Information · Air Infiltration
Air Infiltration
Air Infiltration and Energy Use
Air infiltration is outdoor air leaking into a building through cracks and gaps in its envelope; exfiltration is indoor air leaking out. Together they are one of the largest and least visible sources of energy loss. Leaky windows waste heating and cooling energy, cause drafts and cold spots near the glass, carry moisture into walls where it can condense, and let in noise, dust and pollen.
New windows are not all alike. Code-minimum windows may leak ten times as much air as the best, and how a window is installed matters as much as how it is built.
How window air leakage is measured
Window air leakage is tested in the laboratory under ASTM E283, by sealing the window in a chamber and measuring the air that passes through it at a set pressure difference. Results are given in cubic feet per minute per square foot of window (cfm/ft²), at either:
- 1.57 psf (75 Pa), equivalent to about a 25 mph wind, used for most residential and commercial ratings, or
- 6.24 psf (300 Pa), equivalent to about a 50 mph wind, used for the highest-performance AW class.
Under the North American Fenestration Standard (NAFS, AAMA/WDMA/CSA 101/I.S.2/A440), windows in the R, LC and CW performance classes must leak no more than 0.3 cfm/ft² at 1.57 psf, and AW-class windows no more than 0.1 cfm/ft² at 6.24 psf. Because leakage rises with pressure, 0.1 cfm/ft² at 6.24 psf is roughly 0.04 cfm/ft² at 1.57 psf. NFRC 400 can also show air leakage on the NFRC label.
What codes and standards require
| Requirement | Window air leakage |
|---|---|
| IECC, residential windows and sliding doors | ≤ 0.3 cfm/ft² at 1.57 psf |
| IECC 2021, commercial windows | ≤ 0.20 cfm/ft² at 1.57 psf |
| IECC 2021, curtain wall and storefront | ≤ 0.06 cfm/ft² at 1.57 psf |
| GlasFëns GR85, tested by Intertek | 0.03 cfm/ft² at 1.57 psf (operable) |
Our GR85 windows leak one-tenth of the residential code maximum and well under the commercial limit. Multi-point German hardware pulls the sash into continuous EPDM gaskets on all four sides, so the seal is even all the way around. See the performance data.
The building, not just the window
Today’s best practice treats airtightness as a property of the whole building, not of individual products, and tests it on site:
- Whole-building testing. A blower door pressurizes or depressurizes the building to 50 Pa and measures the leakage, in air changes per hour (ACH50) or cfm per square foot of envelope. The IECC limits most new homes in climate zones 3 to 8 to 3.0 ACH50, and recent commercial codes require testing for many building types.
- Passive House targets are far stricter: 0.6 ACH50 for the Passive House Institute and 0.060 cfm per square foot of envelope at 50 Pa for Phius. See Passive House & Phius.
- The window-to-wall joint. Most window-related leaks are at the installation, not through the window. The window frame must be sealed continuously to the wall’s air barrier with tapes, membranes or sealant. See Air Barrier Interface and Installation.
- Field testing. Installed windows can be checked on site for air leakage (ASTM E783) and water penetration (ASTM E1105) before the interior is finished.
- Build tight, ventilate right. An airtight building needs planned fresh air. Balanced mechanical ventilation with heat or energy recovery (HRV or ERV) supplies it while recovering most of the energy that leakage would waste.
Estimating the energy cost of air leakage
The energy needed to heat or cool infiltrating air has two parts. The sensible load is the energy to bring the air to indoor temperature:
Hs = 1.08 × Q × (Tin − Tout) Btu/h
The latent load is the energy to remove the moisture it carries, in humid weather:
HL = 4,840 × Q × (Wout − Win) Btu/h
Where Q is the infiltration airflow in cfm, T is temperature in °F, and W is the humidity ratio in pounds of water per pound of dry air. The constants combine the density and specific heat of air (0.075 lb/ft³ and 0.24 Btu/lb·°F) and the latent heat of water (about 1,060 Btu/lb) with 60 minutes per hour.
Laboratory leakage rates are measured at a fixed pressure, but real buildings see varying wind and stack pressures, so tested values must be converted to average operating conditions. Whole-building energy models such as EnergyPlus, PHPP and WUFI Passive handle infiltration at the building level, using a measured or design airtightness and hourly weather data, rather than window by window.
Typical leakage of existing windows
For replacement projects, the difference between old and new windows can be large. Typical assumed leakage rates for existing windows, from the ASHRAE Handbook of Fundamentals, are:
| Existing window | Air leakage at 1.57 psf |
|---|---|
| Hung or sliding, no weatherstripping | about 2.5 cfm/ft² |
| Hung or sliding, weatherstripped; awning or casement, no weatherstripping | about 1.0 cfm/ft² |
| Awning or casement, weatherstripped | about 0.5 cfm/ft² |
Replacing a weatherstripped double-hung window at 1.0 cfm/ft² with one of ours at 0.03 cfm/ft² cuts its tested air leakage by about 97%.
Beyond energy
Air leakage affects more than the energy bill. It drives the stack effect in tall buildings, carries moist indoor air into cold wall cavities where it can condense and cause mold or rot, creates drafts that make perimeter space uncomfortable, and lets in outdoor noise and pollutants. An airtight window, properly sealed to the wall, addresses all of them.