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10 Rules for Designing Vapor Barriers

Building envelope and moisture control specialists Richard Keleher and Judd Peterson provide 10 important rules for designing effective vapor barrier systems.

June 01, 2007 |

Improper use of vapor barriers is one of the leading causes of moisture-related issues in buildings today.

Vapor barriers, or vapor retarders, are designed to prevent wall assemblies from getting wet. But when these systems are not designed and installed properly, they can actually prevent assemblies from drying, which can lead to unwanted moisture intrusion.

“Diffusion can be a problem, especially when vapor retarders are used inappropriately—for example, when vinyl wall covering is used in the Southeast,” says Richard Keleher, AIA, CSI, LEED, senior architect at Richard Keleher Architect, Concord, Mass.

Keleher says the function of a vapor barrier is to retard the migration of water vapor. Specifying a vapor barrier's permeability and its proper location within the assembly are a function of climate, interior conditions, and the characteristics of the materials that comprise the assembly.

“Condensation occurs when water vapor in the air meets a surface that is below the dewpoint,” says Keleher. “This tends to happen at thermal bridges in uninsulated or poorly insulated assemblies, and when the interior humidity is unusually high.”

To help AEC professionals get vapor barriers right, Keleher and fellow building envelope expert Judd Peterson, AIA, president of Judd/Allen Group, Edina, Minn., offer these tips:

1 The fundamentals of creating a proper vapor barrier include developing the thermal section of exterior wall or roof; determining the dewpoint for the interior environment, locating the dewpoint in the wall construction; verifying the vapor barrier location and the thermal performance of the insulation relative to the dewpoint location. For relative temperature, humidity, and dewpoint readings, you need to refer to the psychometric chart.

2 Vapor barriers come in several varieties. They include:

Class I vapor retarder – 0.1 perm or less

Class II vapor retarder – 1.0 perm or less and greater than 0.1 perm

Class III vapor retarder – 10 perm or less and greater than 1.0 perm

(Per Canadian General Standards Board)

Vapor impermeable – 0.1 perm or less

Vapor semi-impermeable-1.0permorless and greater than 0.1perm

Vapor semi-impermeable-10permorless and greaterthan1.0perm

Vapor permeable – Greater than 10 perms


3 In climates with extreme cold temperature, make sure to provide a true, impermeable vapor barrier that is rated 0.10 perm or less on the warm side of the insulation. However, it's important to note that most uncontrolled moisture in exterior walls is conveyed by airflow, not vapor transmission.

4 Check vapor barrier continuity at the interior plane of insulation around the entire building envelope, looking for penetrations, corners, and junctions. Continuously seal all laps and penetrations.

5 Provide under-slab vapor barrier sheets using polyolefin membranes, not polyethylene, and continuously seal all laps and penetrations. Place the vapor barrier directly under the concrete slabs on grade and cover the slabs with wet burlap to reduce the risk of curling during drying and curing.

6 Avoid using vapor barriers where vapor retarders would provide satisfactory performance. Consider stepping down in performance if the physics of the environment allow it; this encourages permeability for evaporation and drying.

7 In general, avoid double vapor barriers where possible and where enclosure vapor barriers cannot be perfected because damaging moisture can accumulate between flawed vapor barriers. However, note that double vapor barriers are common where they can be perfected, such as in curtain wall construction, glazing, and roofing.

8 Consider using SVR (smart vapor retarder) membranes, which are polyamide sheet material that vary their permeability depending on ambient humidity of the adjacent spaces. Also be sure to note restrictions on using SVRs, which exist due to relatively heavy humidity requirements.

8 Consider using spray foam insulation material to provide a vapor barrier in awkward or hard-to-reach locations. For a vapor barrier that also provides a fire/smoke seal, consider using additional spray coatings.

9 Beware of inadvertently creating a vapor barrier finish in a location or plane where one does not belong—vinyl wall covering on an exterior drywall surface, for example.

Author Information
Richard Keleher, AIA, CSI, LEED, is senior architect at Richard Keleher Architect, Concord, Mass. He can be reached at 978-369-4550 or
Judd Peterson, AIA, is president of Judd/Allen Group, Edina, Minn. He can be reached at 952-224-5050 or

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