What exactly is a VCL?

Vapour Control Layer (VCL) Explained

One of the most commonly used, and widely recognised, acronyms in construction is VCL, which stands for vapour control layer. A VCL is a critical building component designed to protect the building from potential degradation (or poor performance) by managing the passage of water vapour within a building structure. In other words, it is used to manage condensation risk. Condensation is formed when warm moist air condenses into a liquid on contact with a colder surface. A vapour control layer is typically installed on the internal side of the insulation to control the passage of warm moist air (water vapour) entering the structure. However, as a simple acronym, there is a problem because in most applications a specific level or type of performance is required, as a ‘catch all’ acronym VCL is wide open to error.

Condensation on the internal side of a window, showing that there was plenty of water vapour in the room, which has condensed onto the cold internal window pane. This is a good example of how much water vapour can be available to penetrate and condense within a building structure.

Unfortunately, and perhaps part of a wider misunderstanding, to many the term VCL is a synonym for polythene sheet, add in only a basic understanding of how condensation forms (as described above) and it is easy to see how the use of the term VCL can cause considerable confusion and anxiety. This is made worse by a large number of alternative terms such as vapour check, vapour permeable membrane, vapour barrier, vapour retarder, ACL, AVCL, vapour diffusion retarder, variable diffusion membrane, monolithic membrane, vapour diffusion barrier, airtight membrane, vapour tight membrane, microporous membrane, breather membrane all used to describe products covered by the acronym VCL …………………frankly it’s no wonder people get confused by all of this. In this article, we will try to bring some clarity to the description and use of internal membranes to help you decide which type to use, where and how. A companion piece is in preparation about external membranes.

Broadly put there are 3 types of internal membrane.

Type 1: An impermeable barrier such as polythene, this lets nothing through.  It’s a vapour barrier or a vapour block, has only one function: to stop water, in all forms. An impermeable barrier cannot let water vapour back out of a wall when generated by solar gain - see point below - so this type of membrane has serious limitations in all but a few circumstances. Needless to say, the installation of an impermeable barrier needs to be 100% perfect for it to work.  So, no holes, no gaps at the joints or overlaps, no accidental cuts or nicks and no major scrapes.

Type 2: A membrane that acts as an impermeable membrane most of the time but has some capacity to allow vapour transfer in certain circumstances.  These are often described as a “vapour retarder” or a “vapour check” and they are designed to work under specific conditions where the inherent properties of the membrane can be relied upon.  Since they only allow the movement of water vapour under these narrow and specific conditions, it is essential to apply them in appropriate situations.

Type 3: A membrane with variable permeability is often called an intelligent membrane. These are vapour control layers whose ability to allow moisture vapour to pass through depends on circumstances.  Also known as variable diffusion membranes (VDM), most allow vapour movement in both directions, depending upon relative humidity either side of the membrane.

Some membranes in categories 2 and 3 can be described as breathable, you can read more about breathability here.

The choice of a membrane can largely be determined by location and build type so a vapour barrier (type 1) can be used under a concrete slab but an intelligent membrane (type 3.) is better to line the inside of a warm roof space.

The terminology used to describe membranes in buildings is hugely confusing and often ends up being concentrated into the single three letter acronym ‘VCL’ appearing on a drawing, but we know that VCL is a catch-all acronym it means nothing without some context or explanation, for example, we often notice people using the terms VCL and ‘breather membrane’ interchangeably, particularly with regards to pitched roofs. Whilst they have a similar purpose, there are a couple of important differences between the two.

So why do we need either? Quite simply – the vapour control layer is there to prevent condensation, which can cause a number of problems, including:

  • Structural damage due to rotting timber, whether this be a timber frame, joists or rafters
  • Insulation losing its thermal performance due to having absorbed the moisture
  • Mould, which not only looks unsightly but can also lead to respiratory problems and other health issues

People generate moisture inside their homes, through breathing, through cooking and particularly by washing themselves and their clothes.  To prevent condensation, we need to eliminate this water vapour from inside the building. We also need to get rid of moisture that is outside the habitable zone but within the building envelope.  This might be water from construction – fresh concrete, for example, takes many months to dry out fully - or perhaps rainwater that seeps through tiled roofs or is wind-driven up under the eaves.

Traditionally we have eliminated moisture by ventilation; for example, by ventilating the space between the insulation and the slate or tile on a pitched roof. However, studies have shown that ventilation directly above an insulation layer can reduce its thermal efficiency, which means more and more people are opting for an unventilated roof.

Some Definitions may help

  • Airtight layer- prevents the movement of air which may/ may not act as a Vapour Control layer
  • Vapour Control Layer- a material which can limit both movements of vapour by diffusion, and air movement
  • Breather Membrane- defined as a membrane with a vapour resistance less than 0.6 MNs/g situated on the external side of the insulation acts as a weatherproof layer whilst still allowing water vapour to be passed to the outside.

Understanding your walls, temperatures and condensation

With a plethora of membranes on the market, each designed to do a different job and behave in a subtly different way it is easy to be confused about which membranes are required to create a dry and airtight building structure.

Starting with the basics, when insulating walls you create a temperature gradient across them with the warmest being on the inside and coolest on the outside during the winter months. You can imagine a graph of the temperature showing a fairly steady decrease in temperature as you move closer to the outer surface of the wall.

When you take warm, moist air and cool it (as it will moving through a building structure) you find moisture condenses at a point known as the ‘dew point’ or ‘condensation point’.  This will typically be the intersection of an impermeable or low permeability surface with the temperature falling low enough for water vapour to become liquid.  This is how damp accumulates inside the fabric of your walls or your insulation, to the severe long-term detriment of your building.

