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Airtightness of Buildings - Evaluation of Leakage-Infiltration Ratio and Systematic Measurement Error due to Steady Wind and Stack Effect.

The fan pressurization method that is widely used to measure the airtightness of buildings is known to have quite large measurement error. It is made up of random measurement error (precision) and systematic measurement error (bias). The first part of the present article explains, with analytical evidence, why steady wind and stack effect generate systematic measurement error and it assesses this error through Monte Carlo simulations. Tables with practical results are provided for common zero-flow pressure difference intervals. For example, they show that with a typical zero-flow pressure difference of 5 Pa (0,104 lb/ft²), the systematic measurement error at a reference pressure difference of 4 Pa (0,084 lb/ft²) is in the order of - 1,1% to -2,2%. Possible improvement of the test method is also suggested and assessed. The second part of this article calls on the Monte Carlo simulations to assess the variability of the leakage-infiltration ratio. A simplified analytical formula has been developed to allow comparison and it shows very good correlation. Tables with practical results are also provided. For example, they show that the well-known value of 20 can be associated with a natural pressure field of [-3 Pa; +3Pa] ([-0,063 lb/ft²; +0,063 lb/ft²]).