Air Infiltration Energy Usage
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  • For the purposes of energy modeling, all new fenestration systems perform comparably with respect to air infiltration. However, the same is not true when replacing old, leaky windows. LBNL’s COMFEN software does not consider air infiltration.
  • From the ASHRAE “Handbook of Fundamentals,” air infiltration rates for various types of windows can be assumed to be:
    • 0.1 cfm/sqft at 6.24 psf for fixed windows or curtainwall
    • 0.1 cfm/sqft at 6.24 psf for new AW Class operable windows
    • 2.5 cfm/sqft at 1.56 psf for existing non-weatheripped hung or sliding windows
    • 1.0 cfm/sqft at 1.56 psf for existing weatheripped hung or sliding windows or non-weatheripped awning or casement windows
    • 0.5 cfm/sqft at 1.56 psf for existing weatheripped awning or casement windows
    • NOTE: 0.1 cfm/sqft at 6.24 psf is equivalent to 0.04 cfm/sqft at 1.56 psf
  • For calculating energy impacts of air infiltration, tested air infiltration rates are adjusted for wind velocity at the window face.
  • Site monthly day/night weather data includes both average wind velocity and direction – Only windows on windward elevations exhibit air infiltration at any one time.
  • Uncontrolled air infiltration through cracks and voids in the building envelope increases building energy consumption. By putting untimely heating and cooling load into return air, and/or adversely affecting comfort and air quality in the conditioned space, uncontrolled infiltration creates HVAC load.
  • Conversely, “controlled ventilation” is usually defined as the regulated and relatively steady air supply necessary to maintain indoor air quality, make up for equipment exhaust, or balance exfiltration due to positive building pressure.
  • As long as the exterior average air temperatures for the time period in question are known (from meteorological data), it is relatively easy to calculate the energy necessary to raise or lower infiltrating air to the targeted interior temperature. These are called “sensible” energy expenditures.

Hs = c . ρ. Qw . (Tin - Tout) (BTU/hr)

Where :
Hs = Sensible energy to raise/lower air from Tout to Tin (BTU/hr)
c = Specific heat of air ~ 0.240 BTU/° at STP
ρ = Standard density of air ~ 0.075 lbs/cuft at STP
QW = Air infiltration adjusted for wind at the window face (cuft/hr)
Tin = Interior ambient temperature target (°F)
Tout = Exterior ambient temperature (°F)


  • Assuming humidity control is present in the building’s HVAC system, the energy necessary to dehumidify infiltrating air in the summer is called “latent” energy expenditure, because of the need overcome water’s latent heat of evaporation.

HL = L . ρ. Qw . (Win - Wout) (BTU/hr)

Where :
HL = Latent energy to add moisture to, or remove moisture from,
infiltrating air to go from Wout to Win (BTU/hr). Summer Win is
derived from 60% Relative Humidity maximum for comfort.
L = Latent heat of evaporation H2O ~ 1060 BTU/°F/lb
ρ = Standard density of air ~ 0.075 lbs/cuft at STP
QW = Air infiltration adjusted for the wind at the window face (cuft/hr)
Win = Interior ambient Humidity Ratio target (lbs H2O per lb of dry air)
Wout = Exterior ambient Humidity Ratio (lbs H2O per lb of dry air)

  • A 12-hour operating schedule is appropriate for summer air infiltration calculations in an office building occupancy.
  • During daytime hours on cool, sunny days in the “swing seasons,” the uncontrolled exterior air entering leaky existing windows may provide some temporary natural cooling, to offset solar heat gain, especially in buildings equipped with economizers. For conservatism, use only those months’ energy savings due to reduced air infiltration when it’s fairly certain that HVAC load would never benefit from air leakage.
    • CZ 1 and 2: Cooling months: April through September
      Heating months: None
    • CZ 3, 4 and 5: Cooling months: June, July, August
      Heating months: December, January, February
    • CZ 6, 7 and 8: Cooling months: June, July, August
      Heating months: November through February
  • For replacement window projects, Wausau's Energy Modeling Tool adds air infiltration savings to the other COMFEN-calculated energy savings, to arrive at a more accurate comparison.
  • While not reflected in the calculation procedure described above, excessive air leakage can also adversely impact building “stack effect,” condensation, thermal comfort, and draftiness. Only vision areas are considered, not perimeter interfaces or wall membrane.