Material and System Requirements
The two primary requirements for an effective vapor retarder are a sufficiently high resistance to
water vapor diffusion (low permeance) and continuity of the retarder over the building envelope.
The permeance of a material is the rate at which water vapor diffuses across a unit area subject to
a unit water vapor pressure difference. The SI unit for permeance is ng/Pa-s-m . The inch-pound
unit, referred to a the "perm", is grains/hour-ft -in Hg. (1 perm = 57 ng/Pa-s-m .) The 1989
ASHRAE Handbook of Fundamentals contains a good discussion of vapor retarder properties and
their use in buildings, including a table of permeance values for many common building materials.
Other material requirements for vapor retarders include mechanical strength, elasticity, fire and
flammability resistance, and ease of installation. Vapor retarders include rigid materials such as
sheet metal and some insulations, flexible materials such as metal foils, treated papers, coated felts
and plastic films, and coatings such as mastics and paints.
A vapor retarder is sometimes considered to be any material with a permeance of less than 57 ng/
Pa-s-m (less than 1 perm), but actually the perm rating required for an effective vapor retarder
depends on the specific envelope design and the expected vapor pressure difference across it. In
some applications, a permeance much less than 57 ng/Pa-s-m is required. It is also critical to
consider the fact that all envelope components have some resistance to water vapor transport,
even if they have not been designated as vapor retarders. The performance of the designated
vapor retarder must be considered in relation to these other materials. In order to determine the
adequacy of a particular vapor retarder one needs to conduct an analysis of the temperature and
vapor pressure profiles within the envelope as discussed in the ASHRAE Handbook.
Another important vapor retarder system requirement is the continuity of the vapor retarder over the
building envelope. As discussed below, sealing small penetrations and joints between vapor
retarder elements is generally not crucial. However, the vapor retarder treatment must be installed
over the entire building envelope. For example, if the vapor retarder is the interior finish of vinyl
wallcovering, this wallcovering must be installed over the entire interior surface including wall areas
that are hidden from view, such as above suspended ceilings or behind convector cabinets.
Exterior ceilings and soffits are other areas where vapor retarder continuity must not be forgotten.
Transport by Diffusion versus Air Leakage
Diffusion is one mechanism of water vapor transport through a building envelope, the other being
air leakage. Airflow through leaks and openings in the building envelope can transport much larger
quantities of water than diffusion alone, and in order to truly address the potential for condensation
in the building envelope one must control air leakage. Quirouette calculated the water vapor
transport through 1 square meter of an insulated stud wall with a brick veneer due to both diffusion
and air leakage. For a 10 square centimeter penetration in the wall, assuming that only 10% of the
moisture contained in the exfiltrating air actually condenses in the cavity behind the veneer, he
found that more than 200 times as much water transported by leakage condensed in the cavity as
compared to amount which would condense due to diffusion Although air leakage can easily
dominate the transfer of water vapor, it is still important to control diffusion with a vapor retarder.