A vapor barrier installed in a wall before the insulation and drywall is added.

Having a vapor barrier helps protect the structure of your home from moisture and water damage.

However, these barriers aren’t always required, and unnecessary installation can contribute to damaging your home instead.

A vapor barrier is recommended for your walls when your home is located somewhere with a colder climate to prevent moisture problems from developing. However, installing this material isn’t necessary when there isn’t much moisture in the surrounding environment.

This article will help you understand the circumstances that may require you to install a vapor barrier on your home’s walls.

I’ll also discuss some frequently asked questions regarding vapor barrier installation.

And if you’re looking for a calculator to see what installing a vapor barrier might cost you, try out our Vapor Barrier Cost Calculator here.

How Do Vapor Barriers Work?

Before diving into when to install a vapor barrier, let’s look at how they work.

The main function of vapor barriers is to control the flow of moisture vapor. However, how do they achieve that?

The best way to understand how vapor barriers work is by first understanding the science of moisture diffusion in cementitious materials. Moisture moves from regions of high humidity to those with lower humidity in cementitious materials. This flow can occur in either direction.

In a building, moisture typically moves from the warm and humid interior to the cooler and drier exterior when it gets cold. The moisture moves in the opposite direction when it gets warm.

This is where a vapor barrier comes in handy. It creates a strong barrier that blocks the flow of moisture-laden air. By limiting the movement of water vapor, these barriers prevent excessive moisture from infiltrating the building envelope.  

When a Vapor Barrier Is Recommended

Certain conditions and criteria must be met for a vapor barrier to be required or recommended for your walls.

Aside from asking for advice from a professional contractor or consulting your local building regulations and codes, you can go through this list.

Very Hot, Very Cold, or Very Humid Climate

Extreme climates often mean that your home is more likely to employ temperature-controlled home heating for cold weather during the winter and air conditioning for hot weather or summer months.

When the temperature or environmental conditions vastly differ between the exterior and interior of your home, water vapor and moisture are more likely to form and move through wall cavities.

A map of the different climate zones of the US, shaded in blue, purple and red.
Stilfehler, CC BY-SA 4.0, via Wikimedia Commons

These can condense on cold surfaces and get trapped between walls and cavities, which can cause rot and mold to form.

The list below discusses the best location for your vapor barrier, depending on your area’s climate.

IECC Climate Zone 5-8 and Marine 4

The International Residential Code (IRC) recommends that homes built in climate zones 5 through 8, as well as Marine 4 zones, should install Class-I or -II vapor retarders on the interior (warm) side of the building.

The International Energy Conservation Code (IECC) divided the United States into different climate zones.

These zones are numbered 1-8 and are further classified into Marine (C), Dry (B), and Moist (A) zones. You can take a closer look at this map on Energy.gov.

Aside from the general Class-I or -II recommendation for these zones, here are a few more details:

  • Vented cladding or continuous insulation is recommended.
  • Walls with vented cladding without continuous insulation material: Class II vapor barrier is best—Class I is not recommended for homes with air conditioning operating for extended hours.
  • Walls without vented cladding or continuous insulation: Class II in zones 4C and 5

For a summary of these requirements, you can check out TechNote by the National Association of Home Builders.

Warm and Humid Climate

The IRC doesn’t require vapor retarders or vapor barriers when homes are in a warm-humid or mixed-humid climate.

However, if installing these materials, you should install them on the exterior.

This will prevent outside moisture from condensing on cold surfaces within the wall assembly, such as the outer face of the drywall of an air-conditioned room.

A window pane covered with condensation.
If your windows are dripping with condensation, this indicates high humidity in the home. Without a properly installed vapor barrier, the condensation on the window could be forming inside your wall assembly and cause moisture problems, including dampness, rot, and mold.

It is also recommended that you choose a vapor retarder with higher permeability so that moisture can flow through enough to not get trapped. This also helps to keep the structure and wall assembly dry.

Homes in humid climates that require more months of heating should install interior barriers.

