A vapor barrier is often an essential piece of the construction puzzle, whether you are developing a new building or renovating an existing structure. So, what circumstances make it a consideration?

A vapor barrier is used to protect a structure from a build-up of moisture due to condensation and is a required detail in all but the driest climate zones in the US. Some types of insulation, when installed correctly can double as a vapor barrier, so installing an additional barrier is not necessary. 

Here’s when, and where you should use a vapor barrier, and when it may not be required.

When Do You Need a Vapor Barrier?

Let’s take this back to first principles. Insulation acts as a thermal barrier to prevent warm air from moving to an area of colder air. This means keeping interiors warm in winter and cool in summer.

Warmer air generally holds more moisture than cold air, and so, through its movement from a warm area to a cooler one, it brings moisture with it. As it passes through a wall construction, should it meet a cold surface, then it will likely condense, introducing water into the structure.

Clearly, this is not a good thing, and in some circumstances, the likelihood of this makes a vapor barrier a necessary part of a wall assembly.

Using Vapor Barrier With Fiberglass Insulation

Fiberglass is naturally mold-resistant, however, if it retains moisture it could quickly become a breeding ground, so it is essential that fiberglass is protected from water vapor. In cooler climates the humidity that could affect your insulation comes from inside the home, so an internal vapor barrier is essential.

In warmer climates, when cooling the interior is more usual, then the vapor barrier should be fitted on the external side of the insulation in order to prevent moist, warm air, from being drawn into the wall assembly.

For more details regarding fiberglass and vapor barriers check out this article.

Using Vapor Barrier with Rigid Foam Board Insulation

The short answer to this is that rigid foam board, when installed to the correct specification, acts as a vapor barrier on its own. The temptation to fit an additional barrier should be avoided as it would most likely lead to trapped moisture within the wall assembly.

If your foam board is a minimum of two inches thick, and properly sealed with low expansion spray foam and weathertight tape, then you are good to go.

For more details regarding foam board and vapor barriers check out this article.

Using Vapor Barrier with Spray Foam Insulation

Should you use a vapor barrier with spray foam insulation? The answer to this question is pretty straightforward. Closed cell spray foam, at a depth of two inches, will give you a perm rating of 0.9, the equivalent value of a Class II vapor barrier.

Open cell spray foam, however, is way more moisture permeable and is rated over 30 perms at two inches, therefore a proprietary vapor barrier should be used in tandem with open call spray foam.

For more details regarding spray foam and vapor barriers check out this article.

Vapor Barrier Classifications and Building Codes

When building or renovating it is important that we comply with the expectations of the local building inspector. There are broad guidelines and clear expectations in the code, but local conditions might determine any variation.

Vapor Barriers are rated according to their permeability, i.e. the ease with which moisture can pass through them. Class I is the highest-rated, lowest permeable material such as 6 mil polyethylene, or unperforated metal foil. It has a perm level of 0.1.

Class II materials include Kraft-faced fiberglass batts, with a perm level between 0.1 to 1.0 perms, while Class III materials, permeable up to 10 perms, include half-inch OSB or Plywood sheathing, or most plastic house wraps.

Vapor Barrier in Cooler Climates

The International Building Code tells us that a vapor barrier is a must-have in zones 5 through 8 and marine zone 4. The vapor barrier can be Class I or Class II and should be installed directly behind the drywall so that the assembly dries to the exterior.

The wall cavity insulation should be permeable, allowing the movement of moisture through the wall assembly. Cladding must be vented with a water-resistant barrier, timber, or vinyl siding, for example.

In colder zones continuous external insulation such as mineral wool, that is moisture and mold-resistant will help to keep the sheathing and timber framing dry.

Vapor Barrier in Warmer Climates

Life is a little simpler in the lower numbers. From 1 to 4B, there are no requirements under the code for the use of vapor barriers or retarders. However, this does not mean that we should not be mindful of moisture and the issues it can cause.

In the zones where a vapor barrier is not a requirement, an open wall design is the most sensible way to go. What does this mean? Basically, none of the materials in the wall assembly should have a perm rating lower than 2 in order to ensure the free drying of the structure.

Vapor Barrier Smart Systems

It gets more complicated if you want to use a class 3 or a ‘smart’ vapor barrier. These are usually classed as vapor retarders and limit the movement of moisture rather than stopping it completely.

But if you want ultimate control, then a smart vapor retarder will limit outward moisture movement during the heating season and allow drying to the interior when the aircon is used to cool the internal space.

Vapor Barrier Problems to Avoid

Some insulation materials can be classed as vapor barriers due to the fact that their makeup makes them impermeable, or that they may have an impermeable facing, or that the depth of the insulation means that moisture cannot move freely and dry out rapidly enough to avoid mold growth.

The best case scenario is that you don’t inadvertently fit an additional, second, vapor barrier that will trap moisture.

Permeability Ratings of Materials

To avoid installing vapor impermeable traps in a wall, attic or crawl space construction it is important to consider the permeability ratings of the insulation intended as part of the assembly.

Type of Insulation x thickness1”2”3”4”5”6”7”8”
Polyiso with foil-facing0.
Closed-cell spray foam1.
Polyiso with paper-facing2.
Open-cell spray foam60.
Mineral Wool Batts88.849.529.622.217.814.812.711.1
Fiberglass Batts120       

As can be seen from the chart there are a number of insulation materials that could be used as a vapor barrier within the construction assembly of a wall, and this should be kept in mind when considering whether an additional vapor barrier is required.

For example, using cellulose in a colder climate, it is advisable to use an internal vapor barrier as the permeability of the insulation, even if dense-packed, means that moisture can freely move through the material. Drying to the outside of the building would be the right thing to do.

Fiberglass batts that are neither paper nor foil-faced are perhaps the most moisture-permeable of all the insulation products and should always be used with a vapor barrier in cooler climates.

Foil-faced polyiso, when used as a continuous external 2” insulation layer automatically becomes a Class I vapor barrier, while paper-faced polyiso, XPS, and closed-cell spray foam are rated Class II.

However, it is important that continuous insulation installations meet the minimum R-value code requirements to prevent condensation, whether permeable (mineral wool) or impermeable (polyiso or XPS) and drying to the internal or external face of the structure.


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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