What are Breathable Roofing Membranes?

Breathable roofing membranes
Nick King writes about breathable roofing membranes
Nick King, Area Account Manager Klober

With so much choice available when it comes to roofing membranes,  it’s common to be confused about the different types. The term ‘breathable’ is quite often misunderstood. Nick King, Area Account Manager at Klober, separates fact from fiction when it comes to breathable roofing underlay, along with discussing their added benefits and applications.

Roofing membranes, or underlay, are barriers installed underneath the roofing covering to reduce the effect of wind loading. They 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. They both provide a barrier to protect the property from unexpected water leaks, but they are different.

Breathable Vs Non-breathable

British Standard BS 5250: ‘Code of practice for control of condensation in buildings’ gives rules for avoiding problems of high moisture levels and condensation in buildings.

How airtight the building is 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 more risk of condensation. This means it is crucial that adequate roof ventilation is installed to allow moisture to escape.

Breathable roofing membranes

A non-breathable membrane does not have the same air permeability as a breathable membrane. Breathable options have less vapour resistance so water vapour can escape more freely, potentially without the need for separate ventilation above the roofing insulation.

The obvious advantage with a breathable type of membrane is that the risk of condensation is lowered when compared to its non-breathable counterpart. While breathable underlays might not need more traditional forms of secondary ventilation, it is likely that more ventilation such as eaves or ridge ventilation will still be used.

Installing breathable roofing membranes

Because the term ‘breathable’ implies that the membrane is itself ventilating, we often find that builds do not have additional ventilation measures. This is not recommended. Only in cases where the membrane has been third-party approved as not needing additional ventilation should this be the case. Most roofs will benefit from ridge and eaves ventilation. This will increase the flow of air through the roofspace making it almost impossible to form condensation in the roof.

My advice is to be extra careful 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.

Air Open and Vapour Permeable Membranes

Membranes that are the most breathable are known as air open and vapour permeable. These types of very breathable roofing membranes have been developed through extensive testing and innovation over the years.

For example, all Klober membranes are put through a series of rigorous tests using specialist equipment to prove performance claims. Klober also emphasises durability, with ageing tests designed to make sure the product will continue to perform in the future.

Minimising the Risk of Condensation

While these types of membranes cost more, they are growing in popularity. Being both air open and vapour permeable minimises the risk of condensation forming. This is particularly a risk 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. It also helps in simplifying the ventilation design and installation process.

Roof Penetrations

Another key benefit of using an air open vapour permeable membrane is that they are ideal for complex roofing designs. For example, where there are lots of breaks in the roof line structure. Roof penetrations are caused by rooflights or compartmentalised roofs for apartment blocks. More design freedom is made possible in these cases using air open vapour permeable membranes. It means that the roofing design does not need to factor in additional ventilation.

Ventilating Home Extensions

Another practical use for the most breathable roofing membrane is ventilating the roofs of a ‘lean to’ house extension. Often it is a struggle to install high-level ventilation in these home extensions. This problem is easily solved by choosing an air open vapour permeable membrane, while remaining compliant and effective.

Some buildings, requiring a certain roof style, need a traditional mortar ridge line instead of the modern dry ridge system. Where you cannot fix any ventilation through the ridge line, having a vapour permeable membrane to take care of the ventilation is an advantage.

Building and roofing contractors should be taking condensation in buildings seriously and avoiding it. They need to have the best products available to ensure the most robust and high-performance roofing. This is why Klober has developed Permo Air, the most breathable membrane on the market. This premium air open vapour permeable membrane is the ideal solution. It’s a great choice where both simplified and compliant roofing protection and ventilation is required.

Choosing the right membrane

The whole roofing industry can benefit from manufactures continuing to innovate the choice of roofing components, ventilation and accessories. But don’t get confused by the numerous breathable roofing membranes on the market. As condensation remains a real roof risk, professional roofing contractors will consider their options carefully. They will opt for breathable roofing membranes backed up by technical performance. So be sure to choose membranes with the testing and certification which prove their credentials.

A varied choice of products for different roofs means that no matter the roof there will always be a solution.

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