Methodological errors in determining thermal protection of building envelopes in natural conditions

The process of thermal engineering surveys of buildings is considered. This process includes in-kind measurements of heat flux density and temperature on the surfaces of building envelope. Based on the obtained measurement results, the reduced heat-transfer resistance is calculated and compared with the required design values for building. It is shown that when calculating the heat-transfer resistance of the building envelope based on the survey results, methodological errors may occur, which are caused by external natural thermal effects on the building and internal non-stationary heat fl ows in the building envelopes. During in-kind thermal engineering surveys of enclosures of buildings of any type, these methodological errors are a priori unknown, since the measurement conditions differ from the conditions of thermal engineering tests in special climatic chambers, where a stationary difference in air temperature is maintained on the internal and external surfaces of the building envelope. To assess the methodological errors in thermal engineering surveys of buildings in natural conditions that begin on any day of the calendar year and last for several days, physical and mathematical modeling of unsteady heat transfer in the building envelope is performed. The following thermal effects, close to real ones, are taken into account during the modeling: outside air temperature; direct and diffuse solar radiation; radiant heat exchange with the environment. Enclosing structures with low and high thermal protection and thermal inertia are analyzed. Possible values methodological errors of heat engineering surveys are calculated using archived meteorological data for the Moscow region. It is shown that, depending on the season and weather conditions, their values can vary from several units to several tens of percent. The conditions for conducting surveys are determined taking into account different levels of methodological errors. A comparison of two standard methods for calculating heat transfer resistance is carried out and it is shown that the methods give different levels of methodological errors. The results obtained will be useful for specialists conducting heat engineering surveys of buildings and structures using non-destructive testing methods.