Vapour Barriers

Vapour Barriers, as their name would suggest, serve as a 'barrier' against moisture, inhibiting the passage of water vapour between the outside and inside of a building structure. When insulating a property internally, a vapour barrier is an absolute necessity; the only exception to this is if your insulation already boasts a vapour-resistant layer. Vapour barriers are usually installed in conjunction with insulation to ensure a successful building enclosure. 

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What Is A Vapour Barrier? 

As its title would suggest, a vapour barrier's purpose is to prevent moisture from penetrating a building structure. 

There are a host of vapour barriers available on the market, each with its own advantages and disadvantages. However, the distinction between each is often muddied by the widely applied 'VCL' term, which is often incorrectly used to refer to anything from vapour barriers to vapour control layers. 

Let's clear up the confusion: 

Vapour Barrier

The most common type of vapour barrier membrane is a sheet of plastic (polythene) or foil that is installed on the warm side of the structure, typically on the interior of the walls. A vapour barrier is effective at preventing moisture from entering the structure whilst trapping moisture that is already present, which can lead to condensation and mould growth. 

Vapour Control Layer

The product behind all the VCL confusion is of course the Vapour Control Layer. This breathable membrane allows a certain amount of air circulation while still preventing moisture from entering the structure. Permeable in its design, vapour control layers are often used in conjunction with insulation to reduce condensation and improve the overall efficiency of the insulation.

Breather Membrane

Breather membranes prevent water from penetrating the wall from outside whilst allowing water vapour to escape, thereby reducing wind wash. They are usually installed on the cold side of insulation and work as a weather-proof barrier. 

Why Is A Vapour Barrier Essential? 

A vapour barrier is a crucial element in any building construction as water vapour is an inevitability in any building. We produce it when we do our laundry, when we cook our tea and even when we breathe. 


However, its presence can degrade the efficiency of insulation material, causing interstitial condensation, mould and damage to the building fabric. 

Where To Install A Vapour Barrier

Typically, vapour barriers are placed on the wall side that's exposed to higher temperatures and moist conditions: the internal surface in colder climates and the exterior surface in higher temperature, humid climates.

Vapour barriers come in large rolls that can be cut to size using a Stanley knife.

Frequently Asked Questions About Vapour Barriers

What Side of The Insulation Does The Vapour Barrier Go On? 

Vapour barriers are typically installed on the warm side of your insulation. 

Placing a vapour barrier on the warm side of your insulation reduces the risk of condensation forming because it stops warm, moist air from coming into direct contact with a cold surface inside your wall.

Are Vapour Barriers & Vapour Control Layers The Same Thing?

Not quite. Vapour control layers are designed to 'control' and slow moisture penetration, as opposed to blocking it completely. A vapour control layer (otherwise known as a vapour retarder) is airtight in its design and because of this is typically employed in areas where airtightness is a concern. 

When utilised in a roof space, VCL is usually employed alongside a breather membrane to stimulate air flow. 

Vapour barriers, on the other hand, serve as a 'barrier' that fully obstructs the movement of vapour and air.

What Is Water Vapour Resistance Factor?

The water vapour resistance factor of a material is measured as its µ-value, or "mu-value". This factor has no units and essentially determines the resistivity of building materials to water vapour in relativity to still air.

How Does Moisture Pose A Problem Within Buildings? 

Timber frame structures in particular are largely affected by moisture build-up. Due to timber's hygroscopic nature, any moisture from humidity and condensation will be absorbed by the timber structure. This absorption of water massively increases the chance of rot, and in turn, a loss of structural integrity. 

What's more, damp and lack of vapour control foster the growth of mould and mildew. Mould can greatly affect the health of building occupants, causing a whole range of issues from respiratory complications to lung inflammation; a particularly grave risk for those with asthma and pre-existing health conditions. 

How Does Condensation Form? 

Condensation forms when water vapour comes into direct contact with a cold surface. The water vapour then condenses to a liquid and is deposited on either an internal or external surface. 

What Is The Difference Between A Breather Membrane & A Vapour Control Layer? 

Although the two terms are often used interchangeably, each product serves an entirely different purpose. 

A breather membrane is applied on the outer side of insulation to allow a small amount of water vapour to pass through the building fabric, banishing the need for ventilation systems. The minute holes in the material allow the vapour to travel through whilst acting as a barrier against condensation and water ingress.


Due to this ability, membranes are often used to prevent rainwater from infiltrating roof space. 

Alternatively, vapour control layers (VCL) are fitted to the internal side of insulation to reduce water vapour transfer.

We stock a comprehensive range of breather membranes here

How Is Vapour Barrier Thickness Measured? 

Vapour Barrier thickness is expressed mu or gauge.