Heating floors
- Main factors influencing the use of heated wooden floor
- Additional factors influencing the use of heated wooden floor
- General rules for preparing the heated surface and laying floor on it
1. Main factors influencing the use of heated wooden floor
This section provides a short description of the main factors which influence the use of any type of wooden floor giving special attention to the aspects characteristic to the heated floor.
1. 1. Quality of subfloor installation
Subfloor is one of the main elements of the wooden floor construction. It should be given special attention as the quality of subfloor installation influences the further use of the wooden floor. Wooden floor should be laid on a stable, dry and properly waterproofed surface installed in observance of technological requirements. A suitable floor installation construction should be used. Additional requirements apply when installing a heated subfloor.
Important for heated floor! When using wooden floor on heated subfloor, it is very important that temperature of the entire area of the wooden floor would be changing simultaneously and stay the same. If the temperature of wooden floor changes unevenly or varies in parts of the floor, different conditions created for parts of the floor result in more intense and rapid shrinking (or expansion) of one part of the floor with respect to the other one. With time such conditions might cause the appearance of cracks or waving on the floor surface and visual changes between areas where different heating has been applied, etc. In order to avoid such consequences, installation of subfloor heating system should be properly performed with heating elements in the subfloor installed evenly and at the same depth. Specialists installing heating system must be informed that wooden floor will be installed over the heating system. Whereas, wooden floor layers should be aware of the smoothing compounds used over heating elements so that they could precisely measure the moisture of the subfloor and choose suitable materials.
1. 2. Quality of works and products used for installation
It is recommended to use products of well-known wooden floor products manufacturers that make up one system. Floor installation works should be trusted to professional specialists that provide guarantee for the works performed. Wooden floor installation and products use instructions should be observed in the process of installation works.
Important for heated floor! When installing wooden floor on heated subfloor, special attention should be given to the choice of glue. The suitability of glue for heated floor should be specified in the glue instructions. Adhesives which are not suitable for heated floor may not be used. Wooden floor installed on heated subfloor might move (expand or contract) much more than wooden floor on unheated subfloor. For this reason, the adhesive should be able to resist high tensions and not break off the subfloor while the floor moves. From adhesives that can be applied to the heated subfloor we recommend to choose a new generation reactive adhesive (it works by chemical reaction forming a polymeric bonding component). Reactive adhesives are usually polyurethane-or silane-based, elastic and ensure a strong bonding effect. As far as exploitation qualities are concerned, such adhesives are far ahead of many disperse, solvent and other types of glues. The choice of proper glue is of great importance especially if heated floor is planned to be used for a period of 10-15 or more years.
When installing wooden floor without factory cover (i.e., solid parquet or solid parquet boards), attention should be also paid to cover products that will be used. When installing heated floor, the effect of gluing together board sides should be avoided. Such effect might occur if certain water based lacquers are used: they glue the sides of boards together preventing the free movement of individual elements of the flooring with respect to each other. This results in additional tensions on the floor surface that might cause the appearance of more prominent cracks in some places. This problem may be easily solved by using special primers prior to applying lacquer: they prevent board sides from bonding with each other. Alternatively, we recommend to treat the floor with oil and wax.
1. 3. Microclimatic conditions during installation and use of wooden floor
Humidity fluctuations are the worst enemy of the wooden floor. They cause the appearance of cracks or waving on the floor. For this reason, all wet construction works in the premises must be completed prior to installing wooden floor on heated or unheated subfloor. Relative air humidity must be between 40 and 60%, and temperature must be +18 to +24ºC. The observance of the above conditions is extremely important when laying factory covered floor which will not be polished after installation. The same conditions apply also for the whole period of the wooden floor use.
Important for heated floor! When using wooden floor on heated subfloor, it is very important to maintain correct relative air humidity and temperature since in this case consequences of improper conditions are more prominent and appear more rapidly than in the case of wooden floor installed on unheated subfloor. This happens because wooden floor installed on heated subfloor must endure much more significant and frequent temperature and humidity fluctuations than floor installed on unheated subfloor.
It is also important to keep the amplitude of floor temperature fluctuations to the minimum. The recommended floor temperature fluctuation is maximum 4ºC per day both when the floor temperature is increased or decreased. When the heating is on, the temperature of the floor surface must never exceed +27ºC.
1. 4. Wooden floor maintenance and protection
Maintenance of heated wooden floor is the same as of any other wooden floor. Maintenance products and requirements depend on the wooden floor cover and usually should be indicated in the floor maintenance manual. General wooden floor maintenance and protection requirements are given below.
For cleaning it is recommended to use a vacuum cleaner or another dry method. If need be, floor may be cleaned with a damp well-wringed cloth. It is also recommended to use mats which ensure sand and dirt collection at the entrances to rooms. Special protective plastic mats can be used in order to protect the floor from swivel chair scratching. Legs of furniture must be protected with special protective pads.
Following the above conditions, the use of many kinds of heated wooden floors will be the same as that of wooden floor installed over unheated subfloor. However, as seen from practice, it is often impossible to keep ideal conditions for use of wooden floor due to seasonal air humidity variations that take place in many premises. In such case heated subfloor becomes additional risk factor influencing the use of wooden floor. Therefore, in order to minimise the risk of cracks or waving of the wooden floor or breaking off the subfloor, it is recommended to take into account additional factors influencing the use of wooden floor and avoid having all the risk factors together.
