A dominant opening in a windward wall, which generates large internal pressures in a building, is a critical structural design criterion. The internal pressure fluctuations are a function of the dominant opening area size, internal volume size and external pressure at the opening. In addition, many buildings have background leakage, which can attenuate internal pressure fluctuations. This study examines internal pressure in buildings for a range of dominant opening areas, internal volume sizes and background porosities. The effects of background porosity are incorporated into the governing equation. The ratio of the background leakage area $A_L$ to dominant opening area $A_W$ is presented in a non-dimensional format through a parameter, ${phi}_6-A_L/A_W$. Background porosity was found to attenuate the internal pressure fluctuations when ${phi}_6$ is larger than 0.2. The dominant opening discharge coefficient, ${kappa}$ was estimated to lie between 0.05 to 0.40 and the effective background porosity discharge coefficient ${kappa}^{prime}_L$, was estimated to be between 0.05 to 0.50.