Validating Numerical Calculations against Guarded Hot Box Measurements

Numerical calculation programs are powerful tools for both analysing and optimising the thermal aspects of building envelope constructions. CEN suggests that these programs should be used, and within a short period of time they will be accepted as applicable projecting tools in the European countries. Only few surveys have been made to validate calculation results against measurements, and there is a need to take further this validation to ensure that calculation results are both reliable and useful. Measurements were carried out using a Guarded Hot Box with a measuring area of 1.2 m by 1.8 m. The constructions analysed were typical lightweight walls containing different types of linear thermal bridges. The Guarded Hot Box that was used was built to meet the criteria given in EN ISO 8990 'Thermal insulation - Determination of steady-state thermal transmission properties - Calibrated and guarded hot box' (European Committee for Standardisation 1996a). Numerical calculations were done using two simulation programs, HEAT2 and HEAT3, both of which have been validated according to EN ISO 10211-1 'Thermal bridges in building construction - Heat flows and surface temperatures - Part 1: General calculation rules' (European Committee for Standardisation 1995). The validation has proven very successful and there seems to be no apparent problems in translating the physical problems directly into model descriptions, as long as material properties are chosen as design values. All comparisons show that calculated values of thermal transmittance is within a narrow margin of measured results, and that deviations primarily depend on the magnitude and complexity of the thermal bridge in the wall. Deviations range from approximately 1 to 5 %.