled room at 15°, the relative humidity shoots up to over 80%.

These erratic swings in humidity make wood change in size. If it's not properly installed it can then easily buckle.

Turgon has 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 (T) - this is shown to the right and below. It was then possible, given the temperature, to show the relative humidity inside a home during the Winter and Summer.

We used average temperature and humidity statistics for London in Summer and Winter provided by the Met Office.
Winter: average outdoor temperature = 4°, average relative humidity = 80%
Summer: average outdoor temperature = 17°, average relative humidity = 60%

Saturated vapour density (g cm^-3) = 5.018 + 0.32 T² + 0.00031 T³.

Graph 1 shows the relative humidity of this air indoors plotted against room temperature for Winter, and Graph 2 shows the same thing for Summer.

The graphs show why indoor air in Winter often feels so dry; it has been taken in from the outside and then heated up without any additional moisture being added. And of course the reverse is true in sweltering Summers.

A room held at 20° all year would swing from 30% relative humidity in Winter to 50% relative humidity in Summer. Such swings play havoc on the wood in floors, because the change in length of a piece of wood is directly proportional to the change in moisture content. Depending on the type of wood, the change can be anything up to half an inch for a 10% change in moisture content.

How to Deal with Relative Humidity

First, the site must be thoroughly checked for moisture levels before installation. Second, wood floors must be installed with an awareness and allowance of relative humidity.

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
(a) build with wood conditioned to the average equilibrium moisture content that it will see in service, and
(b) finish wood with low-permeability coatings.
For this reason it is beneficial to allow a room to acclimatise before use.

Turgon have the skill and expertise to ensure you are protected as far as possible, and in any eventuality your wood is guaranteed for two full years from date of installation.

Take our advice - get it done properly.