Efficient design

Design to save energy

Design is the most important phase in the construction a building. Its level of energy efficiency depends largely on the construction techniques introduced during the design phase and on the materials used.

It is possible to achieve a good level of energy efficiency both for new builds using bioclimatic design techniques, and for existing builds through energy improvement works. It is therefore extremely important to be aware of the techniques and materials being used to construct the building. For example, choosing good insulation when constructing the skin (outer shell) of a building helps mediate the cold of winter and the heat of summer, thereby increasing quality of life. In the case of new builds, orientation is a crucial component of energy efficiency. Good orientation in respect of the cardinal points of the compass allows the construction to make the most of the solar radiation to increase its contribution during the winter months, therefore allowing significant energy savings.

Fully-integrated and high-performance system engineering is fundamental to the building system.

An efficient design must consider both the gains as well as the losses. Therefore, to determine the extent of the work, it is important to take stock of the energy losses of each component of the building‘s skin, but bearing in mind that energy efficiency also depends on the thermal behaviour of the building as a whole, as shown in the diagram:

The importance of the systems

System engineering plays a key role in design: efficient systems ensure significant energy savings and a high level of perceived comfort.. In order to create a high performance system it‘s not enough just to choose a high-efficiency generator and maybe a heat pump; it‘s important to make sure that all the other components, including the control system, the heat-transfer distribution system for the heating elements, and heating elements, are all high-performance too.

Let us assume that we have a high-efficiency heat generator (98 %), control system (98 %) and heat-transfer distribution system for the heating elements (98 %), but that the heating elements have a lower efficiency, we get the following product 0.98 x 0.98 x 0.98 x 0.85 = 0.80. This shows that just one low-efficiency component seriously compromises the efficiency of the entire system. If, however, all of the system components, including the heating elements, are high-efficiency, the efficiency of the system will also be high, and we get 0.98 x 0.98 x 0.98 x 0.98 = 0.92.

High-efficiency, low-consumption radiant heating and cooling systems are the ideal solution to improve a building‘s energy efficiency and energy rating.

While traditional radiators and/or convection heaters require periodic cleaning and painting, radiant panel systems require little or no maintenance, primarily due to the tubing used for the rings, which is plastic, and therefore not subject to wear due to corrosion.

We can further optimise a system‘s fuel efficiency and reduce CO2 emissions by using alternative energy sources, such as thermal solar, photovoltaic or geothermal energy. Geothermal systems use heat pumps to extract the energy from the ground to meet heating requirements (heating mode) and cooling requirements (cooling mode).

Radiant system performance

The most important factors affecting the efficiency of a radiant heating and cooling system are:

  • The performance of the insulation layer
  • Maximum and minimum acceptable temperatures
  • The mechanisms of exchange between the water in the tube and the surface of the room
  • The heat exchange coefficients between the radiant surface and the environment

The performance of the insulation layer

The performance of a radiant system depends particularly on the layer of thermal insulation used to limit heat loss between the tubes and the space behind them. Standard UNI EN 1264-4 sets out limit values of thermal resistance for systems that work as both heaters and coolers, and recommends them for systems dedicated to cooling only.

The amount of thermal resistance, indicated by the letter R, depends on the material (on the thermal conductivity) and on thickness, temperature and humidity. The designer can use the thermal resistance value obtained from the thermal conductivity data declared (λd) by the manufacturer (which refers to precise standard values of temperature and humidity), or can correct the value on the basis of real-use conditions , defining a design thermal conductivity for the project (λp) (in accordance with UNI EN 10456).

The limit values of thermal resistance depend on the temperature of the room adjacent or below, as shown in the following table:

Room adjacent or below

Insulation resistance [m2K/W]



Not heated


Heated but not continuously


Directly on the ground


Design external temperature ϑe > 0°C


Design external temperature -5°C <ϑe < 0°C


Design external temperature -15°C <ϑe < -5°C


Maximum and minimum acceptable temperatures

The maximum and minimum acceptable temperatures for the internal surfaces of a room are derived from considerations of comfort or surface condensation. UNI EN 1264, based on the outcome of UNI EN ISO 7730 for environmental comfort, suggests maximum surface temperatures for heated floors, walls and ceilings.

For radiant floor systems, UNI EN 1264 sets a maximum temperature of 29°C for occupied areas, and a maximum temperature of 35°C for perimeter areas (with an air temperature of 20°C). In bathrooms, the maximum surface temperature must be no more than 9°C above room temperature (approximately 24°C).
For heated walls, the maximum surface temperature is 40°C, which corresponds to a temperature difference between the wall and the room of 20°C.

