If you’re building or remodeling your home, there are two important terms that you need to know: vapor barrier and vapor retarder. These two terms sound very similar, but they serve entirely different purposes in the construction of your home, and thus one might be better suited to your project than the other, depending on what you’re planning to do with your house.
A vapor barrier doesn’t allow moisture to pass through it, while a vapor retarder only slows the movement of moisture through the material. A vapor retarder has a perm level between 0.1 and 1 while for a vapor barrier the permeability level is above 1. So, a vapor barrier is more effective than a vapor retarder in moisture control.
Here’s a detailed explanation of these terms and how they apply to your house’s construction.
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Why do we need vapor barriers and vapor retarders?
Most building codes require some form of vapor barrier or retarder in homes. Efficient use of these products prevents ground moisture from passing through the floor slabs. In the absence of these barriers and retarders, moisture will reach the wooden floor, which can lead to mold and mildew growth, rot, and other structural damage.
What is the difference between vapor barriers and vapor retarders?
Regarding home construction, the terms vapor barrier and vapor retarder are often used interchangeably. Both control moisture diffusion, thus serving the same purpose more or less. However, there is a big difference between the two.
According to International Residential Code (IRC), a vapor retarder is any material with a perm rating of 1 or less. This material can be in the form of a sheet, a membrane, or a covering. The term “vapor barrier” is not mentioned by the IRC supplement. However, the construction industry generally uses the term “vapor barriers” to mention an impermeable material that falls in the Class-I vapor retarder category.
Vapor retarder classification based on permeability
Permeability measures how easily water vapor can move through a material. The higher the permeability, the more easily water vapor can pass through. A vapor barrier is a material with low permeability, while a vapor retarder has a higher permeability.
The US Department of Energy suggests installing vapor barriers under the concrete slabs is suitable for most American climates. When choosing a vapor retarder for your home, it’s important to know the difference between a vapor barrier and a vapor retarder. Failing to choose the right product to prevent moisture penetration can lead to drastic consequences for your home.
There are three classes of vapor retarders according to IRC:
Class-I vapor retarder (less than 0.1 perm)
A Class-I vapor retarder is a material that has a permeance of less than 0.1 perm. It means that it can completely resist the passage of water vapor through it. This class comprises materials considered vapor barriers because vapor retarders with a permeability of 0.1 or less are considered impermeable.
Some examples of vapor barriers, i.e., vapor retarders that do not allow water vapors to pass through them, are:
- Non-perforated aluminum foil
- Polyethylene sheet
- Rubber membrane
- Sheet metal
Class-II vapor retarder (between 0.1 perm and 1.0 perm)
Class-II vapor retarders are considered semi-impermeable vapor retarders. Crawl space and radiant barriers belong to Class-II vapor retarders. A typical example of this vapor retarder class is Kraft paper facing found on fiberglass batts. However, the Kraft paper acts as a Class-III vapor barrier on higher humidity levels.
Other commonly used Class-III vapor retarders are:
- Unfaced expanded polystyrene
- Fiber face polyisocyanurate
- 30-pound asphalt-coated paper
Class-III vapor retarder (greater than 1.0 perm and less than or equal to 10 perm)
Materials having a permeability of more than 1 perm but less than or equal to 10 perm are also considered semi-permeable but less effective in preventing moisture transfer compared to Class-II vapor retarders. Many types of paints come under Class-III vapor retarders. Glossy and acrylic paints are generally semi-permeable; however, flat paints and most latex paints are considered permeable. You can always check the perm rating on the paint box to be certain of the effectiveness of the paint as a vapor retarder.
Some common Class-III vapor retarders are:
- 15-pound asphalt-coated paper
- Cellulose insulation
- House wrap
- Unfaced fiberglass insulation
- Gypsum board
Choosing between a vapor barrier and vapor retarder according to climate
When it comes to your home, you want to ensure that it’s as comfortable and energy-efficient as possible. As far as insulation is concerned, you’ll find lots of conflicting opinions on vapor barriers and retarders, making it hard to know how best to use them in your insulation project.
However, according to experienced home builders, choosing between a vapor barrier and a vapor retarder depends on the climate zone, not just the material itself. Understandably, different climates require different moisture control considerations when deciding on the type of insulation to use. When installing vapor retarders in new constructions, follow these guidelines:
In mild climates, Class-III vapor retarders are enough to control moisture movement. For example, gypsum boards and plaster wall coatings can be used effectively in new constructions as their perm rating is suitable for controlling low moisture in mild climate zones.
Continuous and completely sealed vapor retarder installations are required for optimal results in extremely hot, cold, and humid climates. Any gaps or cracks can lead to condensation in the insulation, leading to mold and wood rot. An impermeable vapor retarder or, in other words, a vapor barrier is recommended to be used in harsh conditions. In addition, for cold climates, a vapor barrier should be installed on the interior side of the walls, and for hot and humid climates, it should be installed towards the exterior of the walls for best performance.
Installing vapor retarders in existing construction
Installing vapor retarders or barriers in an existing home is not easy unless it is an extreme makeover. In such cases, sealing leaks is the best way to minimize and control water vapors from entering the insulation and inflicting any damage to the building structure.
If you live in a mild climate zone, applying multiple layers of vapor retarder paint to your house exterior is your best option to lower moisture movement. Check the perm rating when choosing paint for vapor retarding purposes. If you can’t find a perm rating on the paint container, look for the solids percentage in the paint ingredients. Also, apply multiple thick layers of paint for better prevention of moisture penetration.
If you live in a humid climate, you’ll want to use a vapor barrier to prevent moisture from seeping into your home. On the other hand, a vapor retarder is best used in milder climates to prevent water vapors from damaging the building construction. It is also vital to use the vapor barrier and retarders on the interior or exterior side of the building according to cold or hot conditions for optimal prevention.
However, if you’re unsure which product is right for your home or how to install them, consult a professional before making your final decision. They can help you choose the best option based on your specific needs. In the meantime, educate yourself on the differences between vapor barriers and retarders so you can make an informed decision.