Homes that experience more warm months should install an exterior vapor barrier.

Very Cold Climate

These include homes in locations that have icy winters despite experiencing hot summers. The barrier should be installed on the interior of the insulation.

A recommended arrangement would be to use a polyethylene plastic vapor barrier between your insulation and the interior wallboard. The exterior face of your wall should be permeable so that moisture is allowed to dissipate and does not get trapped.

a small wood cabin with snow all around and a blue sky in the background
Vapor barriers are important in cold climate regions to prevent moisture problems.

Absorptive Building Cladding

Unless well-sealed or adequately finished, the material of your home’s walls will allow at least a small amount of moisture to pass through.

Some building cladding materials are also absorptive and tend to retain and release moisture. These building materials include wood siding, brick, stucco, stone, and fiber cement siding.

When your home has this type of cladding, especially in a mixed climate, a vapor barrier should be installed on the exterior of the insulation.

Though vapor barriers are not required for homes in a dry climate, having absorptive cladding materials may make the need for a barrier a necessary one.

Wall Location

Where your wall is or what area surrounds it can also dictate if a vapor barrier is needed.

Exterior walls located at the perimeter of your home have an increased chance of needing a vapor barrier or retarder.

Exterior walls have the house’s interior on one side and the outdoors on the other. There is often a significant temperature difference between the air on either side of an exterior wall.

As water vapor passes through the wall assembly, it may encounter a cold surface and condense onto that surface, potentially leading to problems with moisture build-up. A vapor barrier will help prevent this from happening.

A red brick wall.
Exterior walls are where a vapor barrier is most needed. Internal walls of high-humidity rooms, such as shower rooms, would also benefit from a vapor barrier.

High moisture level rooms are recommended to have vapor barriers, even if they are interior walls. This is to prevent moisture from the room from entering other areas of the house. Bathrooms are the most common example of this type of room.

Walls below grade need vapor barriers to prevent moisture from the ground from seeping through the concrete. Walls below grade usually refer to areas like basements. A barrier will help prevent such spaces from getting damp and musty.

When installing a vapor barrier, it should be placed against the concrete surface before insulation, wood framing, or any finishing.

Signs That Your Home Needs a Vapor Barrier

To this point, it’s clear that a vapor barrier is crucial for a home’s safety because it controls the movement of moisture in and out of a building’s walls. Therefore, installing a vapor barrier can be essential to prevent moisture-related problems like mold growth, rot, and energy inefficiency. 

If you’re wondering whether to install a vapor barrier or not, here are the telltale signs that your home needs a vapor barrier:

  • Condensation issues: If you notice condensation forming on the interior surface of your walls, windows, or other cold surfaces, it’s a sign that moisture is infiltrating your home. This can lead to mold growth and damage to the walls over time.
  • Mold or Mildew Growth: The presence of mold or mildew on your walls or ceiling is a clear indication of excess moisture. A vapor barrier can help prevent moisture migration into wall cavities, reducing the conditions conducive to mold growth.
  • High Humidity Levels: If your home is in a region with high humidity, especially during hot and humid seasons, it may benefit from a vapor barrier to prevent excessive moisture from infiltrating the walls.
  • Rot or Decay: If you notice signs of rot or decay in your walls, such as soft or crumbling wood, it’s a strong indicator that moisture penetrates the building envelope. A vapor barrier can help protect your home’s structural integrity.
  • Stains or Peeling Paint: Water stains on walls or ceilings, as well as peeling or bubbling paint, are signs of moisture issues. A vapor barrier can help keep moisture out and protect the interior surfaces.
  • Musty Odors: A musty or damp odor in your home is often associated with excessive moisture. Installing a vapor barrier can help prevent this odor by reducing moisture infiltration.

When a Vapor Barrier Is NOT Recommended

Though a vapor barrier can be beneficial for preserving your home’s structure, there are certain situations where a barrier would not be recommended.

A red house in the desert with cacti in the garden.
In very dry areas, a vapor barrier is not recommended.

