The unsteady pressurization method to measure the airtightness of the building envelope is studied. Recording the pressure decay in the building space provides the airtightness of the building envelope. The governing equations are derived and the air leakage is calculated ) 6 ( n p a Q ∆ = Introducing the above Equation into Equation 4 gives the following: ) 7 ( n a p a P RTM dt p d V ∆ − = ∆ If the pressure change is steady state (i.e., if the left side of the above equation becomes zero) at the constant injection rate M, Equation 7 expresses the DC pressurization method. When M in Equation 7 is the pulse injection of the air, the solution of the equation gives the pressure decay of the room and is utilized with the Pulse method. When M is injected in a volume-oscillatory manner, the solution of Equation 7 presents the room pressure response synchronous to the injected air volume oscillation, and is used with the AC method. If M=0, the solution of Equation 7 at the initial condition of a given pressure P0 gives the pressure decay from P0 to zero. If n=1, Equation 7 becomes linear and can be solved: ∆p RTM P a e a P a V t a = − − ( ) ( ) 1 8 Generally n is not a unit, and Equation 7 cannot be solved analytically except when RMT=0.