Construction Concerns: Infiltration

By Gregory Havel

An air leak from outside that enters a building through cracks, doors, and other designed or undesigned openings is called “infiltration.” It is one of the parameters used in designing heating, ventilating, and air-conditioning (HVAC) systems. The more resistant a building envelope is to the passage of unwanted, unconditioned outdoor air, the more energy efficient it will be.

The infiltration rate is measured in cubic feet per minute, or in liters per second. The air exchange rate is the number of times per hour that infiltration changes the volume of air contained in the building. This is also called “air changes per hour.”

In the past, infiltration was of less concern than it is today because fuel was comparatively inexpensive. Infiltration rates in older buildings that have not been rehabilitated could result in 1½ to two air changes per hour, accompanied by higher rates of fuel consumption.

As fuel costs have increased over the past 40 years, building designers and owners have become more concerned and educated about infiltration. Common practices today, required by most state and municipal HVAC codes, include sealing cracks, weather-stripping doors and windows, and installing continuous vapor barriers and air flow barriers in new construction and during major remodeling. Some structures have become so resistant to infiltration that their occupants have become ill because of “sick building syndrome”—the result of too many people and too much activity in a space with too little ventilation.

Many of today’s HVAC codes require mechanical ventilation in buildings where the infiltration rate is less than 0.35 air changes per hour.

Infiltration is usually invisible. The two photos were taken recently in a building that meets the present insulation and infiltration HVAC code requirements in southeastern Wisconsin. Indoor temperatures were 68-70°F (20-21°C), with 30 percent relative humidity. Outdoor temperature was  -15°F (-26°C) with wind speeds of 35 miles per hour (mph) [56 kilometers per hour (kph)] and gusts to 45 mph (72 kph).

Photo 1 shows a pair of insulated doors with insulating glass in an insulated doorframe inside the building and weather conditions described above. The high wind speeds overpowered the weather stripping, and other seals provided against infiltration and pushed the cold air into the building. The white deposits on the threshold, the door, and the frame are frost produced by the contact of the comparatively moist indoor air with the frigid air infiltrating the building.



Photo 2 shows frost deposits along small openings between the parts of a fixed (inoperable) window frame and sill because of the contact of frigid infiltration with moisture in the indoor air.



A better insulated and tighter building envelope and reduced infiltration have the following effects on fire behavior:

  • Additional insulation reduces the rate of heat loss through the walls of the fire room, reflecting more heat back into the room.
  • Reduced infiltration reduces the oxygen in the air that is available to a room-and-contents fire, causing it to convert from the growth stage (free-burning) to ventilation-controlled more quickly than in the past.
  • The fire continues to produce heat and pyrolize fuel, producing fuel-rich smoke that can flash or cause a backdraft when oxygen is introduced, as by breaking a window or opening a door.

These effects, combined with lightweight construction materials and methods with petroleum-based furnishings leads to fires that develop faster and with more heat and with less time before flashover and structural collapse than in the past.

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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.

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