2. Additional factors influencing the use of heated wooden floor
Use of wooden floor is influenced by the following additional factors: timber species; method of wood cutting; and wooden floor construction. The above three factors are mainly responsible for the wooden floor stability: i.e. resistance to humidity changes. When a heated subfloor is used, the stability becomes of special importance. When selecting type of wooden floor to be laid on heated subfloor, it is also necessary to take into account thermal conductivity of the floor which is also mainly influenced by the above three factors.
2. 1. Timber species and cutting type
Different timber is characterised by different stability. For heated floor it is best to choose more stable kinds of timber, i.e. kinds which would be less responsive to fluctuations of relative air humidity and temperature. Stable kinds of timber: oak, red oak, ash, merbau, bamboo, tauari, hevea. Unstable kinds of timber which by most manufacturers are not recommended for heated subfloor: beech, ipe, maple, masaranduba, cumaru, kempas.
Different types of cutting timber which is used to make wooden floor also influence further use of heated wooden floor. For heated floor it is recommended to use boards of radial cut. Everybody knows that wood is a bad thermal conductor and decreases floor heating efficiency. However, it has been determined that thermal conductivity of the wood of radial cut twice exceeds that of the wood of tangential cut. This is due to the fact that thermal transmission along the growth rings is twice higher than across them. The stability rates of wood of radial cut are also higher than those of the wood of tangential cut. See section Hardness and Stability of Wood for more information about wood stability and cutting types.
2. 2. Type of wooden floor
Suitability of wooden floor for heated floor also depends on the type of the floor. Suitability of the most popular types of wooden floor for heated floor is given below:
- Two-layer or three-layer parquet boards of stable or medium-stability wood are the most suitable for heated floor. Due to its construction, such floor is more stable than block solid parquet and solid wood blocks, i.e. is less responsive to fluctuations of relative air humidity and temperature. Also, two-layer parquet boards have the lowest thermal resistance rates, i.e. normally 0.04 to 0.1 m2 K/W for 8-13 mm thickness two-layer parquet boards. Thermal resistance of 14 mm thickness three-layer parquet boards start at approximately 0.13 m2K/W depending on the raw materials used during manufacturing process. In Germany, for instance, the recommended thermal resistance of floor should not exceed 0.15 m2K/W.
- The second best for installing on heated surface after three-layer parquet boards is solid parquet boards. When laying block solid parquet on heated surface, it is best to select parquet with the thickness and width ratio not exceeding ¼. The smaller is the ratio of board thickness and width (i.e. the closer is the board thickness to its width), the more stable the board is and the smaller probability of boat-shape shrinking of the parquet is. Wooden floor made from such boards will be less responsive to microclimatic changes, i.e. the risk of distances appearing between the parts and waving will be reduced. The use of block solid parquet installed over heated subfloor also depends on the pattern of parquet laying. It is best to select a pattern according to which boards are laid in different directions: herringbone pattern, squares and boarded pattern. This is due to the fact that mainly the width of the parquet boards undergoes changes, whereas the changes of length dimension are almost invisible. The laying of parquet boards in different directions reduces the risk of distances appearing between the boards and waving.
- Solid boards may also be laid on heated surface. However when choosing this product, one must know that such floor will be the most sensitive to ambient conditions and will require especially good laying and use conditions. This, firstly, is conditioned by dimensions of the solid boards: the bigger the board is, the less stable it is. Moreover, the board thickness and width ratio often exceeds ¼. For this reason, it is not recommended to lay on heated subfloor very wide boards or boards thicker than 20 mm, as their thermal conductivity is low. For solid boards most frequently the unstable laying mode is opted for, i.e. boards are laid in one direction what increases the risk of distances appearing between the parts or waving.
3. General rules for preparing the heated surface and installing floor on it
Most of wooden floor manufacturers provide the following or very similar requirements for preparing heated surface and laying floor on it:
- During wooden floor installation the humidity of the heated concrete base must not exceed 1.5% CM (for unheated concete base it may reach 2.8 % CM);
- Heating of the subfloor is turned on at least 4 weeks after installation of the concrete base.
- After the heating is turned on, temperature is daily increased by +5° C until the maximum heating capacity is reached.
- The heating period of the concrete base drying must be at least 24 days. The heating period of the new substrate before laying parquet boards – at least 30 days, 20 of which must be heated at maximal approximately +45°C temperature. During the heating-drying period of concrete the premises must be sufficiently ventilated. If the heating floor system is installed earlier and has been used for several seasons, before laying the parquet boards we reccommend to rise the temparature of the substrate gradually till approximately +45°C and to keep such temparature incessantly for a week.
- In any cases before parquet boards laying the temperature of the substrate must be kept approximately at +45°C for at least 5 days. After that the temperature of the substrate every day must be reduced for 5° C, till it reaches +18-20°C. Such temperature of the substrate must be kept also during the laying. Necessary room temperature can be kept with special heating appliances.
- After the floor is laid, the same level of the base temperature must be maintained for at least 5 days (+18 to +20° C) to ensure proper hardening of glue and other substances and bonding of the adhesive to the wooden floor. When laying wooden floor, which is not covered with lacquer or oil in the factory, the same base temperature must be maintained during the lacquering or oiling process and for at least 7 days afterwards. After that, the required temperature is gradually reached.