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 < back to summary 7.6 Principles of heat insulation Introduction The task of heat insulation in structural engineering is creating a pleasant atmospheric environment for the people in the building, both in winter and summer and optimising energy consumption. This requires consideration not only of winter but also summer conditions. Heat insulation previously had a more subordinate role in structural engineering but has become more important in recent years. People started to realise that energy resources used previously were limited and must be used more sparingly. Increased environmental awareness also developed, thus stimulating this trend further. Heat Insulation and Energy-Saving Orders After the first oil crisis in 1973, it was recognised that DIN 4108, Heat insulation in structural engineering, alone no longer met demands. The German Federal Government then enacted the first Energy-Saving Law in 1976, implemented with the Heat Insulation Order of 1977. The second order was passed in 1982 and the third in 1995. The current German “Order on energy-saving heat insulation and energy-saving systems engineering”, known for short as the Energy-Saving Order or EnEV, came into force on 1 February 2002. It is binding under public law and must be adhered to. The definition, i.e. the concrete implementation and application is regulated under the sole responsibility of the country’s various federal states. The EnEV thus represents an important milestone in the package of measures taken in the context of the climate protection programme that was decided upon in 2000. Fig. 7.6.1 shows other elements of this programme.
In existing buildings, the applicable heat insulation/installation-technical demands and obligations to upgrade have been adjusted in line with technical advances. In the framework of these measures, as previously in the Heat Insulation Order, a continued use of the same definition for buildings with normal inside temperatures (> 19° C) and buildings with low internal temperatures. For buildings with normal inside temperatures, the highest values of the annual primary energy requirement depending on the A/Ve building type are to be adhered to. By fixing attention on primary energy, it is intended that a clear reference be made to the political goal of CO2 reduction and distortions of competition for competing energy systems be avoided. Further, the energy quality of the building’s casing must comply with the minimum values prescribed. DIN 4108 DIN 4108 and EnEV regulate national application and harmonised European and international standards. For the adherence to the minimum demands and the assessment energy-saving measures, the following parts of DIN 4108 must be observed:
DIN 4108-2 places the minimum demands on heat insulation of components and thermal bridges in the casing of the buildings and provides heat insulation-technical indications for planning and designing lounges heated to normal temperatures of > 19° C according to their use. Minimum heat insulation means that condensation may not form anywhere on the inner surface of the building’s casing neither will mould form under normal conditions. This applies to the surfaces and the corners. Thermal transfer/thermal conduction In the case of temperature differences within a material or between different materials, heat should always endeavour to create temperature balance. Heat will thus flow until a temperature equilibrium has been achieved. The quantity of heat and energy is measured in joules (J) or watt seconds (Ws). An energy quantity of 1.163 Wh (4,187 J) is required to heat up 1 kg of water from 14.5° C to 15.5° C. This corresponds to the power used by a 100 W bulb burning for about 42 seconds (see Fig. 7.6.2).
Specific heat C The specific heat C in kJ/(kg•K) states how many kilojoules are required to increase the temperature of 1 kg of material by 1 K (1 Kelvin). Thermal transfer in the case of building materials is expressed by thermal conductivity indicated by the symbol λ. This is the amount of heat conducted in one hour through a 1 m2 thick layer of material towards the temperature drop if the temperature of both surfaces amounts to 1 K and the other four surfaces of the cube are protected from heat loss (see Fig. 7.6.3). The lower the thermal conductivity, the better the heat insulation of building material of the same thickness. Thermal admission resistance R The heat insulation capacity of a component is indicated by the thermal admission resistance R (previously 1/Λ). To calculate this, the thickness of the respective layer (in metres) is divided by the material-related thermal conductivity λ in W/(m•K) R for one-layered components
For multi-layered components, the individual value for each layer is calculated according to this procedure. Thermal carriage resistances Rsi and Rse To determine thermal transfer of a component, the inner and outer thermal carriage resistances Rsi and Rse must be known. This is a question of the resistance of the air limit layer against the transfer of heat from the inside air to the component and from this to the outside air.
Thermal carriage resistances are standardised according to the position of the component (vertical or horizontal, etc.) and the outer approach flow (free approach flow, rear ventilated, not rear ventilated). Compare this with the following table in Fig. 7.6.6.
These are calculated for an emission ratio on the surfaces of ε = 0.9 and at a wind speed of v = 4 m/s on the outer surface (see Fig. 7.6.6). Thermal transfer resistance RT The sum of all resistances, the thermal admission resistances of the component layers and the thermal carriage resistances of the air layers gives the thermal transfer resistance RT representing the opposition between the thermal flow and the entire component.
The reciprocal value of the thermal transfer resistance shows the thermal transfer coefficient U as the characteristic component size for the construction thermal protection. U = 1 /R T [ W / (m 2· K)] The U value is of fundamental importance for calculating a building’s requirement for thermal heat. The greater RT is and the smaller the U value, the better the heat insulation is. For a better understanding, the thermal symbols of the physical dimensions in the current usual form and the standardised dimensions to be used today are compared once more.
For more information on this subject, please refer to our GALILEO article 7.7 “Heat insulation in light building”. Leo Oberhofer Literature [1] Koschade, R.: Die Sandwichbauweise [Sandwich panel construction]; Verlag Ernst und Sohn, Berlin, 2000 [2] Achtziger, J., i.a.: Mauerwerk Atlas; Institut für [3] Beuth Verlag: DIN-Normen; Beuth Verlag GmbH, Berlin
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