Dinesen recommends installing underfloor heating under Dinesen floors as a comfortable and durable solution. About 80% of our projects are established with underfloor heating. Dinesen has years of experience with underfloor heating under solid floors, and it is an unproblematic solution, provided you adhere to the conditions described below.
In principle, (water-based) underfloor heating is simply a plastic coil that is embedded in the floor construction. Sending hot water through the coil heats up the construction and thus the room. Depending on the building’s insulation, it may be necessary to use supplementary heat sources in the form of radiators, a heat recovery system or a wood-burning stove.
If the underfloor heating is to be the only heat source, the total heat loss from the room must be less than the heat emitted by the floor surface. That requires a formal energy or heat loss assessment, especially in older buildings or in connection with renovation projects. Even if the building lives up to the insulation requirements in the building code – for example in a new building – the conditions listed below should still be considered in the decision-making process.
4.1.1 Humidity and underfloor heating
Regardless of whether the floor has underfloor heating, concrete moisture will always be very harmful for a plank floor. It is very important to ensure that the humidity content of the concrete does not exceed 85% RH. If heating coils have been embedded in the concrete, the underfloor heating should be switched on after 30 days when the concrete has hardened. Even in the warmest summer and with high outdoor temperatures, the underfloor heating should be switched on at an acceptable temperature for at least 1 month before the floor is installed, and you must always check the concrete moisture before installing the floor. Dinesen recommends a destructive measurement of concrete moisture. An indicative surface measurement – for example a GANN measurement – is not accurate enough. If the underfloor heating is not switched on, the residual humidity will not leave the concrete until after the floor is installed, and the heat is switched on; that may cause severe damage to the floor. A vapour barrier should always be installed on top of the dry concrete floor.
4.1.2 The properties of the floor with underfloor heating
As mentioned before, wood is a hygroscopic material, which absorbs and emits moisture from the surrounding materials. Wood will always adapt to an equilibrium level of humidity in relation to the temperature and the relative humidity of the air. When delivered, Dinesen planks are dried to a moisture content of 8-10%. This corresponds to a relative air humidity (RH) of 40-50%. If the air humidity decreases, the wood will emit moisture and shrink in width, which results in contraction gaps. The higher the surface temperature, the lower the air humidity just above the floor, and the more the wood will contract. Contraction gaps will always appear in winter, and at this stage, the floor has its most beautiful appearance. Generally, however, the goal should be to prevent the air humidity from dropping below 35% RH, cf. Table 1.
The capacity of a material to conduct heat is called its thermal conductivity = (w/m°k). For plank floors, the thermal conductivity depends on the density of the wood (kg/m3). Therefore, Oak has a slightly better thermal conductivity than Douglas.
The thermal conductivity is used to calculate the thermal resistance:
Guidelines values for thermal resistance
| Wood type
|| Thermal resistance
Thermal resistance is calculated based on the thickness of a material divided by its thermal conductivity. Thus, thermal resistance reflects the insulation ability and is normally labelled R.
Thermal resistance, calculation examples
| Thickness and variant
|| Formula and result (R)
| 28 mm Douglas
|| 0,028 / 0,13 = 0,22
| 35 mm Douglas
|| 0,034 / 0,13 = 0,27
| 22 mm Oak
|| 0,022 / 0,17 = 0,13
| 30 mm Oak
|| 0,030 / 0,17 = 0,18
As shown in the examples above, the thickness of the floor affects its insulation ability. The thicker the floor, the better the insulation. Therefore you may have to set a higher flow temperature to achieve a suitable surface temperature. The impact on energy consumption is minimal. The difference in energy consumption between a tile floor and a solid floor with 28 mm thick planks is max. 5-10%. And the difference in energy consumption between floors with 28 versus 35 mm Douglas planks is at most a few percent. See also section 4.1.3.
4.1.3 Heating requirement
When designing private homes, the goal is typically to ensure a room temperature of 20-21 °C in the winter. Over the years, insulation requirements have gone up considerably, and this is reflected in the energy consumption of private homes. The energy consumption for heating in modern buildings will be 35-45 W/m2, compared to 45-75 W/m2 in older buildings. Therefore, you should carry out a formal assessment of the actual heating requirement of the building.
