Vapor Retarders versus Air Barriers
The requirements and properties of vapor retarders and air barriers are often confused. In fact, the
ASHRAE Handbook of Fundamentals describes a vapor retarder as having the requirement of
resisting airflow. Actually, a vapor retarder does not need to control air leakage, assuming that the
envelope has a properly designed and installed air barrier system. Problems can arise when the
function of a vapor retarder is confused with the requirements of an air barrier system, principally an
air barrier's requirements for continuity and structural adequacy. For example, polyethylene sheet
or aluminum foil are not strong enough to withstand wind pressures. Also, it is extremely difficult to
seal these sheet materials around penetrations.
An air barrier system is required in the building envelope for air-tightness, and the air barrier system
must be designed and installed to meet all of the requirements. If the air barrier will also be serving
as a vapor retarder, or if it has a low permeance to diffusion, then its position within the building
envelope must be carefully considered in relation to the other envelope components.
Position within the Building Envelope
The general rule regarding the positioning of the vapor retarder within the building envelope is that
the permeance of envelope materials should increase in the direction of vapor flow. Therefore, the
vapor retarder should be located on the high vapor pressure side of the envelope. In climates
dominated by heating this means the vapor retarder should be towards the interior of the envelope,
and in cooling climates towards the exterior. While these general rules are useful, it is still
appropriate to conduct an analysis of each envelope system. This analysis should consider climate
and indoor humidity levels in determining temperature and water vapor profiles through the
envelope system and assess the condensation potential within the system. The important factor to
determine in such a calculation is whether and where the temperature within the envelope system
will fall below the dewpoint temperature. The ASHRAE Handbook of Fundamentals describes the
steps in such a calculation, and presents an example. The Moisture Control Handbook, recently
published by Oak Ridge National Laboratory, discusses the positioning of vapor retarders for
heating, cooling and mixed climates and discusses moisture transport for several different
residential envelope designs.
If a high vapor permeance material is positioned on the low vapor pressure side of the envelope,
the result can be an envelope with two vapor retarders, a so-called vapor trap. A vapor trap causes
problems when water vapor is able to move into the wall on the high vapor pressure side but is
unable to pass through the vapor retarder on the low vapor pressure side. Rules of thumb exist
stating that if vapor retarding materials are to be used on opposite sides of a wall, the water vapor
resistance of the high vapor side should be from 5 to 20 times the resistance on the low vapor side.
However, rules of thumb are no substitute for a careful analysis of the temperature and vapor
pressure profiles within the building envelope.