Infiltration Rules Of Thumb

The infiltration rate is the volumetric flow rate of outside air into a building, typically in cubic feet per minute (CFM) or liters per second (LPS). The air exchange rate, (I), is the number of interior volume air changes that occur per hour, and has units of 1/h.


  • Below Grade or Interior Spaces——No infiltration losses or gains are taken for rooms located below grade or interior spaces.
  • Buildings that are not humidified have no latent infiltration heating load.
  • Winter sensible infiltration loads will generally be 1/2 to 3 times the conduction heat losses (average 1.0 to 2.0 times).

Heating Infiltration (15-mph wind)

Air Change Rate Method

  • Range 0 to 10 AC/hr.
  • Commercial buildings:
    • 1.0 AC/hr. one exterior wall
    • 1.5 AC/hr. two exterior walls
    • 2.0 AC/hr. three or four exterior walls
  • Vestibules 3.0 AC/hr.

CFM/sq.ft. of Wall Method

Range                              0 to 1.0      CFM/sq.ft.

Tight buildings                0.1             CFM/sq.ft.

Average buildings           0.3             CFM/sq.ft.

Leaky building                0.6             CFM/sq.ft.

Crack Method

Range                            0.12 to 2.8    CFM/ft. of crack
Average                         1.0                CFM/ft. of crack

Cooling Infiltration (7.5-mph wind)

Cooling load infiltration is generally ignored unless close tolerances in temperature and humidity control are required.

Cooling infiltration values are generally taken as 1/2 of the values listed earlier for heating infiltration.