The surface temperature of a plank floor must never exceed 27 °C, and the surface can therefore not emit more than 75 W/m2. The surface temperature in a wellinsulated house will typically need to be 2 °C above the desired room temperature. The less effective the insulation, the higher the surface temperature will need to be.
4.1.4 Heat loss
If the building has insufficient insulation, there may be days when underfloor heating alone cannot provide the desired room temperature. There is a considerable heat loss through floors, walls, ceilings and windows. The building code contains requirements to insulation effectiveness – the so-called U-value – which buildings must conform to.
The U-value for a ground floor with underfloor heating must not exceed 0.10. That typically requires insulation with at least 300 mm polystyrene. The U-value for heat loss through the foundations with underfloor heating must not exceed 0.12. Heat loss through the foundation has a considerable impact on heat energy consumption. You also need to consider thermal bridges and leaks. The building code also states U-values for external walls, ceilings, windows, outer doors etc.
Heat loss through windows, regardless of the type of glazing, far exceeds heat loss through walls, and the size of the glazed area will therefore have a major impact on the total heat loss. Note that the placement of windows in relation to the points of the compass, open areas or the sea may cause above-average heat loss, just as cold downdraughts from windows can be substantial. This should be countered with convectors along windows that go all the way to the floor. The convectors can also be used to draw heat into the room quickly. Convector grids can be made in the same wood as the floor to achieve a beautiful architectural solution.
The number of external walls in a room has a considerable impact on heat loss. The more external walls, the greater the heat loss. Be particularly careful to calculate heat loss in small annexes that have three external walls and perhaps also have large window sections, as the size of the thermally conducting floor surface affects the ability to heat up the room. The area is reduced by kitchen units, cupboards etc. Rugs, mats and runners also reduce the emission of heat and risks bringing the temperature under the rugs etc. above the max. level of 27 °C. Small rooms can therefore be more difficult to heat up than large rooms. High ceilings or cathedral ceilings may also lead to higher heat loss.
4.1.5 Flow temperature and surface temperature
The flow temperature depends on the required output and the chosen flooring. Normally, the flow temperature is 30 to 45 °C, and it should not exceed 50 °C. The flow temperature has a limited impact on the heat energy consumption. If the temperature is raised from 30 to 45 °C, heat energy consumption only increases by 6%, as energy consumption is determined by the difference between the flow and return temperature. It is only marginally more expensive to heat a thick plank floor than a thin engineered wood floor, but it takes a higher flow temperature to ensure the required surface temperature.
Dinesen cannot provide any additional advice on flow temperature, as it depends on the construction and heat loss. The real limitation is the surface temperature. The surface temperature of the plank floor must not exceed 27 °C, as higher temperatures may severely damage the floor. The room temperature is controlled by in-room thermostats. The thermostats turn the heat supply on or off to accommodate additional heat from people in the room, the impact of sunlight, lighting etc. The thermostats are positioned in places unaffected by sunlight, on interior walls 1.5 metres above the floor. In-room thermostats can be either cabled or wireless.
Types of underfloor heating
Underfloor heating is typically water-based. The two main categories are heavy underfloor heating with coils embedded in concrete and light underfloor heating with coils in heat emission plates. In rare cases, electric underfloor heating is used.
4.2.1 Heavy underfloor heating
Heavy underfloor heating is based on heating coils embedded in concrete, where the concrete distributes the heat to the entire floor surface. The concrete has good thermal conductivity properties, but the challenge is that the concrete accumulates 15 a great deal of heat and therefore responds slowly. When there is a change in the heat supply due to sunlight or when there are many people in the room, the concrete stays warm for a long time after the thermostat has turned off the heat supply. When the need for heat arises again, it takes a relatively long time to bring the temperature back up, because a large volume of concrete has to be heated before the heat reaches the plank floor. Heavy underfloor heating therefore makes it harder to control the room temperature than light underfloor heating. The benefit of heavy underfloor heating is that the construction of the floor is simple, and that the heating system can be used to dry out the concrete floor. Heavy underfloor heating is installed by a mason and a plumber.