Dry Climates

In dry climates, there isn’t much moisture that can penetrate your walls. Vapor barriers aren’t necessary or required by the IRC.

However, if you still decide to install a barrier, or it is still recommended by a professional or your local code, your vapor barrier should be installed in the interior. This is where humidity is more likely to come from.

Low permeability is also recommended. This lets moisture from the inside flow out while preventing outside moisture from getting into the walls.

If your home already has exterior wall construction materials with vapor barrier properties, do not install an additional interior vapor barrier.

Avoid Impermeable Barriers in Air-Conditioned Areas

While you may think that entirely impermeable barriers could eliminate the possibility of moisture damage, using them around air-conditioned rooms has been linked to air quality problems and mold.

This is because an entirely impermeable barrier also risks trapping moisture, not allowing it to dry.

A man sitting at a desk with his back to the camera and hands behind his head, feeling a cool breeze from an air conditioning unit mounted on the wall above him.
Air conditioning can make a room feel more comfortable on a hot day by providing cool, dehumidified air to the room. Care must be taken if installing a vapor barrier to ensure it is installed on the warm side of the wall assembly to avoid condensation forming within the wall.

The Wall Already Has an Existing Vapor Barrier

More isn’t always better. If your wall already has a vapor barrier installed, you might think it is worth adding another layer of moisture protection with an additional vapor barrier on the opposite side.

However, this may not be the best idea.

You can do this in certain situations, but avoid a double barrier unless a professional deems it necessary.

A vapor barrier blocks moisture, but a double barrier may actually trap moisture in between. Therefore, the opposite side of your vapor barrier should be permeable enough to let moisture escape and allow your wall to dry out.

Best Practices for Installing Vapor Barriers

Your vapor barrier will only be effective if you install it the best way. Therefore, you must adhere to the best installation tips for results. Here are the best practices for installing a vapor barrier:

  • Assess moisture sources: The first step is identifying potential moisture sources for your home. Common areas to assess include basements, attics, crawl spaces, and areas prone to condensation and water leaks.
  • Inspect the existing conditions: Inspect the current conditions of the building envelope, including the walls, drainage, floors, and ceilings. The inspection should help identify signs of moisture damage. You want to repair any moisture issues like leaks before installing a vapor barrier. You should also fix damaged insulation and address any mold and mildew concerns.
  • Understand the building science: Understand the building’s specific needs and the climate zone it’s in. Different regions have different moisture control requirements. Consult local building codes and consider climate, humidity, and wall assembly materials.
  • Choose the right vapor barrier: Choose an appropriate vapor barrier based on moisture control strategy, climate, and compatibility with building materials. The most common types of vapor barriers include polyethylene, foil vapor, and spray foam.
  • Surface preparation: You must prepare the surface properly for optimal adhesion and performance. The preparation includes cleaning to remove debris, dirt, and any loose materials.
  • Install in an ideal location: Vapor barriers are typically placed on the warm side of the wall assembly, which is usually the interior side. This prevents warm, moist indoor air from reaching the cold exterior walls and condensing.

Other FAQs About Vapor Barriers

Vapor Barrier vs. Vapor Retardant

“Vapor barrier” is a term that is often used interchangeably with “vapor retardant.”

However, a true vapor barrier is entirely impermeable and does not allow any moisture through the material.

Vapor retardants, on the other hand, protect from moisture but do not create an impermeable barrier against the vapor. If a vapor barrier has even the smallest permeability, it is more aptly called a vapor retarder.

Both materials serve the purpose of moisture protection. Possible water damage, mold, and rot are also discouraged by helping to prevent the spread of moisture through your home’s structure.

What Is “Perm” in Vapor Barrier Classification?

“Perm” is a term for measuring permeability in vapor retardant materials. A higher perm value means water and moisture can diffuse through the material more easily.

