VARA Intelligent Membranes

Vapour Control Layers are an important consideration when implementing insulation materials into the structural assembly of a building. When insulation levels are increased, the natural ventilation openings within the property are reduced and this can prevent internal moisture, known as interstitial condensation, from escaping with ease. The purpose of a Vapour Control Layer is to stop internal moisture from entering the insulation and becoming saturated so that timber rot and decay can be mitigated. Heated air carries moisture and as the heat attempts to reach colder surfaces through the path of least resistance, the Vapour Control Layer forms an barrier that diverts heat and moisture to other ventilated openings as opposed to the structural assembly. Even if moisture forms on the underside of the Vapour Control Layer, it will not degrade the structural materials and this is what helps to protect the building from degradation, decay and instability.

There are many types of Vapour Control Layer ranging from plastic sheets, foil, liquid-applied layers and intelligent membranes. Here, we look at the different types of membrane and conclude with our preferred solution.

Plastic & Foil Sheet Vapour Control Layers

Arguably the most common form of Vapour Control Layer comes in the form of a plastic or foil sheet that is stapled across the warm (internal) side of the insulation to stud walls, roof rafters or ceiling joists with airtight tape used as the seams (joins). Depending on the gauge (thickness) of the plastic or foil membrane, they often represent the most robust method of stopping air and moisture from penetrating into the insulation. Additionally, plastic or foil membranes can be the cheapest method of providing a Vapour Control Layer and they are generally easy to fit. Plastic and foil does not degrade or decompose quickly and it can reliably remain in place for decades. This can be in comparison to the exterior structure of a building, particularly at roof level, where low resistance breathable membranes are in place. Unlike plastic and foil, the lifespan of these membranes can be up to 25 years and this means that any degradation, tears or imperfections can lead to exterior moisture penetrating the insulation from above. This can represent a problem as moisture will sit within the insulation, leading to timber decay and due to the plastic or foil membrane covering the insulation and roof structure, it may not be visible until the damage is done. With that said, provided the external roof coverings remain in good condition and are replaced when they reach the end of their life, the risk of damage should be limited.

Intelligent Membranes

When considering a Vapour Control Layer, we recommend intelligent membranes as the solution and in particular the Vara range of membranes that are manufactured by Partel. As an approved supplier of Vara membranes, we conducted hygrothermal studies on their performance and discovered that the variable permeability can lead to better protection for the insulation and structural assembly. Intelligent membranes rely upon a variable permeability which promotes bi-directional drying. The winter time moisture accumulation that forms within the building is blocked by the membrane as it senses increased humidity levels. During summer months, as internal humidity falls but external humidity increases, any moisture that may have penetrated the insulation from external roof imperfections will pass back through the membrane and escape within the building. The bi-directional drying abilities of Intelligent Membranes helps to make them an increasingly used Vapour Control method in modern buildings and one that ThermoFoam supports as a robust method of managing moisture in buildings. There are many different brands of Intelligent Membranes and they can be applied in pre-manufactured sheet format or as a liquid, spray-applied version. The spray-applied version is particularly useful in buildings of irregular shape or areas which are hard to reach when using the sheet version.

Partel Vara Membranes

As an advocate of Intelligent Membranes, we’ve partnered with Partel, an Ireland-based manufacturer of specialist membranes for buildings. The VARA Intelligent Membrane range is now sold by ThermoFoam to end users, approved-spray foam installers and other building professionals. VARA PLUS is a smart humidity variable airtight vapour control layer with an optimum permeability range. Advanced Hygrovariable Technology allows incredible summer drying conditions, but can equally protect the construction in winter. In circumstances where a pre-manufactured sheet style membrane can’t be used, VARA FLUID can be brush or spray-applied to surfaces to create an airtight barrier against air leakage and moisture ingress. Some of the benefits of using VARA include:

  • Innovative liquid airtight membrane that changes vapour permeability according to the ambient relative humidity (RH). This contributes to maximum protection in the building envelope where moisture levels are critical, allowing vapour to escape and dry the structure.
  • Ensures optimum moisture flow through building structures with a Sd range of 0.15m to 16m
  • Hygrovariable Technology – Extremely safe airtight system
  • phA Class Passive House Certified airtight system, featuring an air permeance value of 0.00 m³/(m²h)
  • Flexible – ensures adequate coverage (cracks and joints up to 2 mm)
  • LOW VOC polymer-based liquid
  • Class W1 resistance to water penetration
  • Excellent adhesion on various surfaces
  • Fast application – speeds up construction, especially for commercial projects
  • Single component, ready-to-use product


Get in touch with us on 01208 832602 to find out more about how ThermoFoam and VARA can help your building to achieve strong thermal performance, airtightness and moisture management. Our technical team are on hand to provide product specifications, system designs, hygrothermal calculations and the robust delivery of insulation and vapour management solutions.