Photo by Hugh Stevenson.
When we insulate our homes, it's really important to have an effective strategy for managing interstitial condensation – the build-up of moisture within a building's construction. Without this, problems such as dampness, mould, and structural decay can occur. It can also reduce the insulation's thermal performance.
This is a consideration commonly overlooked in many home improvement projects, especially those without an architect, often leading to expensive building defects.
This article explores the importance of a vapour control layer (VCL), helping you to be in a much better position to create a warm, dry building.
A vapour control layer (VCL) is typically a thin sheet material used to regulate condensation passing through a building.
VCLs usually come in the form of plastic membranes, but other materials such as oriented strand board (OSB) can also be used to control vapour.
The term ‘VCL’ is often used ubiquitously to describe all vapour control membranes. However, it's important to know that there are three main types:
A VCL is installed on the inside side of the building and regulates water vapour movement passing through a building's structure.
A breather membrane, installed on the outside, allows water vapour to escape while preventing rainwater from penetrating the building.
Using both a breather membrane and VCL, especially in rainscreen cladding applications, ensures comprehensive moisture management, safeguarding the building from both interstitial condensation and external water ingress.
Sketch detail drawing through the eaves of an unventilated pitched roof: 3- Breather membrane provides weather protection but allows internal vapour to escape; 6- A fixed or variable diffusion VCL allows low amounts of vapour to pass through to the outside during winter or to the inside during summer, keeping the structure dry.
A VCL's performance is indicated by its Sd-value.
The Sd-value measures the resistance to water vapour movement, equivalent to the resistance offered by a metre of air – in other words, its permeability. The higher the Sd-value, the greater the resistance to moisture, and vice versa.
VCLs with an Sd-value of 100 metres or more are considered vapour barriers, allowing very little vapour to pass through. Many fixed diffusion VCLs have an Sd-value between 5 and 20 metres, offering much greater permeability. Variable VCLs have a minimum and maximum Sd-value, representing its range of permeability as it adapts to changes in humidity levels. These properties can vary from as little as 0.25 metres to as much as 60 metres.
Not to be confused with surface condensation which forms in buildings on walls and windows, interstitial condensation is the build-up of moisture within a building's construction layers.
When insulation is added to an external element, such as a floor, wall, or roof, a temperature gradient forms across the construction. In winter, the warmest side is inside and the coolest is outside, with the reverse occurring in summer. During the colder months, humidity levels – the amount of water vapour contained in the air – are higher inside the building than outside, and vice versa during the warmer times of the year. Vapour travels from hot to cold. As warm, moist air moves through the structure, it cools down, leading to condensation at the 'dew point', the place where water vapour becomes liquid. This moisture build-up can cause significant damage over time.
Installing a VCL helps regulate the amount of vapour entering the construction, thereby reducing the risk of interstitial condensation.
Interstitial condensation is particularly problematic in timber frame structures, where absorbed moisture can lead to rot, structural defects, damp, and mould.
In the UK, until relatively recently, a polythene sheet vapour barrier was frequently installed behind the internal finish, as it was thought to completely block internal moisture-laden air from entering the construction. However, we now know that unless the sheeting is perfectly installed and free of any defects, moisture will get through during the winter months. Additionally, these barriers prevent moisture from escaping to the inside of the building during the summer months, which can cause moisture buildup.
For most timber structures we now recommend a fixed diffusion or variable VCL which allow vapour to pass through in both directions. However, in some applications, most notably warm flat roofs that utilise rigid plastic foam insulation, vapour barriers are still used.
Damage to a flat roof caused by interstitial condensation. Photo by RICS.
Fixed diffusion VCLs, such as Ampatex Sinco (Sd-value of 5 metres), are good all-rounders and are suited to many timber floor, wall, and pitched roof applications. For situations requiring higher vapour resistance, such as hybrid warm flat roofs, a low diffusion membrane like the Ampatex DB90 (Sd-value of 20 metres) is a good option.
Variable membranes, like Ampatex Variano (Sd-value of 0.6-60 metres) and ProClima Intello Plus (Sd-value of 0.25-25 metres), maintain high vapour resistance during winter while adjusting to allow moisture movement in warmer conditions. This ensures consistent building dryness throughout the year. These can be used in a wide variety of applications, but are more expensive than fixed diffusion VCLs and may constitute over-specification in certain situations.
Vapour barriers are recommended when insulating new concrete floors.
There are many variables which can affect the choice of VCL, including orientation, location, site conditions, and construction buildup. Variable VCLs are a good option to cover many eventualities. However, they are extremely expensive compared to other types of VCL. Seek expert technical advice from an architect or specialist supplier to ensure you achieve the best solution for your project.
For the VCL to perform effectively, it must be paired with the appropriate type of insulation.
In timber structures where high permeability is needed, using a vapour-open and hygroscopic insulation, such as wood fibre, along with a fixed or variable diffusion VCL, can enhance vapour control. This combination absorbs moisture and buffers changes in humidity levels.
