Which materials have the lowest ambient temperatures

Baunetz_Knowledge

External components and their surfaces are latently exposed to different environmental influences (e.g. climatic changes in the course of the day and year as well as heating and cooling processes due to temperature changes). This leads to stresses from changes in length in the components, which in the worst case causes damage. The surface temperature that actually occurs is formed from the sum of all individual influences. These include:

  • Conductivity of the material
  • Temperature difference between the inside and outside
  • Albedo (reflectivity) of the surface structure
  • Condition of the surface, e.g. colored coatings
  • Absorption capacity of the material
  • Wind speed
  • convection
  • rain
  • Type of solar radiation, direct or diffuse
  • Heat reflection from the material
The usual building physics calculations, as they are z. B. are necessary in the public law verification procedure for heat and moisture protection, but do not include such detailed evidence of the temperatures on the surfaces. Here you only find the wind speed or convection, which are used in the calculations in simplified form in the form of heat transfer resistances. The influence of the heating of the surfaces of a component from solar radiation, on the other hand, is completely disregarded. Nevertheless, from a structural engineering point of view, it is definitely of interest to determine which surface temperatures occur on components and which risks for the construction can result from changes in length.

Basically, all bodies receive and emit radiation. Some of the radiation is reflected back from the surfaces of the body or building materials, while the part that is not reflected is absorbed and converted into heat. Depending on the material properties of the body, the heat is stored as internal energy and passed on into the component. This process leads to a material-specific and temperature-dependent change in length. You can influence this process with the selection of the material, the surface texture or the color. The influence of the colored coating on a sunlit surface and the resulting surface temperatures become clear.

Comparison of the degrees of absorption of different common building materials according to Baehr and Stephan (Baehr, H.D .; Stephan, K.; heat and mass transfer; excerpt from chapter 5.5 radiation exchange, from table 5.8, p.633)

Hashem Akbari published an overview of different colored sample surfaces, from white to black lacquer and different materials, the same structure and the same dimensions, which were examined with regard to the design of passive cooling systems in urban areas. Depending on the different colors and degrees of absorption, there were increases in surface temperatures from +10 K to +49 K.

Temperature increase of selected building materials and coatings (according to H. Akbari)

Degree of absorption αs of selected building materials and coatings (according to H. Akbari)


Akbari H .; Opportunities for saving energy and improving air quality in urban heat islands ”published in“ advances in passive cooling ”, James and James (Science Publishers) Ltd .; London 2007; P.44

The values ​​of the degree of absorption αs for typical building materials can be used to predict the possible warming of materials. The influence of the temperature range on the emissivity can, however, be neglected for normal use in the construction sector, since after VDI 3789-2 the relevant range only extends from -30 ° C to + 100 ° C. If component surfaces are to assume a low temperature under solar irradiation, the quotient of the degree of absorption and emissivity (as / ε) must be small.


Temperature profile of the measurements in the field test bench on 09/21/2011 in Winningen / Mosel

Studies by the Bergische Universität Wuppertal on slate roofs clearly show how great the influence of weather-related influences can be on a roof. Due to the material properties of the slate, the structure is immediately and quickly heated under solar radiation. The example of the measurements from 09/21/2011 shows how the temperature-wise fluctuating roof structure has a temperature that corresponds to the surroundings. When direct irradiation begins, the temperature on the underside of the slate rises by approx. 12 K between 8:00 a.m. and 10:00 a.m., while at the same time the ambient air is only warmed by about 3K. At lunchtime, the slate surface then reaches a surface temperature of approx. 48 ° C, which is 24 K above the temperature of the surrounding air. If the direct solar irradiation is interrupted by the passage of a cloud field, the cooling process of the construction begins immediately, which then approaches the temperature of the ambient air.

With the changes in temperature, changes in the length or spatial expansion of building materials set in at the same time. For most building materials or components, this is mainly a length-related expansion or shortening, which can lead to constraints in the components. This fact must already be taken into account in the planning and be reflected in the structures in the form of joints; This is particularly important for metallic building materials. So there are DIN 18339 VOB procurement and contract regulations for construction work - Part C for plumbing work the maximum distances from movement compensations. The same specifications can also be found for the execution of clinker brick, tiled facades or facades made of metal panels. Here, either field delimitation joints must be planned in order to compensate for tensions from the construction, or the supports of metal panels with elongated hole constructions must be verified in order to absorb changes in length from thermal changes.

Expertise on the subject

heat protection

Tasks and goals of heat and moisture protection

Structural defects usually result from errors that have a structural physical origin. Anyone who is involved in planning and execution should know the properties of the building materials and the climatic processes.

heat protection

The U-value as a building physics parameter

This value forms the basis for the energetic evaluation of all exterior components that separate heated interior spaces from exterior spaces or unheated interior spaces.

heat protection

Heat transport mechanisms

Since building materials have air-filled chambers or pores, the different heat transport mechanisms overlap.