Vapour Barriers

It is important to understand what you want to do, how vapour barriers or vapour membranes work and why to use them. Installing a vapor barrier depends on the home’s location and misunderstanding the location, how they work and why to use them can lead to structural failures and mould issues.

Homefix supply a range of vapour barriers for large sites and DIY projects. Our range includes vapour membranes, vapour barrier tapes, vapour barrier paintsvapour barrier details and moisture meters.

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What Do We Really Want to Do? 

There are to two different requirements that cause endless headaches even for most engineers and architects:

  • keep water out
  • let water out if it gets in

Water can enter a structure or building as a liquid, solid, vapor and/or adsorbed from the ground. If its rain or ground water drain everything and remember to use vapour control layers, tapes, DPMs or flashing.

Solid water such as ice and snow are relatively easy to remove with a shovel.

But the water vapor is in a class all by itself. Controlling water may seem as simple as keep it out and to let it out if it gets in. However, this is not the case because sometimes the best strategies to keep water vapor out also trap water vapor in. This can be a real problem if the structure starts out wet because of rain or the use of wet materials.

It gets even more complicated when you start to consider the weather, climate and seasons.

In general water vapor moves from the warm side of building to the cold side of building. However due to weather, climate and seasons it can be difficult to decide side which is which.

To add to the mix of confusion you will need to know how much water you need to control.

Finally, confusion can arise when materials can store water. This can be both good and bad. A cladding system such as a brick or metal veneer can act as a reservoir during and after a rain fall. A wooden frame, brick or masonry can absorb the water during a rain fall and then let the water dry out naturally.

Part of the problem is that we struggle with names and terms. We have vapor barriers, we have vapor membranes, we have damp proof membranes, etc. What do these terms mean? It depends on whom you ask and whether they are selling something or arguing with a building inspector. In an attempt to clear up some of the confusion the following definitions are proposed:

Vapour barrier is designed and installed in a structure to hold back the movement of water.

How Vapour Barriers or Vapour Membranes Work

A vapour barrier is a sheet material that is specifically made to prevent moisture or condensation from entering an area where it is not wanted. It can be installed in floors, wall, attics, cellars, ceilings, roofs or DIY projects to prevent moisture cause building materials to be damaged by rot or grow mould.

If a vapour membrane is not installed moisture can get into the flooring, walls, roof or structures potentially allowing damp or rot to occur and making structures unsafe and unhygienic.

A vapour membrane can be as simple as a heavy gauge plastic sheet such as our Radbar Radon gas barrier, barrier paint, or an aluminium foil bonded barrier to your insulation sheets.

No matter what type or style of vapour membrane you choose you will need to understand why you need it, how it works and how to install it correctly. Please see our FAQs for more information on this.

Why do you need a vapour barrier?

These barriers are there to stop water damage, from spreading into the floors, walls, ceilings and roofs. This could be to stop water vapour from entering your house or garden structure from the outside environment such as up from the ground or getting into the inside of walls. A moisture barrier can also prevent water from escaping from inside your home such as wet rooms.

Always refer to current local code recommendations when deciding if and how to install vapor barriers. In our experience authorities recommend vapor barriers in certain situations:

  • In areas with high humidity—such as greenhouses, rooms with spas or swimming pools, and bathrooms—vapor barriers are often recommended. Consult building inspection offices for local recommendations.
  • In very cold climates, the use of polyethylene plastic vapor barriers between insulation and interior wallboard may be beneficial, provided all air gaps into any wall and ceiling cavities are also blocked. The exterior face of the wall or floor cavity should remain permeable in order to allow dissipation of any moisture that does enter the wall cavity.
  • Very hot and humid climates may benefit from an exterior vapor barrier that keeps outside humidity from penetrating into walls.
  • Below-grade walls and floor slabs transmit ground moisture through concrete walls or slabs. A vapor barrier against the concrete surface is generally recommended before wood framing or flooring materials are installed.
  • Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth.

Where does the water vapour come from?

Water vapour comes from a lot of different sources such as from in your home or the outside conditions but the most common are the ground, cooking, washing and drying clothes and bathing.

From inside your home water vapour is released from cooking, bathing, drying clothes, etc creates steam that is released into your home.

From outside your home or structure from weather conditions or from being below the water table.

Why use vapour barriers?

A vapour barrier is important to use to stop the vapour from getting into the walls, floors, roofs, and structures where it could cause water damage

Water from outside your home or structure comes from the natural environment such as rain soaking the ground it then is soaked up by a dry structure that is in or on the ground. Also, as the water dries naturally become vapour and collects on cold surfaces where it condenses into water droplets.

All the water vapour from inside the home is from cooking, bathing, etc and drifts around until it finds a cold surface such as a wall or window frame etc. where it condenses into water droplets.

These water droplets will enter the wall and structures and if left will create rot and mould. 

Rot will lead to the structure becoming unsafe and may lead to the structure falling. E.g., Wood will swell and if left will decay. Bricks are designed to soak up water, however, without a barrier this water will enter further into the structure.

Mould is a fungus that can pose a health problem, especially for people with an allergy, an existing respiratory problem, or a weakened immune.

How To Install A Vapour Barrier?

Vapour barriers are usually best installed on the side of the wall that experiences the hotter temperature and moister conditions: the inner surface in colder climates and the outer surface in hot, humid climates. In existing spaces, vapour-barrier paints offer an effective moisture barrier. Consult building inspection offices for local recommendations.

where to install vapour barriers

At Homefix we provide damp proof barriers, vapour paints, wet room kits, weep trays, cavity trays and accessories to help you get the job done. You could get a pump kit that kicks in when the water from the ground is deep enough to be pumped out.

Which Vapour Barrier to Buy

There are a lot of vapour barriers on the market however not all meet NHBC standard (National House Building Council) or BBA (British Board of Agrément) certified. In our experience it best to know that it meets these before you buy.

The vapour barrier thinness required will depend on the location, architects and the build specification. 


Visqueen Vapour Check

To hold back low level risk of condensation within the structure of factories warehouses, industrial units & storage areas, also works to improve the general air tightness of a building.

Visqueen Vapour Barrier

To hold back medium level risk of condensation within the structure of factories warehouses, industrial units & storage areas, also works to improve the general air tightness of a building.

VCL -Vapour Control Layer

Vapour Control Layer (VCL) is mostly used in timber frame constructions, but can also be used in a wide variety of other applications such as flooring and ceiling projects.



Damp Proof Membrane Vapour Barrier

BBA approved Vapour barrier (waterproof membrane) is easy to lay, durable vapour control layer that prevents moisture seeping into concrete or timber frame. Ideal for buildings, small extensions, shed, outbuildings and garden office.


Visqueen High Performance Vapour Barrier

Visqueen High-Performance Vapour Barrier is used to omit the risk of interstitial condensation within a structure as well as improving the general airtightness of the building.

High Performance DPM

Visqueen High-Performance DPM premium-grade Damp Proof Membrane is a robust co-polymer thermoplastic membrane