Wood Flooring and Underfloor Heating

Installing underfloor heating (UFH) underneath wood flooring can give rise to multiple complications due to the challenges brought on by subjecting the wood to what can be wide fluctuations in temperature. It is commonplace for wood flooring to experience an increase in its moisture content due to increased humidity in the atmosphere during the summer months or whenever the heat is left off for prolonged periods. Due to the wood having been specially cured to accommodate heating, this change in moisture content can cause it to lift or become distorted. This is especially common where the floor blocks were too tightly joined during installation. When heating is turned back on, this will help to reduce the moisture content. This means that with underfloor heating you can expect a wide variation in seasonal moisture content as compared to other types of heating.

Follow our helpful guide on what to do and not do when installing underfloor heating for engineered floors.

What to do:

·       Avoid quick-drying new concrete or screeds above UFH installations. This may cause the subfloor to suffer poor cohesive strength.

·       Verify the cohesive strength of the concrete or screeds before proceeding with the wood floor installation.

·       Confirm that the concrete or screeds have attained less than 65% RH before installation.

·       Apply the correct surface applied moisture barrier as needed.

·       Levelling is required before the installation of the wood flooring. If applying a primer moisture barrier, ensure you add a purpose-made fine graded aggregate over the second coat for one coat membranes. If you are applying a two-coat membrane, ensure the same application over the third coat. Allow all this to fully dry before vacuuming away any loose aggregate. This will help the primer and levelling compound to properly bond. You can use a rapid dry water-mix formula as the levelling compound. This option tends to be stronger than alternatives like latex mixes.

·       Ensure the levelling compounds applied are fully dried and cured before the floor installation.

·       Utilise the recommended permanent flexible adhesive during fixing. Silicon Mode ed polymer or 1-part polyurethane are amongst the recommended options.

·       Choose a fully-bond type of adhesive system for UFH instead of liquid battens.

·       Ensure there is full contact between the adhesive and the underside of the flooring element.

·       Ensure a suitable ambient humidity of between 45-65% RH. You can monitor this condition using a domestic hygrometer.

·       Deploy floor temperature sensors in between the flooring and the UFH system.

·       Where you have wet systems, employ zoning to help ensure proper control of flow temperature.

What not to do:

·       Do not install UFH systems beneath solid flooring. Solid flooring and high movement hardwood species such as maple or beech are not recommended for use with UFH applications.

·       Do not allow humidity that is less than 45% RH or more than 65% RH. Make use of a small humidification unit to maintain these levels during low humidity periods such as winter if required.

·       Do not permit the floor temperature to rise above 27°C. Be sure to maintain this even below rugs or carpet.

·       Do not make use of thick insulating rugs as they can increase floor temperatures.

Choosing wood species to combine with underfloor heating

Different kinds of wood species used for flooring will have different thermal properties and moisture movement. There are limited choices that possess the required characteristics that pair well with UFH. Alternatively, you can seek wood species recommendations from the manufacturers who will have done the necessary research and testing to identify the best options that can be stably installed with UFH. We do not recommend introducing an air space underneath the floor and above the UFH as this can promote undesirable temperature fluctuations. We recommend taking guidance from the manufacturer on the appropriate installation process to use for the flooring with UFH applications.

Precautions to take before installing underfloor heating

Before you undertake the installation of UFH and wood flooring, ensure the following:

·       Ensure the screed is fully dried and cured, as per BS8201:2011. This means the screed should have dried to a maximum level of less than 75% RH or less than 65% RH for cases where the flooring is to be bonded directly onto the screed. The UFH installation should be undertaken strictly according to the UFH manufacturer's guidelines.

·       Once the UFH is installed, do not use it for at least 48 hours after completion. This will give the adhesives used the time to be fully dried and cured, and allow for the flooring to have settled.

·       When the UFH and flooring are fully dried and cured, do not immediately turn up the system to maximum temperatures. Make it a gradual 1°C increase each day until you reach the maximum operating temperature. This should ideally not exceed a temperature of 27°C at the surface.

Consider investing in a dual thermostat system that you can use to monitor both the surface temperature at the floor level and ambient room temperature. Try to avoid rapid or extreme temperature changes as these wide fluctuations can cause the wood flooring to dry out more than it should, leading to changes in the wood's dimensions that cause it to warp and potentially crack the floor. Always adhere to manufacturer recommendations.

Test the underfloor system

Before you proceed with flooring installation, you need to ensure the UFH system is working correctly. This besides any other system that may also end up being located beneath the flooring. Ensure successful testing before proceeding with the flooring installation.

Expansion Provision

·       Ensure a minimum 12 mm expansion area around the perimeter of the UFH, where the flooring reaches the wall and similarly around other obstacles like pillars, hearths and so on.

·       Provide additional expansion breaks at all doorways by using suitable profiles such as T-section thresholds and other transition strips.

·       Provide for additional expansion breaks when installing UFH on large floors.

·       Consider installing the flooring parallel to the longest walls. Such installations ensure that the direction of the greatest dimension does not coincide with the direction of the greatest potential expansion. The direction fog retest potential expansion usually runs across the grain.

·       The predetermined expansion area should be based on the level of moisture content of the wood flooring to be installed, timber species, environmental humidity, and the size of the floor.

Precautions before handover

The installer should ascertain how best to keep the flooring dry and stable from the time of laying to handover.

Start-Up schedule

It is vital to ensure proper consultation with the manufacturer of the flooring system to ensure the floor to be installed overhead will be fully compatible with the UFH application. Details to confirm will include the timber species, moisture content and installation procedure. Measures should also be taken to ensure there are mechanisms in place to monitor surface temperatures so they never exceed 27°C.  When installing the UFH system, it must be overlaid with a fibre reinforced smoothing compound that is not less than 5 mm.

·       Avoid any use of the UFH for 48 hours after complete installation to give any adhesives used enough time to fully dry out and cure, while also allowing the floor to properly settle.

·       Do not immediately turn up the system to its maximum settings after the initial wait. Gradually increase the temperature by 1°C a day until you reach the optimal operating temperature. This should never exceed 27°C.

Consider installing a dual thermostat system that will enable the ongoing monitoring of the surface temperature of the floor and ambient temperature of the room. Also, avoid rapid or wide temperature variations as this can cause the moisture content of the wood to alter and cause the flooring to split or crack.

NB:

Before carrying out any installation of a UFH system or flooring, be sure to check that the materials to be used are compatible. This includes the types of moisture barriers, adhesives, primers, aggregates and levelling compounds.

Tips for when using underfloor heating with your wood floor

Engineered floors are compatible with underfloor heating. Before the wood flooring is installed, it is important to ensure that any UFH or other services that will lie beneath the flooring have been fully tested to ensure they are in good working condition. Because the use of UFH may cause fluctuations in temperature that could damage flooring, we recommend these tips for later use by the property owners.

·       Wait at least 48 hours after UFH installation to allow the adhesives used to fully cure and the floor to settle.

·       When you first turn on the UFH system, do so gradually. Make daily 1°C increments to the temperature till you achieve optimal operating temperature, which should not exceed 27°C. Install a dual thermostat system that will allow you to monitor the surface temperature and ambient room temperature.

·       Maintain humidity between 45-65% RH. A small domestic humidifier can be used to increase humidity levels if they drop during prolonged heating such as during the winter months.

·       Ensure surface floor temperatures do not exceed 27°C.

·       Avoid the use of insulating rugs that can increase surface floor temperatures.

·       Avoid turning off and on the heating suddenly. This is to avoid wide and rapid fluctuations in temperature that could affect the moisture content in the wood, causing it to crack or split.

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