What is a vapour barrier?

A vapour barrier (sometimes referred to as vapour retarder) is typically a plastic or foil sheet used for damp proofing to prevent interstitial condensation from forming in various building assemblies such as walls, roofs, foundations and floors. In a typical commercial building or home, vapour barriers or vapour diffusion retarders can improve energy efficiency and comfort, while also preventing problems from moisture and dampness. (Source: U.S. Department of Energy.)

An introduction to vapour barriers and vapour retarders

The purpose of vapour barriers

A vapour barrier is an important component in building construction. Its purpose is to help prevent water vapour from reaching building walls, ceilings, attics, crawlspaces or roofs, where it can condense and cause building materials to rot or grow mould.

Damage from water condensation due to water vapour movement (called “water vapour drive”) can wreak havoc on even the sturdiest of built structures, and threaten the effectiveness of insulation. You can save yourself this costly headache by learning when, how, why and where to install a vapour barrier in your next project.

An introduction to vapour barriers and vapour retarders

What is water vapour?

Water vapour is water in its gaseous state (instead of as a liquid or solid) and is totally invisible. Water vapour constantly diffuses through building materials from the warm, humid interior of a house toward the cold, dry exterior. As water vapour passes through a wall, ceiling or other barrier and meets a surface that has a temperature below the dew point (when water vapour condenses), then it becomes condensation – and a threat to the integrity of your building materials. (Sources: Ecohome.)

An introduction to vapour barriers and vapour retarders

According to sustainability expert and architect Daniel Overbey, water vapour transmission is an important but rather confusing issue. The difference in vapour pressure between two sides of a building envelope assembly is the driving force behind vapour transmission.

As the Canadian Mortgage and Housing Corporation (CMHC) points out, many everyday human activities such as laundry, cooking and bathing release water vapour into a building and increase its humidity. This air then naturally seeks to find a way out of walls, ceilings, etc., through diffusion. The same is true for commercial buildings, even though the activities taking place inside may be different.

Building in a cold climate? Take note.

Some may ask, is a vapour barrier necessary? As a builder, your first step is to consult your local and provincial/state building codes. In many colder North American climates, vapour barriers are a required part of building construction.

You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good. This circumstance may prevent water vapour from drying, which in turn can cause rot and mold. (Source: Dupont.)

If you are unclear about the building requirements, you may need to consult with other contractors in your region, or calculate your building’s needs according to criteria laid out by established professional organizations. For example, the National Roofing Contractors Association (NRCA) recommends vapour barriers on the interior side of a roof in any climate where the outside average January temperature is below 40 F (4 C) degrees and the expected interior winter relative humidity is 45 percent or greater.

What does a vapour barrier do?

Vapour barriers are installed along, in or around walls, ceilings and floors to prevent moisture from spreading and potentially causing water damage.

A true vapour barrier is one that completely prevents moisture from passing through its material, as measured by the “moisture vapour transmission rate.” If even a small amount of permeability is in the material but the barrier still provides protection from moisture, this is called a vapour diffusion retarder. (Source: U.S. Department of Energy.)

Vapour retarders also are commonly referred to simply as vapour barriers. The barrier terminology is less accurate because, in most cases, the products don’t completely barricade the vapour.

An introduction to vapour barriers and vapour retarders

What can I use as a vapour barrier?

There are a wide number of materials available to create effective vapour barriers, including:

  • Elastomeric coatings.
  • Aluminum foil.
  • Paper-backed aluminum.
  • Polyethylene plastic sheet.
  • Asphalt-coated Kraft paper.
  • Metallized film.
  • Vapour retarder paints.
  • Extruded polystyrene or foil-faced foam board insulation.
  • Exterior grade plywood.
  • Sheet-type roofing membranes.
  • Glass and metal sheets.

(Source: U.S. Department of Energy.)

The International Residential Code (IRC) classifies materials by their permeability. They measure this in a unit called a “perm”. As clarified in a study published by the University of Alaska Fairbanks (UAF) Cooperative Extension Service: If a material has a perm rating of 1.0, we know that in 1 hour, when the vapour pressure difference between the cold side and the warm side of the material is equal to 1 inch of mercury (1 inch Hg), 1 grain of water vapour will pass through 1 square foot of the material. One grain of water is equal to 1/7,000 of a pound.

Vapour retarder materials are categorized into one of three types:

Class I vapour retarders (0.1 perms or less):

  • Sheet metal.
  • Polyethylene sheet.
  • Rubber membrane.

Class II vapour retarders (greater than 0.1 perms and less than or equal to 1.0 perms):

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

Class III vapour retarders (greater than 1.0 perms and less than or equal to 10 perms):

  • Gypsum board.
  • Fiberglass insulation (unfaced).
  • Cellulose insulation.
  • Board lumber.
  • Concrete block.
  • Fifteen-pound asphalt coated paper.
  • House wrap.

(Source: U.S. Department of Energy.)

Where do I need a vapour barrier?

The IRC divides North America into eight climate areas for the purposes of determining when a vapour barrier might be needed in a building.

An introduction to vapour barriers and vapour retarders

The IRC recommends builders install a Class-I or -II vapour retarder on the interior side of homes in climate zones 5 (Cold) and north, and in the Marine 4 zone. However, if you air-condition your house in the summer, your might trap condensation in your roof or walls for part of the year. If this is the case, be sure to use a Class-II vapour retarder on the interior of the wall. You also can use a Class-III vapour retarder on the interior paired with spray foam insulation on the interior of the wall or roof. When building in hot, humid climates (zones 1 to 3), you shouldn’t have a vapour retarder on the interior side of the wall. (Source: Fine Home Building.)