4.2.2 Light underfloor heating
Light underfloor heating is based on heat emission plates in aluminium, where the heating coils are placed in curved grooves. The heat emission plates are placed just underneath the plank floor, and because aluminium is a good thermal conductor, the heat is quickly distributed throughout the floor surface. Aluminium responds quickly to changes in the demand for heating, so the room temperature will feel more stable than with heavy underfloor heating. The construction is slightly more complicated but provides a more pleasant floor to walk on, as the substructure is usually a joist construction. Light underfloor heating is installed by a carpenter and a plumber.
4.2.3 Electrical underfloor heating
Electrical underfloor heating is typically constructed with heat cables distributed in a joist construction. As with the other types of underfloor heating, the surface temperature must not exceed 27 °C. The cost of electricity should be included in considerations to install electrical underfloor heating. In the winter months, the electrical underfloor heating must be switched on all the time to provide basic heating for the house. It may be an advantage to combine electrical underfloor heating with radiators. Electrical underfloor heating is installed by a carpenter and an electrician.
4.3.1 Underfloor heating in concrete
The heating coils are tied onto a mesh and embedded in a 10 cm thick layer of concrete on top of the insulation. The construction is simple and only requires that the heating coils are closely attached to the mesh. The surface of the concrete floor must be level with a maximum deviation of 2 mm, measured across a 2-metre straightedge. The concrete moisture must not exceed 85% RH, and a vapour barrier must be used. Insulation must conform to the building code.
188.8.131.52 Directly on concrete with heating coils
Place a sound-absorbing vapour barrier on top of the concrete. Dinesen recommends Platon Stop. Make sure that the vapour barrier is impenetrable to irregularities in the concrete. Fasten the floor planks directly to the concrete with a Dinesen screw and Dinesen rawlplug as described in Dinesen’s 'Installation' instructions. Make sure to push the planks firmly into the substructure. You should be aware that this type of floor is relatively hard to walk on and is therefore generally less comfortable than other installation types. Installing the floor directly on the concrete layer requires knowing exactly where the heating coils are placed to avoid drilling or screwing into them. 16
Sinking screws into concrete is more time-consuming than other types of installation. An alternative option is to use full-surface gluing to install the planks on the concrete, cf. Dinesen’s 'Installation' instructions and the supplementary 'Full-surface gluing' instructions.
Figure 1: Cross-section, direct installation on concrete with heating coils
184.108.40.206 Plywood/chipboard on concrete with heating coils
On concrete floors, always use a vapour barrier of at least 0.20 mm PE foil with 20 cm taped overlay. Let the vapour barrier continue up the wall and cut it off behind the skirting board. Make sure that the vapour barrier is impenetrable to irregularities in the concrete. Lay out grey flooring cardboard (400 g/m2) on at least 22 mm plywood or chipboard. The planks are installed with hidden screws or screwed from above as described in Dinesen’s “Installation” instructions. The advantage of this type of installation is that it is relatively simple. In addition, the solution ensures an effective separation between the concrete and the wood construction. Due to the thickness of the planks, the flow temperature needs to be slightly higher, but as mentioned above, this only has a marginal impact on the heat energy consumption, as long as there are no cavities or irregularities that may block the conductivity of heat.
Figure 2: Cross-section, chipboard on concrete with heating coils
4.3.2 Underfloor heating with heat emission plates
Underfloor heating with heat emission plates provides a very responsive underfloor heating construction and a pleasant floor to walk on. All in all, it is a very comfortable solution.
220.127.116.11 Heat emission plates on laths on top of joists
The floor planks are laid in the same direction as the joists. We recommend installing the planks lengthwise in the building. With this solution you therefore need to install the joists lengthwise, so that the laths can be laid across the joists. Install the joists at approximately 60 cm intervals with insulation in between. On top of the joists, install a layer of planed 21 x 100/28 x 120 mm laths at approximately 30 mm intervals to make room for the grooves in the heat emission plates. Next, install the heat emission plates, fastening them with Floor plank Cardboard Vapour barrier Concrete Heating coil Floor plank Cardboard Chipboard Vapour barrier Concrete Heating coil 17 nails on one side. After installing the heating coils, lay out grey flooring cardboard (400 g/m2), and fasten the floor planks to the laths with screws. Pay attention to the placement of the coils.
