Airtightness of Buildings – Considerations Regarding the Zero-Flow Pressure and the Least Square Regression

This paper discusses two particular points of the buildings airtightness measurement method (EN ISO 9972:2015) in relation with the calculation of the combined standard uncertainty; Zero-flow pressure difference and Line of organic correlation. The zero-flow pressure difference is measured before and after the test in order to calculate the change of pressure caused by the blower door. Actually the zero-flow pressure difference fluctuates during the test in function of the wind and the temperature difference between outside and inside of the tested building. One could therefore consider that all measured values have the same probability of occurrence during the test (rectangular probability distribution) and adapt in consequence the way of calculating the ‘mean’ zero-flow pressure difference. The paper shows how it could be done and taken into account in the calculation of the combined standard uncertainty. The air flow coefficient and air flow exponent are generally determined using an ordinary least-squares regression technique (OLS). This is however generally not the most appropriate technique because there are uncertainties in both the measured air flow rates and the pressure differences. The paper shows how the line of organic correlation (LOC) could be used in order to take these uncertainties into account.