The construction industry is increasingly coming to the view that breather membranes in high-rise and higher-risk buildings should have a minimum Euroclass A fire safety rating in line with the law for cladding, but there is less clarity over the question of whether to use an A1-rated system or an A2-rated system. Matt Thompson, Sales Director at ITP Ltd, explains the differences between the two options.
The Euroclass European classification EN 13501-1:2018 has become the standard reference in relation to fire safety regulations in building. The current minimum Euroclass rating required by law for breather membranes in high-rise buildings is Euroclass B-s3, d0. However, safety specialists and advocates of best practice advise a minimum Euroclass A to match the standards required for cladding and other external wall elements. When an A-rated option is available, it makes sense to maximise fire safety throughout the building envelope.
That approach seems straightforward at first glance, but complexity arises when you consider the detail behind systems manufactured to meet A1 and systems manufactured to meet A2. Due to differences in their design and composition, A1 membranes are considered the better option for some buildings while A2 membranes are more suitable for others.
A1 denotes a non-combustible material and is the highest rating a product can achieve, defined as a material that does not contribute to fire at any stage, including a fully developed fire. A2 denotes limited-combustible material in England and Wales (somewhat confusingly, it denotes non-combustible material in Scotland). The s1 denotes little or no smoke propagated and the d0 denotes no flaming droplets or particles. These categories are essential references for any building specification, but those responsible for choosing building materials must also consider the classifications in the context of how they might affect other areas of the material’s performance.
Breather membranes have two primary functions. Firstly, they allow the building to ‘breathe’ with the release of excess moisture vapour into the atmosphere, thereby preventing the build-up of condensation in the insulation layer. In many buildings, they perform a second primary function with resistance to the inward passage of water from rain and damp air. A breather membrane’s ability to perform this function is measured by its water tightness, classified in ratings of W1, W2 and W3. W1 is the highest rating, providing the best protection in the most demanding conditions.
Membranes used in safety critical installations comprise a base layer, which gives the resistance to fire, and a coating layer which gives the water protection. Euroclass A1 membranes have been developed for the highest level of fire protection. However, the outer coating layer needs to be as thin as possible to achieve the A1 rating. This results in a W2 rating for water tightness. A Euroclass A2-s1, d0 membrane has a heavier coating, enabling a W1 rating. These differences mean that A1 is considered the optimum solution for closed facades but A2 is considered the optimum solution for buildings in locations with high exposure to wind-driven rain and buildings with open-jointed façade cladding or leaky cladding.
As well as understanding the difference between A1 and A2 membranes, it’s also vital to be aware of the testing and certification requirements for Euroclass ratings. The quality of testing and data within the construction industry is highly variable. Some suppliers and manufacturers will go to the nth degree to demonstrate that their products meet and exceed performance requirements. Others will cut corners, obfuscate and omit.
Façade design is an area of construction that illustrates the problems caused by this variation. A product’s classification can be misleading when separated from the context of its durability. A Declaration of Performance (DOP) should show the tests results before and after ageing as specified in the harmonised standard BS EN 13859-2, but some manufacturers supply a DOP without the results after ageing. It’s impossible to make an informed decision about membrane specification without that data, particularly in relation to open-jointed cladding and permanently open facades which demand higher levels of UV resistance as well as W1 water tightness to prevent degradation in other areas of performance such as fire safety.
Architects and contractors should also be wary of crucial omissions within lab testing methodology. For example, some membrane manufacturers supply fire test data for their textile, but not for the integrated adhesive materials they supply to fix it to the façade or to make overlaps. It is always worth asking manufacturers about the fire safety credentials of any accessories that they supply as standard with their products.
The arrival of the Building Safety Regulator ushered in a new era of more robust regulation and much greater scrutiny of fire safety data. Knowledge of best practice in façade fire safety and related product testing will be a vital resource for all dutyholders involved in the design and construction of high-rise projects.