A phenomenon called vapor drive moves water vapor from areas of high density to low density. Vapor drive naturally diffuses moisture vapor into and through wall structures, resulting in condensation on cool surfaces. To understand how air barriers impact building comfort and integrity, it is critical to understand how moisture vapor interacts with wall structures.
The degree of vapor drive is controlled by the porosity of the wall and moisture and temperature gradients. Moisture vapor will naturally move from a region of high vapor concentration to an area of lower concentration, until the concentration is in balance. If vapor pressure is high outside the wall and low inside the wall, vapor drive will be directed inward (and vice versa). The greater the difference in this vapor pressure, or “concentration gradient,” the greater the vapor drive.
Moisture vapor will naturally move from the warm side of a wall to the cooler side. If the temperature is high inside the building and lower outside the building, vapor drive will be directed outward (and vice versa). The greater the difference in this “temperature gradient,” the greater the vapor drive.
The movement of moisture via diffusion is a result of differences in vapor pressure that are related to the temperature and moisture content of the air. Temperature is the greater factor affecting vapor drive. In fact, when the temperature differences between indoors and outdoors is greater than 20°F, the vapor drive can be quite strong. Add a significant difference in humidity, and the vapor drive becomes even stronger.
What this means is that vapor drive will act differently relative to a wall depending on the climate, or even the time of year.
Cold climate: vapor drive primarily from interior to exterior;
Mixed climate: vapor drive approximately equal in both directions;
Hot, humid climate: vapor drive primarily exterior to interior. (Santos, 2007)
Understanding vapor drive provides insight and direction in helping achieve the desired performance of the wall assembly as well as placement of the permeable or non-permeable air barrier.
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 firstname.lastname@example.org.