Wood is a hydroscopic material. This means it can attract and hold water. Its moisture content (MC) depends on the amount of moisture in the air, and whatever it’s in direct contact with.
The normal MC of wooden flooring is between 8 and 10%. When fitted with under floor heating it will be lower at 7-8%. Changes will cause floor boards to expand as moisture is absorbed and contract as it’s given off. A 1% difference in the MC of solid wood flooring can produce an expansion of 2mm. If your floor wasn’t fitted correctly, this can have devastating consequences.
The Equilibrium Moisture Content (EMC) of wood is the point at which it should neither gain nor lose water. For example, a wooden floor in a room kept at 20 degrees Celsius with a relative humidity of 50% has an EMC of 9.3%. Flooring needs to be laid and maintained at within 2-3% of EMC. This is why boards must be acclimatised – to reach ‘equilibrium’ with the environment before installation.
Variations in temperature and humidity can affect the moisture levels in your timber floor. Like a sponge, wood absorbs moisture from the air, and then gives it off when humidity levels are low. To prevent movement, cupping and gaps your installer will calculate an expansion allowance (normally 10mm).
It is perfectly normal for small gaps to appear between boards in the winter. This is because humidity levels fall due to central heating. These should naturally close up again in the summer months.