What is a vapor barrier? Do my walls really need one?

If you have any major home improvement activities planned that involve either stripping down existing walls to their studs or building new walls, how you’re going to insulate those walls is definitely something your contractor will talk to you about. At some point of time, you are bound to hear the words “vapor barrier� or “vapor retarder�. If you’re a novice to building practices and building materials, you’re bound to ask, “What is a vapor barrier? Do my walls really need one?� Well, this short, informative blog ought to help clear the air.

What is a vapor barrier?

Simply put, a vapor barrier or a vapor retarder is an impermeable barrier used to prevent water vapor from entering walls, ceilings, and floor cavities. Weather is the major cause for moisture build-up in walls, and vapor barriers are your defense against potential damage caused by this water vapor build-up, such as mold growing on building materials and rot.

This often raises the question of whether there are any differences between air barriers and water barriers. Considering air barriers diffuse exterior air from cavities in walls, floors, and ceilings, and that air is often laden with water vapor due to differences in air pressure, air barriers are also technically vapor barriers. However, the job of a vapor barrier is not to stop air from flowing through the cavities. That is the job of an air barrier. Therefore, air barriers need to be a lot more tightly sealed than vapor barriers. 

However, do all walls need a vapor barrier? Let us delve deeper into the subject.

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What types of climates need a vapor barrier?

If you live in an area that experiences mixed humid climates, which means warm and humid months as well as cold months that require a lot of heating, your walls definitely need vapor barriers. 

Homes located in cold climates also definitely need vapor barriers as a part of their home insulation.

If you live in mixed dry climates, you may not always need to invest in vapor retarders. However, it’s always safest to check local building codes while renovating.

Those living in hot and humid climates are usually advised to place vapor barriers at the exterior, outside of the insulation in their walls.

What kind of walls need vapor barriers?

If your walls are made out of absorptive wall assembly materials, such as brick, stone, stucco, fiber cement, or brick, then the potential of water vapor entering the inner wall cavity and damaging the structure is higher. These walls will definitely need the protection offered by vapor barriers.

However, vinyl siding walls do not face this issue.

Generally, all external walls in the colder, as well as more humid climate zones, are better insulated and experience higher energy efficiency with vapor barriers installed.

Ideally, walls, attics, crawl spaces, and ceilings are the parts of your home most likely to need vapor retarders.

What are the different types of vapor barriers?

Vapor barriers are made out of a variety of materials. Here’s a comprehensive list.

1. Aluminum foil
2. Metal and glass sheets
3. Aluminum with paperback
4. Special vapor retarder paints
5. Elastomeric coating
6. Plastic sheets
7. Roofing membranes, especially sheet types
8. Exterior grade plywood

How are barriers classified?

Vapor barriers are classified according to their permeability, and this unit is called a perm. 

If a barrier has a perm rating of 1.0, it means that when the vapor pressure difference between the warm side and the cold side of the material is equal to one milligram of mercury in one hour, then one grain of water vapor will pass through a square foot of the barrier.

Materials are classified into three different classes.

1. Class I barriers include materials like rubber membrane, polyethylene sheet, and sheet metal. These have a perm rating of 0.1 or less.
2. Class II barriers have a perm rating of more than 0.1 perms and a maximum of 1 perm. Materials in this class include expanded polystyrene, 30-pound asphalt paper and bitumen coated Kraft paper.
3. Class III retarders have perm ratings ranging from more than 1.0  to a maximum of 10. These materials include fiberglass batts, cellulose insulation, gypsum boards, concrete blocks, and board lumber. 

Which class of barriers do you need?

The continent of North America has been divided into eight different climate zones. Where you live will be the major deciding factor in what class of vapor barriers your home needs.

If you live in the Marine-4 zone, climate zones 5 or north of them, Class I or II vapor barriers are recommended. 

However, you will need to use a Class II barrier for the interior side of your walls if your home is air-conditioned during the summers, since that may trap some amount of condensation in your walls and ceiling. Alternately, you could pair a Class III retarder with spray foam insulation for the inside of the walls and roof.

If you live in climate zones 1 to 3, your barriers should not be on the interior of the walls. 

Pro tips

1. If your home has concrete walls, you ought to choose a Class I barrier.
2. Polyethylene sheeting is the most popular type of vapor barrier for crawl spaces.
3. Experts advise that most damage caused by condensation is caused by air leakage and not by vapor diffusion. Make sure you have effective air barriers in place to prevent this.
4. There are products available in the market that are a combination of both vapor barriers as well as air barriers. These products are ideal for moisture-laden humid southern climates.

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Written by Narayan Shrouthy. December 6, 2021

Narayan is a content writer for Kukun. With experience in both content writing and editing, Narayan mixes his writing and love for reading with his passion for music. As an active musician, He aims to be a key player in Kukun's content experience.

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xavier.chavez1@outlook.es(@xavier-chavez1outlook-es)

It is incredible all the kinds of insulation you can find nowadays. I am building my own house step by step, and this is going to help me a lot…many thanks for the info¡¡