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

A vapor retarder is a material that prevents water vapor from penetrating building assemblies such as walls and ceilings. Moisture can condense inside walls and cause structural problems. 

So, on which side of a wall should you install a vapor retarder?

If you want to learn the answer, make sure to keep reading.

Where on the Wall Should a Vapor Retarder Be Installed?

You should install vapor retarders on the wall’s exterior if you live in a hot climate and on its interior if you live in a cold region. The idea is to place the barrier on the wall’s more humid and warmer side.

Climatic conditions are not the same everywhere. So understanding how vapor barriers work can help you better determine where to place them. 

Explaining How Vapor Retarders Work

First, let’s clarify the concept of vapor before explaining how vapor barriers work.

States of Water

Vapor is gaseous water or water in the gaseous state. Science informs us that matter can exist in three forms – solid, liquid, and gas. We know that water is a liquid in its natural state. However, it can transform from a liquid state (water) to gas (vapor) at a specific temperature. 

So, how does water vapor constitute a problem in building assemblies?

Water Vapor Condenses in Dry Colder Regions

Water vapor diffuses from a warm damp region to a colder dry area. It moves through building assemblies, such as walls and ceilings, condensing where the temperature is below the dew point.

Condensation will cause damping within building assemblies, leading to many problems such as corrosion, deterioration of building fabric, and mold formation. Mold formation can induce allergic reactions in humans, among other potential health problems. 

Vapor Retarders Block Water Vapor

Vapor retarders, also known as vapor barriers, prevent water vapor from entering and condensing in building assemblies. Barrier materials have no air spaces and don’t absorb water, so they don’t allow the passage of water. 

A vapor barrier roll, staple gun, and some staples resting atop a piece of plywood

However, knowing how water vapor travels will help you understand where to install it. Since humidity travels or diffuses from a hot moist region to a cold, dry region, understanding the climate in your area will help you determine the pattern of water vapor travel and decide which side of the wall you should install the barriers on.

Vapor Barrier Installation Tips

Installing vapor barriers in your home isn’t enough. You have to install them correctly, or it will be a futile effort and a waste of money. Here are some critical installation tips to help you get the best results.

Know Your Climate

This consideration is essential to help you decide the direction of water vapor. If you live in a cold zone, it will usually be warmer inside than outside. Human and electrical heating factors will contribute to the warmth inside. 

In this case, water vapor will travel from the warm interior to the cold exterior. If this is the prevailing situation in your region, you must install a vapor retarder inside. This application will block the passage of water vapor into the walls, where it will condense and cause damping.

On the other hand, the exterior will likely be warm and damp if you live in a hot region. Although human heat will add some warmth, the prevailing situation will be a colder exterior. So, to effectively block water vapor, you must install it on the outside.

Photo of the exterior of a home under construction with a vapor retarder installed, waiting for exterior cladding.

Don’t Install Vapor Barriers on Both Sides of the Wall

Some might be tempted to install vapor retarders on both sides of the wall for foolproof damp protection. But, unfortunately, that might be counterproductive.

It’s just as critical to allow moisture to dry out as much as it is to prevent it from coming in. Since a vapor retarder blocks humidity from entering one direction, it should also allow its escape from the opposite direction to keep the inside of the walls dry at all times.

So installing vapor retarders on both sides of the wall will prevent moisture from escaping in one direction. The result is the damping you were trying to avoid in the first place.

Block Air Gaps, Cracks, and Holes Before Installing Vapor Retarders

Building assemblies are not completely permeable to moisture. They have gasps that allow the passage of air. Installing vapor barriers may not completely shield the house if there are cracks and holes in the walls.

To make your work more effective, check for air gaps, holes, and cracks in your wall and block them before installing a retarder. If you’re using a polyethylene barrier, use sealing tapes for the best results.

a crack in a concrete foundation

Don’t Use Completely Impermeable Barriers on Air Conditioned Spaces

Every vapor retarder has some level of permeability to water. While some are completely impenetrable (vapor barriers), others still allow moisture to a certain degree.

In air-conditioned spaces, installing completely impermeable vapor barriers may not be ideal. The reason is that if you completely block our moisture, there’s also no way for the air from your AC to escape.

It could lead to mold growth and also affect the quality of air. The remedy would be to air out your house once in a while to let some air escape and allow fresh air to come in. Because this may not be possible for everyone, it’s best to use a semi-permeable material as a vapor retarder.

How Long Do Vapor Retarders Last?

If properly installed, retarders should stay useful long enough, although they won’t last forever. At some point, they would break down and require replacement. 

Vapor barriers should last five to ten years on average. However, some remain effective beyond this time, even up to twenty years. 

The lifespan of a vapor retarder depends on several factors, such as:

  • Proper installation 
  • Climate 
  • The permeability of the material

Factors Affecting the Lifespan of Vapor Barriers

We expect the vapor barriers in our homes to last a long time.

However, this depends on the following factors:

  • Pest invasion
  • Human activity
  • Wrong vapor barrier code
  • Age

Pests Invasion

Rats, mice, and other rodents can find shelter in your crawl space and tear the vapor barriers while trying to make room for themselves. Their droplets can also cause the place to smell and pollute the air in your home. 

If you notice the presence of pests, call an expert to have a look and rid your home of all rodents and their excreta. Then, seek out and block all possible entrances replacing vapor barriers.

Human Activity

Humans can also damage a vapor retarder. They may either damage it while installing it or fixing or installing other equipment at home.

