Theory of thermal performance of buildings


Department of Buildings Physics

The main orientation in the range of the thermal performance of buildings theory is the mathematical modeling of the heat, air and moisture transfer in heterogeneous structures, taking into account non-linear effects following from the dependence of transfer coefficients on moisture and temperature.
In connection with this activity, the programs for calculation of one- and two-dimensional non-staedy temperature-moisture fields in structures and for calculation of two-dimensional fields of air flow in permeable structures have been developed. These programs results from the work within the framework of the projects IEA-Annex 24 HAMTIE and IEA-Annex 41 MOIST-ENG and they are unique of this kind in Slovakia. The mentioned programs are used to predict the defects of thermal insulation properties (the condensation of water vapour diffused or transported by convection) of insulated constructions and to analyze processes of material degradation (corrosion, biological degradation).
In connection with modeling the transfer phenomena, the workers in the field of thermal performance of buildings theory deal with determination of the boundary conditions in the heat, air and moisture transfer in specific types of indoor and outdoor climate.

These problems include:
• creating the design climatic reference years for analyses of hygrothermal behaviour of buildings (from the aspects of the comfort, energy requirements, the damage risk, durability…) for a given climatic zone.
• determining the indoor climate parameters, from the moisture production and ventilation analysis of the assessed space.
In the field of the theory of thermal performance of buildings the department has a many-years tradition in measurements of the material properties from the aspect of heat, air and moisture transfer. For this purpose it possesses the set of equipment and apparatuses and the corresponding methods for the following standard tests:
• determining the thermal conductivity by the guarded hot plate method
• determining the thermal resistance by the method of heat flow meter in a warm chamber
• determining the coefficient of diffusion of water vapour by the method without temperature difference
• determining the equilibrium moisture content of materials.
Besides these tests, the methods for the following non-standard tests and measurements have been developed:
• determining the thermal diffusivity and specific heat capacity in the temperature and humidity ranges characteristic for building constructions using the method of the regular regime of 1st kind with the boundary conditions of 3rd kind. This test allows to determine the mentioned parameters quickly, using small samples, for the non-metallic materials with density from 300 to 2600 kg/m3
• determining the moisture diffusivity, based on a capillary suction test
• evaluation of the homogeneity of the structures based on the non-contact measurement of surface temperature fields.
The specific group of problems solved in the field of theory of thermal performance of buildings consists of the monitoring and analysis of hygrothermal behaviour of buildings and groups of buildings. The mentioned measurements can be performed at need either in situ or in laboratory.
For the purposes of laboratory measurements, the climatic chambers with the range of long-term maintainable temperatures and relative humidities are available at the department. In these climatic chambers the following tests were being done:
• the investigation of heating systems behaviour in relation to their control
• the long-term monitoring of hygrothermal behaviour of envelope structures, analyzing the influence of air convection and moisture transport on heat transfer and volume changes in them.
The thermal measurements in situ provide generally an extensive set of data on: temperatures, heat flow rates, relative humidities, flow velocities and pressures of air, emissivities of surfaces, intensities of long-wave and solar radiations, moisture contents in materials. The experimental and analytical identification of the building thermal parameters on the bases of the short-term measurements, enabling the simulation of their hygrothermal behaviour for different purposes (assessment of the energy requirements, diagnostics of defects…) can be consequence of such measurements.