Airtightness of buildings - Calculation of combined standard uncertainty

The paper presents a calculation method for the combined standard uncertainty associated with the buildings airtightness measurement done in accordance with the ISO standard 9972:2006 (or EN 13829).

The method consists in an application of the law of propagation of uncertainty (JCGM 100:2008) combined with a linear regression (y = a x + b). It goes from the measured values to the air leakage rate and the air change rate.

The ordinary method of least-squares and these of weighted least-squares (with and without negligible uncertainties of the x values) are presented. For each of these methods, the standard uncertainties of the constant a and b of the regression line and their estimated correlation coefficient are given.

The pertinence of the different methods of least-squares for the buildings airtightness measurement is discussed. It seems that the conditions of application of the unweighted method of least-squares are generally not met in the framework of the buildings airtightness measurement; it is therefore advisable to use the weighted method of least-squares.

Real measurement’s data show that the combined standard uncertainty of the pressure differences is not always negligible compared with these of the airflow rates. The consideration of these two uncertainties in the calculation of the weights seems therefore necessary even if it requires a resolution by iteration.