Photo of the interior of a home under construction with exposed insulation partly covered by a vapor retarder

Whatever the cause, you’ll need to change the vapor retarder in the affected parts. But you’d have to change the entire thing if the barrier is damaged beyond patching.

Wrong Vapor Barrier Code

Some countries specify the type of vapor barrier used for every building type. If you’ve used the wrong one, the retarder may not last.

The barrier will eventually break down and manifest the signs explained earlier. Before you install a vapor barrier in your house, acquaint yourself with the correct building codes for vapor barriers.

Old Age

Age is a natural cause of deterioration. Vapor retarders will eventually become weak over time. However, as mentioned, the deterioration speed will depend on several factors, like proper installation, the permeability of the material, and the prevailing climatic conditions.

Types of Vapor Retarders

The three main types of vapor retarders include:

Type 1 Vapor Barriers

Type 1 retarders are impermeable and don’t allow water vapor penetration. They are rated 0.1 perms or less (the lower the permeability, the more resistant it is to vapor).

Type 1 vapor barrier materials include:

  • Polyethylene sheet (visqueen)
  • Rubber membrane
  • Sheet metal
  • Unperforated aluminum foil (FSK)

Type 2 Vapor Retarders

Type 2 vapor retarders are rated 0.1-1.0 perm. They’re semi-impermeable: water vapor may penetrate only a small amount.

Materials with this rating include:

  • Bitumen-coated Kraft paper
  • Unfaced extruded or expanded polyethylene
  • Thirty-pound asphalt-coated paper

Type 3 Vapor Retarders

Type 3 retarders are rated 1.0-10 perms. They’re semi-permeable, allowing partial penetration of water. 

Materials with a rating above 10 perms can no longer be classified as vapor retarders because such materials are porous and cannot function as barriers.

Semi-permeable materials include:

  • Concrete block
  • House wrap
  • Cellulose insulation
  • Gypsum board
  • Fifteen-pound asphalt-coated paper
  • Board lumber
  • Fiberglass unfaced insulation
  • Latex paint

Smart Vapor Retarders

If you live in a region with a mixed climate, the direction of water vapor diffusion will change as the weather changes. It can be confusing to decide which type of retarder to install.

Smart vapor retarders take care of this problem. They can adjust their permeability as the humidity level changes in your home.

Signs That It’s Time To Replace Vapor Retarders

The following signs indicate that a barrier in your wall has broken down and requires a replacement.

Water Infiltration

Vapor retarders should prevent or slow water vapor penetration into your walls. When water vapor begins to penetrate, the barriers installed are no longer effective and would need replacement.

Water infiltration may manifest in the form of wet walls, peeling or fading paints, and wood rot if the wood is attached to the wall. Water infiltration will make your walls a potential breeding ground for mold and mildew.

Mold Growth

Dampness in your walls will eventually lead to the growth of mold and mildew. Sometimes, they manifest on the outer wall, while others form from within. Mold growth can cause allergic reactions and breathing problems, among many other health-related issues.

A picture of a corner of a room showing black mold growing on white walls and ceiling.

How To Replace Vapor Barriers

Replacing your retarders isn’t a complex task. You can do it yourself if you have the zeal, which will be fun and save you money.

Getting professional help will cost you extra bucks, but you’ll have the guarantee of a properly executed job. 

Whichever option you choose, the most important thing is to install the retarders properly. 

Do-It-Yourself (DIY) Tips

If you want to do the replacement yourself, the tools you’ll need include:

  • Hand gloves
  • Nose mask or any good respirator
  • Coverall
  • Blade or knife or scissors
  • Tape
  • Hammer
  • Light
  • Knee braces
  • Vapor barrier
  • Measuring tape

It’s always a good idea to keep these items close by to save the time and stress of looking for them when needed.

With your tools handy, follow the steps below to replace the vapor barrier in your house.

  • Identify the areas that need replacement unless you’re changing the entire stretch.
  • Cut out affected parts, or remove the old vapor barriers.
  • Measure the areas that need sealing. Then, measure and cut it out of the length of the roll you have.
A worker measuring a window frame.
  • Place the vapor retarder on the affected areas and seal with tape. If you’re changing everything when measuring the vapor barrier, leave extra inches so it can overlap when sealing.

Choosing a Suitable Vapor Retarder

Vapor retarders are essential but not mandatory for every building. In addition, the retarder type used differs from zone to zone due to varying climates.

These zones have their building codes and vapor barrier specifications. Vapor barriers are compulsory for colder zones but not mandatory for warmer zones.

  • Hot or humid zones: They don’t require vapor barriers in the house’s interior wall. However, you can use Type 1 or 2 vapor retarders, but you don’t need Type 1.
  • Cold and north zones (zones 5,6,7): For cold areas, you can use Type 1 or Type 2. However, if you’ll be using an air-conditioner for a better part of the year, you should use Type 2 or Type 3 if the walls are well insulated.
  • Mixed climate (hot and humid): Smart vapor retarders are the best type for mixed environments. It can both absorb and retard water when it should.
  • Marine zone: The same specifications for hot or humid zones are suitable for marine zones.


Water vapor can penetrate building walls and cause damping, leading to mold formation and structural problems. Vapor retarders prevent moisture from entering the building assemblies by blocking or retarding the movement of water vapor. 

However, you must correctly install these barriers to get a damp-proof building. The proper place to install them is on the exterior wall in a hot, humid climate and the inside wall in a cold, dry region.


Leave a Reply

Your email address will not be published. Required fields are marked *