Masonry Magazine November 2012 Page. 20
Air Barriers: Impermeable or Permeable?
BY JANE WU
AIR INFILTRATION OF BUILDINGS CAUSES SIGNIFICANT HEATING/COOLING ENERGY LOSS and permits moisture and condensation to collect in the walls. This interstitial condensation creates significant problems for buildings and occupants alike, including poor air quality, mold and mildew, and even structural damage.
To prevent these destructive outcomes, no building design today is complete without a continuous air barrier. However, architects and engineers are faced with the challenge of selecting an air barrier from the many types available, including the choice between vapor permeable and vapor impermeable air barriers.
Understanding vapor drive
MOISTURE VAPOR naturally diffuses into and through wall structures a phenomenon called "vapor drive." The degree of vapor drive is controlled by the porosity of the wall, together with environmental factors, especially moisture and temperature gradients. Moisture vapor will naturally move from a high concentration to a lower concentration and from the warm side to the cooler side. The greater the difference of the moisture gradient and temperature gradients, the greater the vapor drive.
What this means is that vapor drive will act differently, relative to a wall, depending on the climate, or even the time of year. (See climate map.)
Northern Cold Climate-Vapor Drive Interior to Exterior
Moderate Climate-Vapor Drive Equal Both Directions
Hot/Humid Climate - Vapor Drive Exterior to Interior
Climate Map
November 2012 www.masoncontractors.org
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