This model describes the one-directional pressure driven air flow through a crack-like opening, using the equation

V̇ = C Δpm,

where is the volume flow rate, C is a flow coefficient and m is the flow exponent. The flow coefficient is

C = L CD,Rat ΔpRat(0.5-m) (2/ρ0)0.5,

where L is the effective air leakage area, CD,Rat is the discharge coefficient at the reference condition, ΔpRat is the pressure drop at the rating condition, and ρ0 is the mass density at the medium default pressure, temperature and humidity.

The effective air leakage area L can be obtained, for example, from the ASHRAE fundamentals (ASHRAE, 1997, p. 25.18). In the ASHRAE fundamentals, the effective air leakage area is based on a reference pressure difference of ΔpRat = 4 Pa and a discharge coefficient of CD,Rat = 1. A similar model is also used in the CONTAM software (Dols and Walton, 2002). Dols and Walton (2002) recommend to use for the flow exponent m=0.6 to m=0.7 if the flow exponent is not reported with the test results.



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