Summary: Problems in Practice
Most vapor retarder problems stem from considering the envelope vapor transmission properties of
the vapor retarder in isolation from the rest of the building envelope and the particular environment
to which the envelope will be exposed.
A vapor retarder is not just a material with a permeance below a specific value, it is a material/
system that has been considered in relation to the entire envelope system regarding its ability to
retard the diffusion of water vapor to locations where it may condense.
A good vapor retarder is not necessarily a good air barrier. And if the air barrier is distinct from the
vapor retarder, the water permeance of the air barrier must be considered.
Vapor retarder continuity is not as essential as air barrier continuity. Small discontinuities in vapor
retarders at joints, intersections and penetrations will not generally have serious effects due to their
small areas, but they should be avoided. Neglecting the installation of the vapor retarder over large
areas may result in more serious condensation problems.
References
ASHRAE Handbook of Fundamentals, Chapter 20 Thermal Insulation and Vapor Retarders - Fundamentals,
Chapter 21 Thermal Insulation and Vapor Retarders - Applications and Chapter 22 Thermal and Water Vapor
Transmission Data, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta,
1989.
Hutcheon, N.B., G.O.P. Handegord, Building Science for a Cold Climate, John Wiley & Sons, Toronto, 1983.
Quirouette, R.L., "The Difference Between a Vapour Barrier and an Air Barrier," Building Practice Note 54,
Division of Building Research, National Research Council Canada, 1985.
Lstiburek, J., J. Carmody, "Moisture Control Handbook. New, Low-rise, Residential Construction," ORNL/Sub/
89-SD350/1, Oak Ridge National Laboratory, 1991.
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