For heated ceilings, UNI EN 1264 recommends not exceeding the safe surface temperature of 29°C to avoid radiant asymmetry. The values refer to a room of standard size and shape. A different room of different dimensions may permit temperatures higher than 29°C. It is important to note that the maximum water temperature must take into account the material into which the tubes are inserted. For plasterboard, for example, the water temperature should not exceed 50°C (UNI EN 1264-4).

When in cooling mode, it is important to note that if a surface is colder than the dew point of the room, a layer of condensation could form on the surface. This is to be avoided since it could damage the structure, cause accidents (slippery floor) and make the air unhealthy due to the formation of mould. It is therefore advisable to keep the surface at a temperature above the dew point.

In addition, the minimum temperature for the floor should be 19°C so as not to cause discomfort to anyone in the room, be they seated or standing.

The mechanisms of exchange between the water in the tube and the surface of the room

Hydronic radiant systems are systems that work with a low temperature difference between the water and the room and, as such, are commonly defined as low-temperature difference systems. The thermal power supplied by the water during the heating phase is able to be released into the room thanks to a heat exchange mechanism between the hot water and the internal surface of the radiant surface, such as the floor for example. The same principle applies to cooling, but in this case thermal power is removed by the cold water circulating in the tubes.

Since the area of the surface next to the tube is most affected by the temperature of the water, it is evident that the closer the tubes (and therefore the narrower the spacing), the greater the efficiency of the heat exchange.

So the thermal power can be easily transferred to the room, it is essential that the tube is in thorough contact with the conductive layer into which it is inserted, keeping air pockets, in the case of dry systems, or contact with insulating systems, to a minimum. In floor systems, the conductive layer also has to perform the function of supporting any loads placed on the floor. Therefore a traditional or self-levelling screed, or a fibre-plaster board of suitable size is used.

In certain cases, it is possible to insert a sheet of highly conductive material, such as aluminium, in the area separating the material above the tubes (as can be screed) and the insulating layer below. This is usually done if the tube is fitted underneath the supporting layer.

Another important parameter for determining the efficiency of the radiant system is the coating material. If a wood covering is chosen - wood being an insulator - rather than a conductive material such as ceramic, higher water flow temperatures will be required to achieve the same degree of thermal comfort, which will involve supplying more power to the water. Similarly, a wooden floor in summer would require lower flow temperatures.

Finally, it is important not to underestimate the conductivity of the tube as a factor. More and more often, plastic tubes are being used in radiant systems. They guarantee long-term reliability, are low cost, are not subject to corrosion and allow versatility in installation. Compared to the copper or steel tubes traditionally used in the home heating industry, plastic tubes have a lower coefficient of thermal conductivity, around 0.3 -0.4 W/(m K) for polyethylene tubes.

Exchange coefficient

Inside the room, overall heat transfer is given by combining the heat transfer by convection and the heat transfer by radiation which affects all the surfaces and anyone in the room.

Convective exchange affects the air inside a room and can be determined by a difference between the temperature of a surface and the air. In a room that has an active radiant surface, the main mechanism of convective exchange is by that surface, although other convective phenomena can exist next to a cold window or above people or a computer usually at a temperature higher than the air. If the room has a primary air inlet (natural ventilation), the coefficients of convective exchange could be locally high if the inlet speed of the air is higher. However, when considering a radiant system‘s efficiency at heating or cooling, only the convective exchange between surfaces with average temperature θsm and an ambient reference temperature is considered.



Cold floor

Warm ceiling

Cold ceiling


hc [W/(m2K)]


1.0 1.0 5.3 2.5

Heat transfer by radiation affects all surfaces of a room and is determined by the thermal radiation emitted by a surface. The magnitude of this heat flow is influenced by several factors: the emissivity of the surfaces (a property of the material), the temperature and area of the surfaces, and the view factor between the two surfaces. The view factor is a purely geometrical factor which depends solely on the mutual position of the two surfaces. When considering a radiant system‘s efficiency at heating or cooling, only the thermal exchange between the active surface and the average temperature of the other surfaces is considered, ignoring the presence of other bodies (such as people) in the room.

Adding up the contributions of transfer by radiation and convection gives a single coefficient, called the threshold coefficient of exchange (hl). According to UNI EN 1264-5 this is:



Cold floor

Warm ceiling

Cold ceiling


hl [W/(m2K)]


6.5 6.5 10.8 8