Vapour Barriers Explained

Last updated: November 15, 2023

What is a vapour barrier? Which materials are best for improving the energy efficiency of your home?

Vapour barriers (also known as vapour retarders) describes the layer used in damp proofing to stop or limit the amount of moisture that can travel through the walls, flooring, or ceiling.

This is done to prevent condensation in cavities which can lead to rotting or mould.

Vapour barriers can also improve the energy efficiency of your home.

However, there is often a lot of confusion as to what the best material is to use for vapour barriers, whether you need it or not, as well as the different terms used to describe vapour barriers.

So in this guide, we’ll explore all of these frequently asked questions.

How does a vapour barrier work?

A vapour barrier controls the movement of moisture through a structure.

In colder climates moisture will diffuse and travel from the warm interior of your home through the structure until it hits a surface colder than the dew point where it will then condense and ultimately become a damp problem.

The opposite can happen in summer and in warm climates when moistures travels from the outside in.

We are used to seeing condensation on windows but the same will happen on walls, floorings and ceilings.

Note that is almost impossible to fully stop vapour diffusion, thus the once common term vapour barrier is a misnomer. You may now see the term vapour retarders used instead.

Where does water vapour in our homes come from?

Water vapour is invisible and is caused by our day to day activities, like drying clothes, showering, cooking and the simple act of breathing all cause moisture in our homes.

You can of course reduce moisture inside by changing your habits, such as ventilating your kitchen and bathroom and not drying washing on radiators. However, it is impossible to completely rid your home of moisture in the air.

In the past we have always been taught to tackle condensation and moisture in our home through ventilation.

The problem with this however means, in cold climates we are letting the warm air out of our homes when our homes now more than ever need to be more energy efficient.

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Do I need a vapour barrier?

In the UK and other colder climates where winter temperatures can get colder than 4 degrees you should always install a vapour barrier on the inside of interior insulation.

If you live in a warmer climate the vapour barrier should be on the outside side of the installation.

It is however a good idea to consult your local building code as if installed on the wrong side in a certain climates a vapour barrier could end up trapping moisture leading to damp and rot within the walls and roofing.

Note: Some insulation will come with a vapour resistant layer in which case the gaps should be sealed with special vapour barrier tape.

What is the best material for a vapour barrier?

There are many materials that can be used as a vapour barriers.

Which you choose will come down to which is best for your climate.

Vapour retarders are classified by how permeable they are, and colder climates will want the least permeable material.

The International Residential Code (IRC) classes vapour retarders into class l, class ll or class lll based on their permeability.

The lower the permeability the less water vapour can travel through. Class l is 0.1 perms or less.

Class I vapor retarders (0.1 perms or less):

  • Glass
  • Sheet metal
  • Polyethylene sheet
  • Rubber membrane

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

  • Unfaced expanded or extruded polystyrene
  • 30 pound asphalt coated paper
  • Plywood
  • Bitumen coated kraft paper

Class III vapor 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 
  • Brick
  • 15-pound asphalt coated paper
  • House wrap

Materials such as rigid foam insulation, reinforced plastics, aluminium and stainless steel can also be used as they are relatively water resistant but should also be sealed at the joints.

Source: U.S. Department of Energy.

Things to consider when installing a vapour control layer or vapour barrier/vapour retarder:

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Your climate

In milder climates materials such as gypsum wall board, and plaster walls coatings could be enough to act as a vapour retarder.

However in colder climates class l or class ll vapour retarders are advised in all new constructions such as glass, sheet metal and polyethylene sheets.

Make sure it is continuous

Any gaps, tears or any kind of damage to the vapour barrier means moisture is able to get through and potentially damage the installation or the structure of the building.

Making sure it is continuous and all gaps are sealed is essential to prevent problems down the line.

If the building is air conditioned

If air conditioning is used in warmer climates, it is best not to use fully impermeable materials.

Using impermeable materials when combined with air conditioning has been linked to mould build up inside the building.

Do not use on two sides

You want the structure to be able to fully dry out.

This is why it is not advised to put a vapour barrier on two sides of the cavity or installation.

Different terms used for a vapour barrier

A Vapour Barrier– This term is now less commonly used because it implies it blocks all moisture from travelling through a structure.

However, most materials will not completely block all moisture and is why a vapour retarder or a vapour control layer better describes the overall term.

A Vapour Retarder– This term is now the more common term used instead of Vapour Barrier.

A Vapour Control Layer– Describes a number of materials that control the amount of moisture allowed to diffuse through.

Vapour Checks– Another term used instead of a vapour barrier. But means the same thing.

Can I add a vapour barrier to an existing home?

Unless you are adding an extension or re-modelling walls it is hard to add a vapour barrier to an existing home.

Though your home may not need it, as numerous layers of paint effectively work as a vapour barriers.

However, you can buy vapour resistant paint and some paints will even display the perm rating on the label.

If not look for the formula to see the percent of pigment in the paint.

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Glossy or acrylic paints work well but it is best to look for “vapour diffusion retarder” paint and follow the instruction on the tin.

Does a vapour barrier improve the efficiency of a home?

A bonus to vapour barriers is that they can make your home more energy efficient, lowering your household bills.

Having excess moisture in the home can affect the temperature, making it colder in winter and more humid in summer.

When the moisture is diffused out through the walls of the structure it helps to regulate the temperature.

This not only stops the installation from getting moisture damage it will reduce your household bills.

Air barriers vs vapour barriers

The difference between an air barrier and a vapour barrier is often one of confusion.

A vapour barrier is there to diffuse moisture through a structure. Whereas an air barrier is there to prevent air leakage.

The confusion can come because a vapour barrier can act as an air barrier but an air barrier should not stop vapour from defusing.

Air flow can carry moisture into the cavities but only moisture diffuses though a vapour barrier.

Air barriers can be placed on the interior or exterior side of the structure to prevent air flow. This need to be completely sealed.

There are some materials that will act as both an air barrier and a vapour retarder such as polyethylene plastic sheets, builders foil, foam board insulation and other exterior sheathings. Materials that include any of these will also work.

Having both an air barrier and vapour retarders is important in more southern warmer climates to prevent humid air from the outside entering the cooler wall cavities.

The key to making them work effectively is sealing all areas, including around windows, air vents and electrics.

Failure to do this will not only make them less energy efficient but could cause damage to the structure particularly during the cooling season.

Also remembering that any damaged sections should be filled and repaired, or if too large completely replaced.