TABLE OF CONTENTS It is practically impossible to accurately predict infiltration on theoretical grounds. However, it is possible to develop relationships to describe the general nature of the problem. With experience and some experimental data, it is possible to put these relations in convenient table and graphical form, useful in estimating infiltration rates. Much of the following material describes how the graphs herein were made and the restrictions placed upon them.
Infiltration is caused by a greater air pressure on the outside of the building than on the inside. The amount of infiltrated air depends on this pressure difference; the number, the size, and the shape of the cracks involved; the number, the length, and the width of the perimeter gaps of windows and doors; and the nature of the flow in the crack or gap (laminar or turbulent). The relation connecting these quantities is
| (6.1) |  |
| Q | = flow rate of leaking air, cfm. |
| ?p | = pressure difference between the inside and the outside surfaces of the building, in. of water gauge. When the outside pressure is greater than the inside pressure, Q, a positive value, is the flow rate of air leaking into the building. If the inside pressure exceeds the outside pressure, Q is the flow rate of exfiltrating air. |
| n | = flow exponent. If the flow in the crack is laminar, n = 1.00; if turbulent, n = 0.50. Usually, the flow will be transitional, thus n will be between 0.5 and 1.0. |