  • Class I – Impermeable Vapor Retarder (Vapor Barrier): Less than 0.1 perms. This includes glass, polyethylene sheets, rubber membranes, and sheet metal.
  • Class II – Semi-Permeable Vapor Retarder: 0.1 to 1.0 perms. This includes bitumen-coated Kraft paper, plywood, thirty-pound asphalt-coated paper, and unfaced expanded or extruded polystyrene.
  • Class III – Permeable Vapor Retarder: Over 1.0 perms to 10 perms. This includes fifteen-pound asphalt-coated paper, board lumber, cellulose insulation, concrete, unfaced fiberglass insulation, and gypsum board.

A material with a perm rating over 10 isn’t considered a vapor retarder and is highly permeable.

Which Side Should You Install Your Vapor Barrier On?

It’s possible to install your vapor barrier on your wall’s interior or exterior side. However, certain factors will affect the best location for your barrier.

The best way to determine this is to install the barrier closer to the warm side of the wall’s structure.

In cold climates, install the barrier toward the interior side, where there will likely be heating.

In warmer or wet climates, install your barrier toward the exterior to prevent the diffusion of outside moisture and humidity.

A vapor barrier installed in a wall assembly during construction.
The best time to fit a vapor barrier is during construction of a wall, or when extensive renovations are being done.

When Should You Install a Vapor Barrier?

Vapor barriers are usually installed on the structure of a wall after insulation and before applying the finishing material.

Therefore, it is best to plan for installing a vapor barrier early on. This way, it is installed during the construction of your home.

Unless your home undergoes extensive remodeling, installing a vapor barrier in an existing home or structure is very difficult.

If you are contemplating the installation of a vapor barrier or retarder in your existing home, obtain an energy assessment from a professional.

That way, you get a professional opinion on whether you need the barrier or not to seal your home against moisture leaks.

You can read more on when you should install a vapor barrier when installing insulation here.

Does a Vapor Barrier Give an Absolute Seal Against Moisture?

Properly installing the correct vapor barrier or retardant in your home decreases the chances of moisture permeating your walls.

However, the seal is not absolute even when using a true vapor barrier.

Walls usually have openings like windows, doors, and electrical outlets. All seams and penetrations must be sealed well to prevent excessive moisture and air leakage through these openings.

You can use spray foam insulation to seal the gaps and spaces around any electrical outlets, lighting fixtures, or other cracks in your wall cavity.

This helps strengthen the seal against moisture penetration and increases energy efficiency in your home.

Loctite Tite Foam Gaps & Cracks is a bright white spray foam ideal for this. It adheres firmly to most building materials, such as:

  • Wood,
  • Concrete,
  • Brick,
  • Metal,
  • Stone,
  • and PVC.

It expands and sets quickly, so wear gloves when you work. It can even be painted over, making it a better choice for indoor applications.

A vapor barrier with a torn hole in it and a boy peering through the gap.
A damaged vapor barrier must be repaired properly otherwise it will let moisture through.

What To Do When Your Vapor Barrier Is Damaged?

When installing a vapor or air barrier in the interior or exterior side of your wall cavity, damage can sometimes be unavoidable.

Some caulk, polyethylene, or foil tape can repair small punctures, tears, and holes. Remember, when covering the damage, be sure to extend the application of your sealant so that it doesn’t only exactly cover the hole.

If the damage is more extensive with large tears or holes, the vapor barrier or retardant should be replaced entirely.

Proper repair or replacement is essential. Any break in the seal of your vapor barrier can provide an entry point for moisture, which can cause damage to your structure.

Final Thoughts

Vapor barriers and retardants are recommended mainly for cold climates.

Though they can be beneficial for warmer and humid climates, they are not required. Likewise, vapor barriers are mostly unnecessary for dry climates.

You must install these materials properly on the right side of your structure for them to function optimally.

A damaged or incorrectly installed vapor barrier can cause more damage than good.

Consult professionals and your local code to know what’s what if you plan to install a vapor barrier for your home.

For more information on the pros and cons of fitting a vapor barrier, please read our article on the topic here.

Should you forego the help of a professional, here’s our DIY-friendly guide to installing vapor barriers in crawl spaces!

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