Timber floors over under floor heating


Both wood-based and vinyl-based flooring can be installed over underfloor heating and in recent years owners in the cooler southern states of Australia and in New Zealand are more frequently including this heating option in their dwellings. There are two systems that are commonly used – being a hydronic system where warm water is passed though pipes beneath the floor, and an electrically heated mat system. This information sheet will discuss aspects relating to the flooring products used and installation aspects that need to be considered. Firstly though, an understanding of the heated environment and movement experienced in these flooring products will be explained. 

Environments suitable for underfloor heating 

The cooler climates in Australia covering the likes of Canberra, Melbourne, Adelaide and Tasmania are well suited to underfloor heating systems. These climates are characterised by cold winters with heating needed for perhaps five months of the year, and they also experience dry hot summers. New Zealand is similar, but not experiencing the degree of heat during summer months. The following graphs, based on morning and afternoon weather data show the temperature and relative humidity for Canberra and Christchurch. You will note that in mid-winter, the graphs indicate low temperatures of 7°C or 8°C and high relative humidities of 70% to 80%.At times owners in the likes of Sydney or even Brisbane may request information on underfloor heating, but these climates are very different to those above, and although vinyl-based flooring products may be considered, these climates are much less suitable for wood-based flooring due to high humidity warm summers conducive to floorboard expansion.

 The internal environment 

With wood-based flooring we include solid timber flooring, engineered flooring, laminate flooring and bamboo flooring. All these products have cellulose fibres which expand and contract with changes in the relative humidity and temperature of the air.For solid timber flooring this relationship is well established and the following table, known as an Equilibrium Moisture Content (EMC) table outlines this relationship. As an example, what this tells us is that if solid timber is held long enough in air that has a relative humidity of 60% and temperature of 20°C the timber will approach 11% moisture content.

In our colder climates it is important to note that internal climate within a heated dwelling is very different to the external climate outside the dwelling. The first diagram below illustrates the effect on the relative humidity and temperature inside the dwelling, with heating over a five-month period. It is evident that outside, the temperature may be 7°C and the relative humidity 80% and, from our table above, timber in such conditions would increase its moisture content to near 16%. With an increase in moisture content the board also expanding as shown in the second diagram. But internally, if the room is heated to 20°C the relative humidity will drop to about 40% and the moisture content of wood-based flooring in that room will decrease to near 8%, and with associated shrinkage of wood-based products.

This is why wood-based floors are more suited to climates with cold winters and drier summers. In winter the moisture content will be low and in summer it will remain moderately low. Yet in climates with warm humid summers such as Sydney, when heating is not in use high levels of summer floor expansion can result in associated expansion problems in the floor. Also note that the relative humidity is the main factor determining the moisture content that timber approaches and temperature affects how quickly it attains that moisture content. With warm humid summers the rate of moisture uptake is much more rapid than in cold humid conditions. Regarding vinyl-based flooring such as Hybrid, SPC, WPC and LVP, they also change dimension seasonally but are only affected by temperature changes and not changes in relative humidity. Hence, if we can reduce the seasonal temperature variation in the flooring, we will also reduce the seasonal movement. As such, these products are very well suited to underfloor heating. With this heating to a surface temperature of no more than 27°C, no longer are these products subject to the cold winter floor temperatures. But be aware that the benefit only remains provided the underfloor heating remains on over winter, noting that this type of flooring cools and heats rapidly and sudden changes in temperature can result in movement-related problems.

A comfortably heated dwelling

 We now need to look at what may be considered comfortable conditions inside a dwelling. For this we need to consider not only room temperature but the distribution of that temperature from floor to ceiling, as well as the floor temperature itself, as we often walk on floors with bare feet. The adjacent diagram shows the difference in heat distribution in a room with wall-mounted radiators compared to underfloor heating. The hot air from the radiator travels toward the ceiling and as it cools it travels back toward the floor, and a circular path is set up. When considering the temperature profile, the air is warmer at head height at 23°C and cooler at floor level at about 19°C. With underfloor heating, the heat is more evenly given off from all the floor surface and the temperature is lower higher up. Hence at head height, 20°C and at floor level 25°C. We also need to consider what is a comfortable floor surface temperature and, in the USA, ASHRAE dealing with heating and cooling systems, have assessed the floor temperature that people feel most comfortable standing on. The optimum as shown in the next diagram is 23°C. They have also assessed room relative humidities of 30% to 50% as being comfortable. In New Zealand and the colder Australia climates we would expect this to be similar to the USA. But, note that in the warmer, more humid locations in Australia internal humidity from 50% to 70% would be considered quite normal. The amount of heating applied is to a degree going to depend on the heat loss from the room, and if the heat loss is high, then you will be tempted to increase the floor temperature to compensate, so as to maintain a reasonable air temperature in the room. So, if you feel the need to raise floor temperature much above 23°C you may need to consider reducing heat loss from the room, and noting that a maximum floor temperature of 27°C is not to be exceeded.

