Calculating Infiltration

Infiltration is a special case of convection between the inside and outside of, a home. The majority of energy is lost by infiltration when outside air leaks into a home, is conditioned, and then leaks out again. Air enters a home through openings in walls, cracks around doors and windows and a variety of other openings. This air flow is caused by a difference in air pressure between the inside of the home and the outside environment. Wind blowing against the side of the home produces a higher pressure than that within the home. Warm air rises to the top of the home by convection and then leaks out of cracks in the upper walls and roof. This creates a lower pressure in the bottom of the home causing air to leak inside. The amount of heat lost through infiltration depends on a number of factors. Although it is somewhat difficult to calculate with precision, this amount of heat loss can be so significant in determining the total energy used within a home that it should be estimated as accurately as possible. There are five factors which affect the amount of energy a home will lose through infiltration. They are:

Volume (V) of Air
The volume of air within a home is constant even though the air itself changes as new air enters and old air leaves. The total volume is equal to the volume of space within the conditioned portion of the home. To determine the volume (V) of air, multiply the height (H) of the space times the width (W) of the space times the length (L) of the space. While this can be done for the home as a whole, it is more accurate to calculate it for each room and then add these volumes. For rooms with vaulted ceilings, approximations can be made to account for the greater volume. Take 1/2 of the difference in height between the peak of the ceiling and the side walls and add it to the height of the side wall to calculate the volume.

Air changes per Hour (AC)
The rate at which the volume of air in a home changes differs greatly from building to building. The number of air changes per hour (AC/h) can range from as low as 0.35 or less to a high of four or five. This wide variation is due to a number of factors: the number and size of openings in the envelope of the home; the average speed of the wind blowing against the home and the protection the home has from this wind; the number and size of chimneys, vents, and exhaust fans and the frequency of their use; and the number of times that doors and windows are opened, which in turn depends upon the occupants, who and how many there are, and how they use the home.

Heating Capacity of Air (HC)
Air can be heated and cooled. A certain amount of heat is necessary to change the temperature of each cubic foot of air one degree Fahrenheit (F). This amount of heat depends on the density of air which varies with temperature and pressure. This figure will generally be within the range of 0.018-0.022 BTUs/cu. ft.°F. In most cases, an average of .02 BTUs/cu. ft. can be used.

Temperature Differences
As with conduction, the difference in temperature is directly proportional to the amount of heat transferred. With infiltration, as with conduction, this value is the difference between the outside temperature and inside temperature.

Time (t)
The length of time infiltration occurs is directly proportional to the amount of heat transferred. Since the air changes are determined on a per hour basis, the total length of time for which the energy loss is to be calculated must also be expressed in hours. For example, if you want to know the amount of energy lost through infiltration in one day, the span of time (t) would be expressed as 24 hours. If you want to calculate the energy loss for a month, you would use 30 days times 24 hours per day or 720 hours for the time (t) span.