12 April

Air Pressures that Cause Air Infiltration and Exfiltration


The infiltration and exfiltration of air in buildings is the primary reason to install an air barrier system.
 When installing an air barrier system it is critical to understand the pressures that occur on buildings which cause infiltration and exfiltration.  There are three major pressures.

  • Wind Pressure
  • Stack Pressure sometimes called chimney effect
  • HVAC Fan Pressure

Wind

Wind pressure tends to pressurize a building positively on the façade it is hitting, and as the wind goes around the corner of the building it speeds up considerably, creating especially strong negative pressure at the corners and less strong negative pressure on the rest of the building walls and roof. Wind pressure on buildings is significant in calculating energy or moisture–related air leakage in buildings. Over the course of a year average wind pressure measures 10-15 mph in most locations in North America (Hutcheon and Handegord, 1983). 

Stack Pressure

Like wind, the chimney effect can move large volumes of air through a building envelope.  It is caused by a difference in atmospheric pressure at the top and bottom of a building due to temperature. Temperature variation causes a difference in the weight of the columns of air indoors vs. outdoors. In the winter, the warm air in a heated building is lighter (less dense) than the cold air outside the building.  That warm air bubble wants to rise up and out. When it does, the flow of air leaving the top of the building draws cold air into cracks at the bottom. The reverse occurs in warm climates with air-conditioning. 

HVAC Fan Pressure

Fan pressure is caused by HVAC system pressurization. Fan pressures can create negative pressure on the building envelope, drawing in cool, dry air in the winter and hot, humid air in the summer. Alternatively, positive fan pressures push warm, moist air into the building envelope in the winter, depositing condensation on surfaces within wall and roof enclosures. 

To control the air bypass caused by wind, stack, and fan pressure, the proper specification and application of the air barrier system is of critical importance.

Contact the air and vapor barrier experts at Carlisle Coatings & Waterproofing for project specific questions, details and applications guidance. They may be reached at 888-229-2199 or technicalservices@ccw.carlisle.com . 

For straight-forward information about how new wall assemblies containing plastic foam insulation and combustible weather barriers can meet tough new NFPA 285 standards, please visit fireresistccw.com.



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