Heat transfer and thermal resistance

 A hydronic system in a slab will be considered along with the thermal resistance of engineered flooring products. These are design aspects that need to be considered by the heating specialist, but it is also important that we understand the concepts. Firstly, to maintain a room air temperature of say 20°C, heat needs to be coming off the floor. The unit of heat is a watt and from a heated subfloor what is of interest is the Watts per square meter (W/m2 ). If the heat coming from the floor is in balance with the heat leaving the room through walls and ceiling, then the room temperature will be maintained. The room air temperature will increase if the heat from floor exceeds the heat loss from the room and if the heat loss is greater than the heat added the room temperature will reduce.

In a hydronic heating system, the heat transferred to the room will be a function of the temperature of the water in the heating pipes, the spacing of the pipes and flowrate of water through the pipes. As such to control room temperature it has been indicated that the most effective way is to control the flowrate. The diagram below shows two scenarios with pipes spaced at 100mm and 200mm and with a constant 50°C water temperature resulting in a room temperature of 20°C. It is the applied heat and thermal resistance (of concrete above the pipes and the floor system) that determines how much heat comes off the floor surface. Where the pipes are more closely spaced, the underfloor temperature is a little higher and so is the heat transfer into the room. So, an important consideration is the thermal resistance of the flooring system and in the example below it shows a typical engineered floor that may be either adhesive-fixed or floated. It shows the thermal resistance (R) of a 15mm thick floorboard at 0.15m2 K/W and as would be expected a lower resistance of 0.11m2K/W for the thinner 12mm thick board. Note that the adhesive would also have a small thermal resistance. When considering the floated floor example, the underlay also has thermal resistance, and it too is small for most underlays. The example does however show that some care is needed as a thicker underlay could have a higher thermal resistance, similar to that of the board. Note that the total thermal resistance of the floated floor system is just the resistance of the underlay added to that the flooring. It should also be noted that a floor system thermal resistance (R value) of up to 0.15m2 K/W works well. If the floor system resistance was high, then the effect would be a higher flowrate of water through the pipes to create a high temperature at the surface of the slab, in order to achieve the necessary heat transfer from the floor surface. At a thermal resistance of 0.15 the temperature of the floor surface and slab surface will be similar and note that the maximum slab surface temperature is also nominally 27°C. As outlined above, the thicker the flooring the higher the thermal resistance; and also note that denser flooring species have better heat transfer than lower density flooring. Engineered flooring often has lower density timber in the core and balancing layers and due to this it is not generally recommended that the board thickness exceed 20mm.

Flooring products over underfloor heating

 As has been indicated, solid timber, engineered that is fixed or floated, as well as laminate and vinyl based flooring (hybrid) can be laid over heated slabs. However, regarding solid timber flooring, although traditionally used, it is not seen as the preferred option and with engineered a better choice.

The advantages of engineered flooring over solid timber flooring are as follow: • An engineered floor system is generally thinner with better heat transfer, noting that for stability reason with Australian species, 19mm thick solid floor is more stable over the longer term than thinner overlay flooring. • Engineered flooring is manufactured at a lower moisture content closer to the in-service moisture content with heating on. Engineered flooring is often manufactured at about 9% whereas solid timber flooring closer to 11%, and has a greater moisture content range at manufacture. • Engineered flooring experiences less expansion and contraction when compared to solid timber. • A width to thickness ratio of up to about 10:1 works well with engineered flooring (e.g. 140mm x 14mm), so quite suited to wider boards. With solid timber flooring narrower quarter sawn boards, if possible, are preferred. There are still solid timber floors that are installed but owners need to be more forgiving with the seasonal movement and greater effect of hotter areas in the floor. As such, during the heating period some cupping and greater gapping can be expected that may also not be even across the floor. Even with engineered flooring care is needed. Not all products are considered by suppliers and manufacturers to be suitable for laying over a heated subfloor. The lamella of some species such as Blackbutt and Spotted Gum being more prone to surface splits, known as checking, are not considered suitable by many suppliers. It also needs to be considered that with engineered flooring board construction varies greatly and that some constructions are more robust and stable than others. As such, board width is not necessarily a good guide as to whether an engineered board is suitable to be laid. Manufacturers and suppliers will outline in product information which of their products can be laid over underfloor heating. Irrespective of the product type, if there is no written information to indicate that a product is suitable for laying over a heated subfloor, then it should not be installed, and another product chosen. Note also that some suppliers and manufacturers will also be specific as to which heating systems their floor can be laid over, which can he hydronic and not an electrically heated mat. Due to variance in advice, if a heating system supplier indicates that the system can be used with a certain product type it must also be checked with the flooring supplier or manufacturer, that the specific flooring product can be used.