For concrete floors and warm flat roofs, plastic foam insulation, such as PIR or XPS, combined with a vapour barrier, is a practical choice.
Foil-faced rigid foam insulation, such as XPS or PIR, can be taped together to form an effective vapour barrier. However, the impermeable nature of this insulation and the vapour barrier make it unsuitable for most types of timber frame construction. It blocks inward evaporation during the summer months, leading to rot and other damp-related defects. Instead, this combination is more suited to modern cavity wall construction, where there is less reliance on the VCL. Masonry is better than timber at absorbing and releasing moisture without defects.
Pitched roof construction showing a variable diffusion VCL with wood fibre insulation between rafters. Image by Pro Clima.
In the UK, the VCL should always be positioned as close to the inner face of the building as possible. This minimises the amount of internally generated moisture, which poses the greatest risk, from entering the construction.
To protect the membrane from defects caused by the installation of building services, interior fixtures, and fittings, a void should be created in front by fixing the interior finishes to counter battens.
In concrete floor construction, the VCL is located between the insulation and screed, also serving as a separation layer.
Maintaining good levels of airtightness is crucial for controlling interstitial condensation. Unwanted gaps within a building's external envelope can create a pathway for moisture laden air to enter the structure. This is why many VCLs also double up as airtightness membranes, referred to as air and vapour control layers (AVCLs)
AVCLs should be taped together and sealed around the edges and any penetrations. Additionally, creating voids for building services will help protect the membrane from damage, helping to maintain its performance.
By installing a proper airtightness layer, you're not only keeping your home dry but also improving its energy efficiency and thermal comfort.
Oriented strand board (OSB) can be used as a cost-effective vapour control layer, particularly in timber frame structures. OSB provides a certain level of vapour resistance and is more robust than thin membrane VCLs, also offering structural benefits. Products like Smartply Airtight OSB are specifically designed for airtightness and vapour control. While OSB can function as a VCL, dedicated VCL membranes may offer more reliable performance, especially where specific vapour control properties are required.
There are instances when a dedicated VCL is not required.
In older buildings with traditional solid wall construction, effective vapour control and airtightness can be achieved by installing hygroscopic and vapour open insulation, such as rigid hemp or wood fibre boards, directly to the masonry and finishing with lime plaster. The insulation buffers vapour as it passes through the construction, while the lime plaster provides an airtight layer.
Breathable internal wall insulation finished with lime plaster can provide effective vapour control and airtightness. Low Energy House by Architecture for London. Photo by Lorenzo Zandri.
1. Create service voids
Battening out internal finishes helps protect the VCL and provides more effective vapour control. It creates a void to run services, reducing the risk of puncturing the VCL, and protects the VCL from internal fixtures and fittings installed after the external envelope construction is complete. For example, fitting a kitchen often requires heavy-duty fixings, which can compromise the VCL.
2. Carefully choose electrical fittings
Selecting fixtures and fittings that minimise the risk of penetrating the VCL will protect it from damage. For example, choose recessed ceiling lights with slim profile back boxes or surface-mounted light fittings to avoid compromising the VCL.
3. Choose a fixed diffusion membrane to save costs
Variable vapour control membranes are excellent but expensive. Specifying a fixed diffusion membrane wherever possible will save on build costs.
4. Use compatible tapes and sealants
Adhesive tapes are needed to join membranes together and seal penetrations, such as where ductwork passes through the building’s external envelope. Taped joints and seals can make up a considerable percentage of the overall vapour control layer, so it's imperative that these are compatible with the membrane for optimum performance and to avoid future defects.
5. Utilise expert knowledge
Insulating a building is complex, with many variables including construction buildup, orientation, exposure to elements, and the type of insulation used. Appointing a skilled residential architect and consulting with specialist suppliers like Back to Earth and Mike Wye will help you avoid costly mistakes by ensuring that you specify the correct construction buildup for your specific project.
6. Minimise wet trades during construction
Wet trades, such as installing concrete, can significantly increase moisture in buildings. Concrete releases moisture as it cures, contributing to overall humidity within the structure and potentially leading to condensation issues if not properly managed. Opting for dry trades wherever possible will reduce the amount of moisture entering the structure, thereby minimising the risk of condensation.
7. Ensure good building ventilation
Ensuring the building has good ventilation will keep internal vapour levels manageable, drastically reducing the risk of condensation-related problems. For deep retrofits or new builds, consider installing a Mechanical Ventilation with Heat Recovery (MVHR) system to help manage internal moisture levels. They also improve energy efficiency and help maintain a healthy indoor environment.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, appoint a skilled residential architect.
About the author
Aron Coates, an architect with over two decades of experience, draws upon a wealth of residential architectural expertise, encompassing projects ranging from the restoration of historic structures to the design of contemporary homes.
