©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
UPDATED on May 15, 2015
Although building science has evolved rapidly over the last 40 years, one theme has remained constant: builders are still confused about vapor barriers.
Any energy expert who fields questions from builders will tell you that, year after year, the same questions keep coming up: Does this wall need a vapor barrier? Will foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. trap moisture in my wall? How do I convince my local building inspector that my walls don’t need interior poly?
To begin a discussion of vapor retarders and vapor barriers, I’ll answer a few of these persistent questions. Since I plan to return to this topic in a future blog , I invite readers to submit further questions.
Q. Why would I want a vapor retarder in my wall or ceiling?
A. Vapor retarders help slow the diffusion of water vapor through a building assembly. During the winter, a vapor retarder on the interior of a wall will slow down the transfer of water vapor from the humid interior of the home into the cool stud bays. During the summer, a vapor retarder on the exterior of a wall will slow down the transfer of water vapor from damp siding towards the cool stud bays.
However, a vapor retarder is a double-edged sword: while under some circumstances it can have the beneficial effect of helping to keep a wall or ceiling dry, under other circumstances it can have the undesirable effect of preventing a damp wall or ceiling from drying out.
Q. How often does water vapor diffusion through walls and ceilings cause problems?
A. Very rarely. In many cases, in fact, an interior vapor retarder does more harm than good. The main mechanisms by which moisture enters a wall are from the exterior (usually due to flashing defects that admit wind-driven rain) and via air leaks that carry “piggy-backing” moisture that condenses in a wall cavity. Vapor diffusion is a relatively insignificant cause of moisture problems in walls. (For more information on why air barriers matter more than vapor retarders, see “Air Barriers vs. Vapor Barriers.” )
Q. Under what circumstances can vapor diffusion cause problems?
A. Although vapor diffusion problems are rare, they can occur. Dangers of vapor diffusion problems are higher:
Even in a home with one of the characteristics listed above, the mechanism for moisture transport into walls and ceilings is much more likely to be air leakage than vapor diffusion.
That said, there are a few types of building assemblies that merit close attention to risks associated with outward vapor diffusion. These assemblies include cathedral ceilings in cold climates that are insulated with open-cell spray foam, and double-stud walls in cold climates. For more information on these types of building assemblies, see The Return of the Vapor Diffusion Bogeyman .
Q. What’s the difference between a vapor barrier and a vapor retarder?
A. A vapor barrier stops more vapor transmission than a vapor retarder. A vapor barrier is usually defined as a layer with a permeance rating of 0.1 perm or less, while a vapor retarder is usually defined as a layer with permeance greater than 0.1 perm but less than or equal to 1 perm.
Q. What does the code say about vapor retarders?
A. Codes vary; older versions of model building codes often included more sweeping requirements for vapor retarders than more recent versions.
The 2006 International Residential Code (IRC) and the 2006 International Energy Conservation Code (IECC International Energy Conservation Code.) both define a vapor retarder as a material having a permeance of 1 perm or less. This definition includes such materials as polyethylene sheeting, aluminum foil, kraft paper facing, and vapor-retarding paint.
In section R318.1, the 2006 IRC requires: “In all framed walls, floors, and roof/ceilings comprising elements of the building thermal envelope, a vapor retarder shall be installed on the warm-in-winter side of the insulation.” It should be emphasized that this code requirement makes no mention of polyethylene; vapor-retarding paint fulfills this code requirement.
The 2006 IRC includes exceptions to the vapor-retarder requirement. It allows a vapor retarder to be omitted:
In section 402.5, the 2006 IECC requires: “Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.”
In the 2006 IECC, the exceptions to the vapor retarder requirement are very similar to the exceptions listed in the 2006 IRC, except for an additional exception: “Where other approved means to avoid condensation are provided.” This last exception gives broad latitude to the building official — and places a heavy burden on any builder intent on convincing a local official that a certain building assembly complies with this exception.
The 2007 Supplement to the IECC and the 2007 Supplement to the International Residential Code (IRC) introduced a new vapor-retarder definition. Vapor retarders are now separated into three classes:
Since 2007, the IECC has required (in section 402.5) that walls in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene.
The exceptions have also been rewritten. Three of the exceptions are listed in section 402.5 of the IECC, which notes that vapor retarders are not required on a basement wall, on the below-grade portion of any wall, or on a wall constructed of materials that cannot be damaged by moisture or freezing.
Further exceptions are allowed in section 402.5.1, which states that in climate zones where a Class I or Class II vapor retarder would normally be required, a less stringent vapor retarder — a Class III retarder like latex paint — can be used under the conditions listed in Table 402.5.1 (see accompanying figure). Only certain types of wall assemblies are worthy of this exception; they must have either an adequate layer of exterior foam sheathing or “vented claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .”
Q. Clearly, I can get in trouble with my building inspector if I omit a vapor retarder in certain climates. Are there any situations where I could get into trouble for including a vapor retarder?
A. Yes. Although it’s perfectly legal to install interior polyethylene or vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). wallpaper in any climate, these products can lead to moisture and mold problems in most of the U.S. Unless you’re building in Canada, Alaska, or somewhere close to the Canadian border, you don’t want interior polyethylene or vinyl wallpaper — especially in an air-conditioned house.
Interior polyethylene and vinyl wallpaper prevent a wall from drying to the interior during the summer, when inward solar vapor drive (a phenomenon associated with so-called “reservoir claddings” — for example, brick veneer and stucco — that absorb and hold moisture) can cause condensation on the exterior side of the wallpaper or poly. Unless the moisture introduced into the wall by inward solar vapor drive is able to dry to the interior, wall damage can result.
Q. When it comes to vapor retarders, what do the experts recommend?
A. Here’s a sampling of statements by leading building scientists on the subject of vapor retarders:
To read more questions and answers on this topic, see Do I Need a Vapor Retarder? 
Last week’s blog: “The Energy-Efficiency Pyramid.”