Reducing Infiltration and Exfiltration

When air leaks into your facility from outside, you have infiltration; when conditioned indoor air leaks out, it’s exfiltration. Both can occur through cracks around windows, doors, dampers, and skylights, or whenever a door or window is opened. The amount of infiltration and exfiltration that occurs is obviously affected by the types of openings your building has along with how good your weatherproofing is, but there are other factors as well. The building’s structure, the direction and duration of the wind, and pressure differences inside and outside the building also affect air leakage.

Pressure. If buildings are designed to minimize changes in air pressure, then the problems and costs associated with infiltration and exfiltration are minimized. For example, an L- or U-shaped building whose center is oriented away from prevailing wind will experience minimal infiltration and exfiltration because those winds are not trapped within the building structure.

The building plan also influences the amount of infiltration and exfiltration. Having fewer doors and windows reduces infiltration, as does using vestibules and wind barriers in front of those entrances subject to high wind velocities.

Infiltration increases as wind velocity increases. In most parts of the northern hemisphere, northern or western sides of a building are exposed to the wind during winter months. The average wind velocity in winter is 10 to 15 miles per hour. In some locations, however, it is considerably higher. In high winds, a negative pressure is often created on the side of a building opposite the wind (referred to as the leeward side). If a building’s northern and western exposures are windowless and/or tight, this negative pressure may force air into the building through openings in the lee side.

The Stack Effect. Pressure differences are created by a building’s air handling system, the wind, and temperature variations. As warm, low-pressure air rises through a building, it tends to draw with it the cooler, high-pressure air from lower levels. This phenomenon, called the stack effect, increases infiltration and is a significant concern when designing heating systems for tall buildings in cold climates. The height of the building and the penetrability between floors determine the impact stack effect may have.

In high-rise buildings with operable windows, stack effect can increase infiltration on lower levels of the leeward side and increase exfiltration on upper levels of the windward side when high winds and low temperatures occur. Even in pressurized buildings with sealed windows and revolving doors, stack effect may cause heat loss or heat gain through elevator shafts or stair towers.

In tall buildings, the stack effect resulting from indoor and outdoor temperature differences also causes air to leak through cracks and openings. Stack effect is always a potential problem for vertical spaces such as service shafts, elevator shafts, and staircases. The density difference between the warm air in the shaft and cold outdoor air causes air to leak into the bottom of the shaft and out the top.

Implement the following procedures to reduce infiltration due to stack effect:

  • In winter, reduce temperatures in stairwells. If necessary, protect piping from freezing.
  • Seal elevator shafts at the top and bottom. Be sure that penthouse machine room doors are weather-stripped and kept closed.
  • Seal vertical service shafts at the top and bottom and, in tall buildings, on every sixth floor.
  • Weather-strip doors to basement and rooftop equipment rooms when they are connected by a vertical shaft that serves the building, and keep the doors closed.
  • Comply with building code venting requirements. Check the fire-resistance rating of materials used. Skylights or smoke relief vents may be required.

Inspection Guidelines: Use the following guidelines when inspecting doors, windows, skylights, and other exterior surfaces for infiltration and exfiltration.

Windows and skylights

  • Replace broken or cracked windowpanes.
  • Weather-strip the operable sash if cracks are evident.
  • Caulk around window frames (exterior and interior) if cracks are evident.
  • Repair windows. Be certain that all windows have sealing gaskets and cam latches that are in proper working order.
  • Rehang misaligned windows.


  • Install weather stripping if none has been installed.
  • Replace any worn or broken weather stripping.
  • Rehang misaligned door frames.
  • Inspect all automatic doors to ensure that they are functioning properly. Consider adjusting doors to close faster.
  • Inspect gasket on garage and other overhead doors. Repair, replace, or install gaskets as necessary.

Exterior Surfaces

  • Caulk, weather-strip, or install gaskets on all exterior joints, such as those between wall and foundation or wall and roof, as well as those between wall panels.
  • Caulk, weather-strip, or install gaskets on all openings used for electrical conduits, piping, through-the-wall cooling, outdoor air louvers, and the like.
  • Where practical, cover window air conditioners and through-the-wall cooling units when not in use. Specifically designed covers can be obtained at relatively low cost.

This article is adapted from the BOMI International course Energy Management and Controls. More information regarding this course is available by calling 1-800-235-2664. Visit BOMI International’s website,