Figure 3: Cross-section, heat emission plats on laths on top of joists
18.104.22.168 Heat emission plates on laths between joists
The floor planks are laid across the joists. This construction can be used on a tier of beams or a new tier of joists with a cc-distance of 60 cm. Place insulation in between the joists. Install at least 45 x 45 mm planed battens/fillets at 60 cm intervals between the joists. Recess them, so the top of the laths is level with the top of the joists. Lay out a layer of planed 21/28 x 120/95 mm laths at approximately 30-50 mm intervals to make room for the grooves of the heat emission plates. Install the heat emission plates, and fasten them with a nail on one side. After installing the heating coils, lay out grey flooring cardboard (400 g/m2), and fasten the floor planks to the joists. Pay attention to the placement of the coils.
Figure 4: Longitudinal section, heat emission plates on laths between joists
22.214.171.124 Self-supporting heat emission plates on joists
Install the joists with a cc-distance of 60 cm with insulation in between. Install the self-supporting heat emission plates directly on the joists, nailed onto the plates through the steel bands that run across. After installing the heating coils, lay out grey flooring cardboard (400 g/m2), lay out the floor planks across the joists, and fasten the planks to the joists with screws.
Figure 5: Longitudinal section, self-supporting heat emission plates on joists
126.96.36.199 Underfloor heating chipboard on joists
22 mm floor heating chipboard can be installed on top of an insulated layer of joists with a cc-distance of max. 60 cm. Grooves have been prepared in the chipboard for the heat plates and 16 or 17 mm heating coils. Lay out grey flooring cardboard (400 gr/m2) and fasten the floor planks to the chipboard with a screw. Pay attention to the placement of the heating coils.
Figure 6: Longitudinal section, underfloor heating chipboard on joists
188.8.131.52 Underfloor heating chipboard on concrete
The surface of the concrete floor must be level with a maximum deviation of 2 mm, measured across a 2-metre straightedge. The concrete moisture must not exceed 85% RH, and a vapour barrier must be used. On the dry and straight concrete floor, place a 22 mm underfloor heating chipboard on top of a vapour barrier of at least 0.20 mm PE foil with 20 cm taped overlay. Let the vapour barrier continue up the wall and cut it off behind the skirting board. Make sure that the vapour barrier is impenetrable to irregularities in the concrete. Lay out grey flooring cardboard (400 g/m2), and fasten floor planks to the chipboard with screws. Pay attention to the placement of the coils.
Figure 7: Longitudinal section, underfloor heating chipboard on concrete
4.3.3 Underfloor heating plates
Underfloor heating plates are used in an underfloor heating system where polystyrene will establish an insulating layer directly on the levelled sand layer. For example, lay out a layer of polystyrene that is 250 mm thick. On top of this, lay out a vapour barrier and a layer of 50 x 50 mm joists at 60 cm intervals. In between the joists, lay a 25 mm layer of polystyrene and, on top of that, 25 mm underfloor heating plates (tracked polystyrene plates with aluminium heat emission plates with room for heating coils). After installing the heating coils, lay out grey flooring cardboard (400 g/m2), and fasten the floor planks to the joists with screws.
Figure 8: Longitudinal section, underfloor heating plate system
4.4 Test, start-up and use
Very important: Start up the underfloor heating slowly and gradually.
It is important that the plumber pressure-tests the system to make sure there are no leaks, that the coils match the room specifications, and that the thermostats are working. The user should also receive a user’s manual and be given a thorough introduction to the operation of the system. During the first week, the flow temperature must not exceed 25 °C. After the first week, it may be increased by max. 5°C every other day, until the required surface temperature has been reached. If the flow temperature is raised too quickly, the planks will warp. Often, the underfloor heating system is left on all year around, since it is controlled by in-room thermostats and therefore only uses energy when the thermostat reaches its pre-set point.