The flow of air through openings in a structure follows laws similar to those describing air flow through orifices and capillaries. A the area in square feet and h the pressure difference in inches of water. For complex openings such as cracks around windows and doors or cracks in walls air flow relationships must be determined by test. Table I - Equivalent Orifice, Areas, sq. Differences occur in air pressure between the inside and outside of buildings because of the effects of wind, temperature differences between inside and outside, and sometimes as a result of the operation of mechanical ventilation and exhaust systems.
Air flow around and over a building causes variations in pressure around it.
Distribution of pressures over building surfaces depends on wind speed and direction, height and shape of the building, and surrounding terrain. In general, pressures are positive on the windward side resulting in infiltration, and negative on the leeward side resulting in exfiltration. Pressures on the remaining sides may be negative or positive, depending on the angle of the wind. They are generally negative over roofs except on the windward side of steep ones. Pressures over the surfaces of buildings are related to the velocity head or stagnation pressures of wind.