Calculation model for airtightness and natural ventilation of buildings. Rakennusten tiiviyden ja ilmanvaihdon laskentamalli.

A multi-cell calculation model was developed for calculation of the interconnections between airtightness, air change rates, pressure conditions and energy consumption. The flow equation used in the model is quadratic, which can be used as well for a single leakage path as for a whole building envelope. For energy calculation the area of wind directions is divided into12 sectors (each 30 degrees) plus one sector for calm wind conditions. The mean values of wind speed and outside temperature applied to each wind sector are calculated from weather data of several years period. The momentary pressure fluctuations of wind are not taken into account in calculation. Steady-state flow equations are applied to each leakage path to solve the mass flow using the mean values of wind pressure coefficients for each wall area and wind sector. The physical realibility of the model is tested by comparing the results of calculations with the results of the measurements in an actual building. The airtightness of various leakage paths was measured with the collector chamber method. The total air tightness of the building envelope calculated with leakage data measures in an actual building agreed very well with the values measured with the pressure test at small pressure differences (<10 Pa) Good agreement between calculated and measured infiltration rates was also obtained in test conditions.