Why you shouldn't use a ‘vapour barrier’ (Type 1. Membrane) in your walls

Different construction and insulation materials cope differently with condensation. Some materials, such as masonry, can absorb and release it again once the weather warms without too much damage. However, when using vapour impervious insulation in timber frame construction, any condensation forming in the walls tends to be absorbed by the timber, a process that can cause rot. Additionally, during the winter months when this condensation tends to occur, driving rain may also enter the fabric of the building, further increasing moisture levels in walls. It is therefore very important to prevent this condensation process occurring in the first place, for the longevity of the building.

One further complication to the above process can be found in the summer months. The temperature gradient is often reversed and the higher temperature is found on the outside of the wall and the lower temperature on the inside. This creates a situation where moisture is driven inwards and condensation can form close to the inner face of the wall instead.

In the UK until relatively recently an impervious vapour barrier was used on the inner face of a timber frame and was thought to prevent condensation formation by simply blocking the flow of moisture-laden air through the wall. However, it has since been found that not only are vapour barriers regularly full of holes which let moisture through during the winter months, they also cause the accumulation of moisture inside the wall during the summer months. This was caused by the barrier preventing moisture from escaping towards the interior of the building.

Water vapour that has condensed against a VCL, in this instance a polythene sheet (type 1) used as a VCL over mineral wool and behind plasterboard. Damage can be seen on the timber stud. This occurred within 2 years of installation. Photo courtesy of SkamoWall.

The high humidity levels and warm temperatures found in these walls combined to form perfect conditions for mould and rot to thrive. This was problematic to both the timber structure, as it rotted, but also to the inhabitants of the building as mould spores are well known to cause respiratory problems and ill health.

Vapour barriers are still useful though. One of the few places above ground level where a complete vapour barrier should be used is in flat roofing when using foil faced PIR insulation. In this case, you need to lay a vapour barrier on top of your flat roof deck before you lay the insulation and your flat roof covering.

Using a vapour control layer to control condensation

The answer to keeping timber-framed walls and roofs dry is to use a layer to restrict the flow of moisture but not to try and stop it. In other words a VCL (or a vapour retarder). A VCL is always used as close to the inner face of a wall as possible and reduces the amount of moisture passing through the layer to low levels, ensuring only insignificant amounts of condensation occurring within the structure. Additionally, this will allow moisture that is driven towards the interior in the summer months to slowly pass back inside the building. This prevents the conditions for mould forming and ensures the longevity of the structure.

Vapour control can be performed very accurately by the many membranes available but it can also be performed at a basic level by OSB, whose vapour resistance (or vapour permeability) is similar to that of some membranes. The benefit of using OSB as a VCL is that it is far more robust than a 0.2mm membrane and does not require the installation of another layer into your timber frame structure if used internally. However, you will need to test the airtightness of the OSB before using it as there is some variation in air permeability. For guaranteed results either use an airtight VCL membrane, such as the ProClima Intello Plus or Constivap or a board such as Unilin Vapour Block or a liquid applied membrane such as Blowerproof. Blowerproof and Intello Plus are both BBA certified.

It is also advisable to try and minimise the amount of moisture that enters your building fabric during construction.  Much of our construction timber, sometimes including expensive windows, roof joist assemblies and even SIPs panels, are commonly stored on site with little or no protection against rain, especially driven rain.   Once wet, they can take a significant amount of time to dry out, contributing to the internal moisture load a new dwelling has to deal with.  This can even delay and degrade the final stages of construction: for example, airtightness tapes on OSB have been known to come off during airtightness testing, not adhering properly because the timber is still too wet.

The latest type of VCL membrane is the 'intelligent' membrane, such as Proclima's Intello Plus membrane. These are very useful products that remain very vapour tight (low vapour permeability) during the winter months when it’s important to try and prevent moisture from entering your structure from the interior. As temperature and humidity in the walls rises the pores in the membrane open and allow moisture to migrate towards the interior of the building. This gives the best of both worlds and ensures your structures remain as dry as it is possible to be.

Watch the video to see how an intelligent VCL works

Using a vapour-check or foil backed plasterboard as a VCL

Vapour control layers are always required whenever you insulate, irrespective of the insulation used. They should be used to form a continuous airtight layer and so all the joints and any penetrations must always be sealed with the appropriate airtightness tapes. Without good levels of airtightness the VCL does not work and moisture levels cannot be controlled inside the structures. Products such as vapour-check, foil backed or insulated plasterboard tend to act as a vapour barrier but with none of the joints or penetrations sealed. These products should not be used instead of a VCL or where a VCL is used.


Always refer to a qualified designer, if in doubt or ask the manufacturer for technical and installation advice, we are always happy to answer any questions about airtightness or vapour control or point you in the right direction.



Thanks to Paul Jennings and Chris Brookman for their contributions to this article.

Paul Jennings has over 30 years’ experience of airtightness testing, in the UK and around the world, and has been extensively involved in the delivery of onerous airtightness specifications in Passivhaus and other low-energy projects. He tested the first UK certified domestic and non-domestic Passivhaus buildings, both in Machynlleth, on the same day more than ten years ago, and recently led the team that used 8 sets of test equipment to carry out the most complex airtightness test carried out in the UK to date, on Agar Grove Phase 1, in London. He trains airtightness testers and pioneered the development and delivery of airtightness champions training courses. He has been instrumental in improving our processes and tools for achieving good airtightness, as well as training sealing operatives and delivering numerous CPDs and conference presentations to a wide range of building professionals on different aspects of airtightness.

Chris Brookman lives in a Passive house of his own design which he built based on his own life principles of low impact, low energy living and human health. Chris runs Back to Earth, is a recognised expert on green building and passive construction, he has written widely on the subject and is a keen blogger on the practical and technical aspects of delivering sustainable construction; Chris also curated the first online Wood Fibre Insulation course

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