Fitting wooden floors safely over under floor heating is simple and effective if some basic rules are followed.
Prior to installation Turgon's basic site conditions must be met before the floor is installed, these are the same site conditions that we insist on for the installation of all our hardwood floors with a few additions specifically for wooden floors fitted over under floor heating. The basic site conditions are as follows:
Air relative humidity should be at a stable point between 40-60 %.
Moisture content of the walls must not exceed 8% Air temperature should be no less than 13 degrees C and the general operating environment should be between 18- 24 degrees C.
All wet trades such as plaster, terrazzo and floor tiling should be completed and dry before wood floors are installed.
Windows must be glazed and outside doors hung before wood floors are installed.
New buildings or extensions should be adequately ventilated and heated for at least 10 days before wood floors are installed.
All radiator and water pipes should be subjected to a water test before installation of wood floors.
Any sub floor must be smooth and flat to within +/- 3mm over a 3 metre radius.
Concrete sub floors must be sound and not loose.
Wooden sub floors must be sound and securely fixed.
Moisture content of wooden sub floors must be between 8-10 % and within 4% of your hardwood floor to avoid over expansion or contraction.
Moisture content of concrete screeds must be no greater than 75% relative humidity
A DPM must be incorporated below any screeded surface.
The additional conditions for wooden floors fitted over under floor heating:
Hot water under floor heating systems must be fully commissioned and pressure tested prior to fitting of the hardwood floor.
Any under floor heating system must be calibrated to restrict the surface floor temperature to a maximum of 27 degrees centigrade.
Under floor heating systems both hot water and electric must have a floor probe to prevent the surface floor temperature from exceeding 27 degrees centigrade.
The under floor heating system must be turned off during installation and only turned back on after a minimum of 48 hours after the installation is complete.
Any wooden floor that is to be fitted over under floor heating must be kiln dried to between 6% - 8% according to BS 8201:2011.
Site conditions at the time of fitting and the environment in which the floor is kept post installation are critical for a successful wooden floor installation, this applies whether you have under floor heating or not.
Wooden floors fitted over under floor heating are in very close proximity to the heat source and are continually heated during the heating season. Your floor literally becomes a giant radiator and your wooden floor is likely to get very dry. There is a direct correlation between the temperature of the air, the moisture content of the air and the moisture content of the wooden floor. You can see this clearly by following the link below;
Air relative humidity, which has such an effect on the moisture content of your wooden floor is an expression of the amount of moisture the air can hold. In other words it is the amount of water in the air divided by the air's capacity to hold water multiplied by 100 to give you a percentage.
Let's look at an example of how this effects a wooden floor fitted over under floor heating.
Let us assume that the air in the room at chest height is 20 degrees centigrade and has a relative humidity of 50%, perfect conditions for our wooden flooring and according to the chart in the link above we would expect the wood to have a moisture content of 9.2%.
However as we approach the heat source which in this case is the wooden floor itself, the air temperature will rise. It is safe to assume that if the wooden floor has a surface temperature of 27 degrees centigrade then the air temperature directly above the wooden floor will be close to 27 degrees centigrade. How will the increase in air temperature effect the relative humidity of the air ?
We already said that Air RH = Water Content / Water Capacity x 100
Water Capacity increases exponentially with increases in air temperature.
At 20 degrees centigrade the water capacity is 14 grams of water per kilo of air, at 27 degrees centigrade this increases to 23 grams of water per kilo of air.
Where air relative humidity is 50% at 20 degrees centigrade we can say that
Water Content (7g) / Water Capacity (14g) x 100 = 50% RH
By increasing the temperature to 27 degrees but not introducing more water the relative humidity drops to 30%.
Water Content (7g) / Water Capacity (23 grams) x 100 = 30% RH
By holding a wooden floor for prolonged periods with an air relative humidity 30% and an air temperature of 27 degrees centigrade the wooden floor moisture content will drop to 6.1%.
This is the reason that British Standards 8201:2011 recommend wooden floors that are to be fitted over under floor heating are kiln dried to 6%-8%.
If you fit a wooden floor with a moisture content above 8% over under floor heating you must introduce more water content or moisture into the environment to stop the floor drying out.
This can be done by installing humidifiers in the property to regulate the air relative humidity.