Heating systems

 There are many types of underfloor heating systems and details will not be covered in this information sheet. Most systems relying on heated water being circulated or mats with electric heating elements, and other systems are also available. To indicate some of the varieties available the following photos are provided. Hydronic systems (photos by Hunt Heating) Electrically heated mat (photo by Coldbuster)

Floor installation and start up procedures with in-slab hydronic heating

 With floors over a heated slab the floor installation and start-up procedures are more exacting and are covered below. However, it needs to be considered that with any of the flooring products outlined in this information sheet, the key is not to have sudden changes in temperature and to also ensure that maximum temperatures are not exceeded. With pipes embedded in concrete it needs to be considered that even though the concrete is ‘dry’ it still has more water in it than it will have when in service as a heated slab. Just as solid timber at 11% moisture content has more water in it than if in service over a heated slab. When the slab is initially heated in a controlled manner moisture will be released from the slab surface, so it is imperative that the flooring has not been installed till after that phase. The complete sequence for a floor, that may also require site sanding and coating, is outlined below and as shown in the diagram.

Pre-heat the slab prior to laying adhesive-fixed wood-based flooring, allowing excess slab moisture to evaporate from the slab surface. 

  • Turn off the heating and follow this by a non-heating period. 
  • Install the flooring and follow this by a non-heating period.
  • Gradually increase the underfloor heating to the normal expected temperature. 
  • Follow this by a non-heating period prior to sanding and finishing (if applicable). 
  • Turn the heating system on gradually to operating temperatures. 

  • Specific advice will vary to a degree depending on both the heating system and flooring manufacturer requirements and their more specific advice should be adhered to. The periods mentioned above can be days or weeks, and heating up at no more than 2°C per day.

How not to have a problem with hydronic heating systems

 The following provides points to be considered with the installation of the heating system:

 • Ensure the heating system has been installed to the manufacturer’s guidelines. 

• The heating system should govern temperature by water flow control valves or similar, not an on/off thermostat. 

• Verify that the heating elements are evenly distributed and of adequate depth. 

• Never install the flooring before the heating system is commissioned. 

• Slab moisture prior to laying should be no higher than 2%.

 • Verify that when in operation the slab surface temperature is maintainable up to 27°C and note that with the floor in place this is not to be exceeded. 

• Look to maintain a room temperature of 20°C and relative humidity in the range 35% to 60%. The following provides points to be considered with the installation of the flooring system:

 • Ensure the flooring is recommended by the manufacturer for use over underfloor heating. 

• At laying, have a subfloor temperature of above 15°C and internal RH of 40% to 65%. 

• Ensure the flooring is not ‘shocked’ with rapid temperature increase – only 1 to 2°C per day. Inform customers that this is just as important when turning the system back on after summer.

 • If the flooring is coated ensure adequate curing times prior to heating the floor.

The maximum floor temperature is not to exceed 27°C noting that 23°C is considered more optimal.

Where problems arise and associated consequences 

The following provides a list of causes of common problems: 

• Operating the heating system with a surface floor temperature above 27°C and rapid changes in temperature (preventing the flooring from adjusting slowly). 

• The heat distribution is not even and hot spots (hydronic and electric). Floor rugs will also raise the surface temperature. 

• High humidity in the dwelling. 

• Cleaning the floor with too much water. The consequences being as follow:

 • Board shape change (warping) and uneven shrinkage movement. 

• Delamination of the face lamella in engineered.

 • A floor surface that does not have an even temperature. 

• Delamination and checking under floor rugs. Below are photos showing where a floor rug has been placed on an adhesive-fixed oak engineered floor. Note that a rug adds to the thermal resistance and would cause increased board surface temperatures under the rug. The consequence of this can be surface checking as had occurred, but also making the boards more prone to lamella delamination, a photo of which is also provided.

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