Air barriers, vapor control and condensation · Lesson 2 of 4

Air barriers, vapor control and condensation: materials, standards and design constraints

What a perm measures, the three vapor retarder classes and the test method behind them, what the 2021 residential and building code text asks for, the air leakage limits in the 2021 IECC, and which kind of analysis fits which question.

You will be able to: Read a perm rating and its test method, place a material in a vapor retarder class, read the vapor retarder and air leakage provisions of the codes this module reads, and choose between a dew-point screen, a code table and a hygrothermal analysis.

16 min · 12 slides · 14 questions

All lessons in this module

Slide 1 of 12

What a perm measures

One perm is one grain of water vapor through one square foot per hour for one inch of mercury of vapor-pressure difference.

There are 7000 grains in a pound.

The higher a material’s perm rating, the more vapor it lets through.

Sources

Insulating Basements: Part 2 - Materials, Builder Brief BB0610 - Pennsylvania Housing Research Center, Penn State
Vapor Open Walls (Building America Building Science Advisor) - Oak Ridge National Laboratory (hosted)

Glossary

Perm: The inch-pound unit of water vapor permeance: one grain of water vapor through one square foot per hour for one inch of mercury of vapor-pressure difference, with 7000 grains to the pound.
Vapor retarder class: A rating by perm: Class I is 0.1 perm or less, Class II is more than 0.1 up to 1.0, and Class III is more than 1.0 up to 10, with the test method stated.
Vapor barrier: Building Science Digest 106 defines a vapor barrier as a Class I vapor retarder, one of 0.1 perm or less.
ASTM E96: The water vapor transmission test method behind perm ratings. Method A is the dry cup and Method B the wet cup; the digest allows either for its classes, and the codes’ one-perm allowance names the water method, Procedure B.
Hygrothermal analysis: A transient analysis of coupled heat and moisture movement, such as WUFI-ORNL/IBP, which includes wind-driven rain. The codes accept an approved hygrothermal design as an alternative to their vapor retarder tables.
Air permeance: The rate of air leakage through a material or assembly, in cfm per square foot at a stated pressure. The commercial 2021 IECC limits (C402.5) are stated at 0.3 inch water gauge (75 Pa).