Nick King, Area Account Manager at Klober, separates fact from fiction when it comes to breathable roofing membranes
Roofing membranes are strong weathering barriers installed underneath the roofing covering to reduce the effect of wind loading and provide a secondary barrier against wind-driven rain and snow. The most common types of roofing membranes fall into two categories, breathable and non-breathable membranes. While both provide the essential barrier needed to protect the property from unexpected water ingress, there are key distinctions between them.
Breathable Vs Non-breathable
British Standard BS 5250: ‘Code of practice for control of condensation in buildings’ stipulates rules for avoiding problems of high moisture levels and condensation in buildings. Depending on how airtight the building is, this will influence the movement of air inside and how much moisture vapour will reach the roof space. With modern buildings developed to be as energy efficient as possible, new homes tend to be more airtight and at a greater risk of condensation. As a result, it is crucial that adequate roof ventilation is installed to allow moisture to escape.
A non-breathable membrane essentially does not have the same air permeability as a breathable membrane. This means that breathable options have less vapour resistance so water vapour can exit freely, potentially without the need for separate ventilation above the roofing insulation.
The obvious advantage with a breathable type of membrane, therefore, is that the risk of condensation is lowered significantly when compared to its non-breathable counterpart. While breathable variants may mitigate the need for more traditional forms of secondary ventilation, it is likely that additional ventilation, such as eaves or ridge ventilation, will still be required.
Because the term ‘breathable’ implies that the membrane is self-sufficiently ventilating, we often find that builders consciously omit additional ventilation measures. This is quite simply a misconception. Only in cases where the membrane has been third-party approved as not needing additional ventilation is this the case. My advice is to be extra vigilant around claims of validity. The National House Building Council (NHBC), for instance, will only accept certain breathable membranes without additional ventilation if they have relevant British Board of Agrément (BBA) approval as being air open as well as vapour permeable.
The most breathable membrane
Membranes that are the most breathable are known as air open and vapour permeable. These types of extraordinarily breathable membranes have been developed through extensive testing and innovation over the years. All Klober membranes are put through a series of rigorous tests, utilising specialist equipment to prove performance claims. Great emphasis is also placed on durability, with additional accelerated ageing tests designed to ensure the product will continue to perform in the future.
While these types of membranes do come at a premium, they are growing in popularity. Being both air open and vapour permeable minimises the risk of condensation forming, particularly during the drying out period of a building. This provides extra reassurance for building and roofing contractors that problems with condensation during this vulnerable time can be avoided, in addition to simplifying the ventilation design and installation process.
Another key benefit of using an air open vapour permeable membrane is that they are ideal for complex roofing designs with lots of breaks in the roof line structure, such as skylights or compartmentalised roofs for apartment complexes. More design freedom is made possible in these scenarios, as by opting for a membrane like this means that the roofing design does not need to factor additional ventilation.
Another practical use for the most breathable membrane would be to support a roofing contractor tasked with ventilating the roofs of a ‘lean to’ extension at the rear of a property. Often it is a struggle to install high-level ventilation in these scenarios. By choosing an air-open vapour-permeable membrane, this problem is conveniently solved while remaining compliant and effective.
For buildings that require a certain roofing aesthetic, a traditional mortar ridge line, instead of the modern dry ridge system, could be a requirement. In these instances, where you cannot fix any ventilation through the ridge line, having a vapour-permeable membrane to take care of the ventilation is advantageous.
Condensation in buildings needs to be taken seriously and Klober’s mission is to ensure that building and roofing contractors have the best products available to ensure the most robust and high-performance roofing. This is why Permo Air, the most breathable membrane on the market, has been developed. This premium air-open vapour-permeable membrane is the ideal solution where simplified and compliant roofing protection and ventilation combined is required.
For further information about Klober Permo Air visit klober.co.uk/product/pitched-roofing/membranes/permo-air