Experts say that most condensation problems occur because of air leakage, not vapour diffusion, so be sure you properly seal penetrations (such as flashings) for air leakage using an air barrier.

An introduction to vapour barriers and vapour retarders

Air barrier vs vapour barrier – How they differ

Some compare a vapour barrier to a raincoat, whereas an air barrier is more like a windbreaker. In a lot of cases you may not need a vapour barrier, but instead use an air barrier to prevent water vapour from migrating through air currents. This is the number one way for water vapour to travel into homes and assemblies (such as walls or roofs). In fact, air flowing through holes and cracks is 30 times more likely to transport water vapour through building assemblies than through simple diffusion of the water vapour. (Source: CMHC, “Canadian Wood-Frame House Construction”, p.18.)

An introduction to vapour barriers and vapour retarders

On the other hand, a vapour barrier helps prevent the second most common form of water vapour movement: vapour diffusion. This is “the slow movement of individual water vapour molecules from regions of higher to lower water vapour concentration (higher to lower vapour pressure).” (Source: Dupont.) Condensation is caused when warm air cools as it moves through building materials like insulation and drywall. (Source: Ecohome.)

A vapour barrier isn’t intended to stop air from flowing or migrating; that’s the job of an air barrier. So, although a vapour barrier must be continuous, unlike an air barrier, a vapour barrier does not need to be as tightly sealed. (Source: CMHC, “Canadian Wood-Frame House Construction”, p.18.)

Some products, such as IKO Industries’ AquaBarrier, act as both a vapour and air barrier. These are often used in humid, southern climates where moist exterior air is frequently found. (Source: U.S. Department of Energy.) Combined vapour/air barriers are also appropriate in any place where both the air barrier and vapour barrier are both located on the warm side of the building assembly. (Source: CMHC, “Canadian Wood-Frame House Construction”, p.38.) 

An introduction to vapour barriers and vapour retarders

Vapour barriers for commercial roofs

Vapour retarders frequently are used in flat roof construction to prevent moist air from the inside of the building condensing onto the roof assembly and potentially causing damage to materials. (Source: NRCA.) These products are an important way to preserve the thermal efficiency of roof insulation, and so make up a crucial part of protecting the comfort and energy efficiency of a home or commercial building. In most cases, when installing a vapour retarder on a roof deck, it should have a perm rating of 0.5 or less.

An introduction to vapour barriers and vapour retarders

To work effectively, the vapour barrier also must be warm enough to stay above the dew point on the exterior side, which means sufficient insulation must be installed over the barrier to maintain the temperature no matter the weather outside. (Source: NRCA.)

In the event you are erecting a “cold building” (such as a refrigeration building) that stays at 32 F (0 C) degrees or less inside, you will need a vapour barrier on the outside of the insultation to prevent warm exterior air from entering and potentially damaging the roof’s insulation. (Source: NRCA.)

An introduction to vapour barriers and vapour retarders

Vapour barriers are especially important as a part of flat roof construction on commercial buildings. Water vapour that penetrates roof materials can cause considerable damage, including:

  • Corrosion of steel materials.
  • Growth of microorganisms.
  • Reduced effectiveness of insulation.

(Source: NRCA.)

A flat roof vapour barrier such as IKO’s MVP Modified Vapour Protector, offers appropriate moisture protection.

Flat roof vapour barrier materials

When constructing a flat roof, there are two types of materials commonly used: Bituminous vapour retarders (asphalt mixed with felt or fiberglass) or non-bituminous vapour retarders (plastic, laminate or aluminum coated).

Is a vapour barrier necessary?

Once you’ve identified the climate where you’re building and the building’s anticipated use, you can determine whether the entire building envelope (including the roof) needs the protection of a vapour barrier.

An introduction to vapour barriers and vapour retarders

Any builder should consider this decision carefully prior to the construction phase, as a properly selected vapour barrier will help ensure the building is compliant with local building codes and support energy efficiency and maximum service life of all materials.

Vapour barrier vs vapour retarder: What is the difference?

Vapour barrier does not allow any water vapour through it, a vapour retarder allows a prescribed amount of water vapour through it. Using a vapour barrier would be more important in a high humidity scenario; swimming pools, changing rooms etc.

What vapour retarder class is a vapour barrier?

A vapor barrier is considered a Class I impermeable vapor retarder, which has a perm rating of less than 0.1. Here are the classes of Vapour Retarders.

Defining Term Vapour Retardance Class
(Lstiburek, 2011)
US Perm Material Examples (perm)
(Building science Corporation, 2015)
Vapour impermeable
(Vapour barrier)
Class I Vapour Retarder < 0.1Perm Polyethylene “poly” sheet (0.03)
Vapour semi-impermeable Class II Vapour Retarder 0.1 – 1.0 Perm VR Primer, Oil-based paints, vinyl coverings
Vapour semi-permeable Class III Vapour Retarder 1.0 – 10 Perm Asphalt Felt -#15, OSB, plywood, latex-based paints
Vapour permeable > 10 Perm Typar® (9-15), Tyvek® (55)

Where can I find IKO contractors to help me with vapour barrier and vapour retarder?

Use the “Find a Contractor” search to find a contractor that can install IKO products at the bottom of our homepage.

Visit our site’s vapour retarder section to learn about our commercial vapour retarder products.