Design - Vapor Barriers and air barriers

There are myriad materials marketed for controlling the flow of moisture and air through wall and ceiling assemblies.  However, it doesn't take much research to become confused about which to use where.  For example, the nearby map shows that, for our lower Midwest climate zone, we need an interior vapor barrier.  As we will see, this would be an unwise choice. When step-son, Keith, and I were building his house in a nearby county, even the building inspector was sufficiently ambivalent to accept the wall construction with or without a polyethylene sheet plastic vapor barrier.


This post is limited to those materials used on the inside of assemblies. The next post will tackle those used exteriorly. Furthermore, this post emphasizes vapor barriers for wall and ceiling assemblies while the next post completes the story by discussing air barriers.

The Problem
Anyone who is involved with building a house would do well to read "Understanding Vapor Barriers" by Joseph Lstiburek on which the following discussion is based.  He says that "Vapor barriers are...a cold climate artifact that have diffused into other climates more from ignorance than need. The history of barriers itself is a story based more on personalities than physics.....It is frightening indeed that construction practices can be so dramatically influenced by so little research...  Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of (wall or ceiling) assemblies prevent assemblies from drying inward.  This can be a problem in any air-conditioned enclosure. This can be problem in any below grade space.  This can be a problem when there is also a vapor barrier on the exterior.  This can be a problem where brick is installed over building paper and vapor permeable sheathing."

Simplified Terminology
Lstiburek proposes simplifying terminology.  He suggests that all of the materials used in wall and ceiling cavities that are capable of influencing the behavior of moisture vapor, such as house wraps, polyethylene sheeting, felt paper, OSB, plywood, foam insulation board with or without foil backing, drywall, latex paint, vinyl wallpaper and all cladding materials should be called "vapor retarders" because they all have the capacity of retarding the movement of water by vapor diffusion.  Vapor retarders should then be sub-classified into four groups according to the rate at which vapor diffuses through them as measured by their vapor permanence or "perm" as follows:

• Class I Vapor Retarder:        0.1 perm or less
• Class II Vapor Retarder:       Between 0.1 and 1.0 perms
• Class III Vapor Retarder:      Between  1.0 and 10 perms

Then Lstiburek goes on to categorize materials generically into four groups based upon the above three classes as follows:

• Vapor impermeable:                Class I Vapor Retarder       (vapor barrier)
• Vapor semi-impermeable:        Class II Vapor Retarder      (vapor retarder)
• Vapor semi-permeable:            Class III Vapor Retarder    (vapor retarder)
• Vapor permeable:                    Greater than 10 perms

Air moves through wall and ceiling assemblies due to differences in air pressure and contains varying amounts of water in the form of vapor.  All of the materials listed below as vapor retarders have some capacity for blocking air movement and, in so doing, might be called "air barriers".  

Examples of Vapor Retarders
The following list comes from Energy.gov:

• Class I:  Glass, metal, polyethylene sheeting, rubber membrane
• Class II:  Unfaced extruded (XPS) or expanded polystyrene (EPS), 30# felt (asphalt coated paper), plywood, bitumen coated kraft paper
• Class III:  Gypsum board, unfaced fiberglass insulation, board lumber, concrete block, brick, 15# felt (asphalt coated paper), house wrap

Choosing a Vapor Barrier

Lstiburek is quite clear as to best practices for choosing vapor barriers  Paraphrased from his work, they are as follows:

• Avoid using vapor barriers where vapor retarders will suffice; avoid vapor retarders where vapor permeable materials will work; thereby "encouraging drying mechanisms over wetting prevention mechanisms
• Avoid vapor barriers on both sides of an assembly so as not to block drying in at least one direction
• Avoid installing on the interior of air conditioned space such vapor barriers as polyethylene sheeting, foil faced batt insulation and reflective radiant barrier foil
• Avoid vinyl wallpaper on the interior of air conditioned spaces

Interior Vapor Barriers

A reasoned summary for when to use an internal vapor barrier like polyethylene sheeting is found in Green Builder and reproduced verbatim below:

• Most buildings don't need polyethylene anywhere, except directly under the concrete slab or on a crawl space floor.
• The main reason to install an interior vapor retarder is to keep a building inspector happy.
• If a building inspector wants you to install a layer of interior polyethylene on a wall or ceiling, see if you can convince the inspector to accept a layer of vapor retarder paint or a "smart" retarder (for example, MemBrain or Intello-Plus) instead.
• Although most walls and ceilings don't need an interior vapor barrier, it's always a good idea to include an interior air barrier.  Air leakage is far more likely to lead to problems than vapor diffusion (the italics are mine).

Both Lstiburek and Energy.gov say that drywall with latex paint on it, when installed

correctly, forms an adequate semipermeable vapor retarder for most of the US. As we sit at the junction of the mixed-humid and hot-humid zones depicted on the above map, this is obviously the best option for our project as well.  The only caveats that make our project different is that (a) we will not have conventional air conditioning for cooling and humidity control as would be expected for our area and (b) our earth sheltered design means our living space is partially below grade.  However, the energy recovery ventilator that we have planned should adequately replace air conditioning for humidity control and the earth contact walls will be not be in contact with moisture because of the French drains and the insulation/watershed umbrella. And the earth contact walls will not be subject to sweating because the earth behind them will be warmed by the AGS system (for info on AGS, click on "Featured Post" in the column to the left).

Vapor Control Varies According to Climate and Assembly Components
Despite the above advice for avoiding interior vapor barriers, there is no universal solution to vapor control for all situations.  Lstiburek's paper discusses various scenarios for exterior wall assemblies and specifies the best climate zone(s) for each (not only for zones depicted by the map above, but for severe cold climates further north as well).  In northern climates, for instance, the best practice is in fact to use an interior vapor barrier.  But, in another paper on air barriers, he warns against using them even in cold climates for air conditioned spaces.

Even though the primary function of air barriers is to limit air infiltration and exfiltration, they are also vapor retarders in that they control the movement of moisture-laden air through an assembly -- as we shall see in a subsequent post.

In the meanwhile, I will devote the next post to the newly-built cathedral ceiling so as to be able to reference it later for the follow-up post on air barriers.