Breather membranes are used in roof and wall installations to ensure long-term protection to the building envelope. Breather membranes offer water repellent properties combined with high water vapour permeability to control condensation within the building’s interior. Specialist breather membranes with additional characteristics, such as high UV resistance, ventilation control and energy efficiency, are also beneficial for the building’s long-term protection.

Breather membranes perform various key functions. Firstly, they allow the easy release of excess moisture vapour into the atmosphere. This functions keeps the internal wall/roofing structures dry. Secondly, breather membranes protect against the ingress of dirt, dust and rain which are especially problematic when coupled with driving rain. Some systems also provide protection against the impact of solar light and heat.

Breather membranes must perform these protective functions without compromising their breathable qualities. Breather membranes are installed between the insulation and the outer wall layers (e.g, cladding or roof tiles). If they protect from external conditions without being sufficiently breathable, the naturally occurring moisture within the building will be trapped in the interstitial layers. This will be highly damaging in the long term.

British and international building standards specify flame retardant materials for specific installations and structures. Flame retardant additives are incorporated within the materials or added as a coating to make them flame retardant; it is the formulation, quality and amount of these additives that determine which flame retardant tests the material will pass and what standards to which they will comply.

A flame-retardant material is one that self-extinguishes; it does not mean that it is flame proof. Flame retardant materials are resistant to catching fire, reduce flammability, and inhibit, suppress or delay the production of flames. Flame proof materials are ones that are not liable to catch fire or be damaged by fire and are not readily ignited or burned by flames.

In 2002, to harmonise the classification of the reaction to fire for building materials, the European Commission introduced the Euro Fire Class System (Euroclass) based on EN ISO 13501-1.

Building regulation in England covering fire safety matters within and around buildings are specified in Approved Document B (Fire Safety) Volume 1 (dwelling houses) and Volume 2 (buildings other than dwelling houses). The documents specify the minimum standards (BS 476 and EN ISO 13501-1) for all materials used in the construction, with specific installation requirements. The full document is available online: www.gov.uk.

On 21 December 2018, changes to the Building Regulations 2010 specified that materials used in the external walls of buildings over 18m must now be Class A1 or A2, as determined by EN 13501-1. This relates to buildings containing dwellings, institutions or rooms used for residential purposes. Class 0 rated materials (as determined by BS476 Parts 6 & 7) no longer qualify. Materials excepted include: door frames and doors, fire stopping materials, membranes, seals/gaskets/ fixings/sealants and thermal break materials.

Current regulations require an external wall construction over 18m (and 11m in Scotland) to have a minimum Euroclass A2-s1,d0 fire safety rating. However, breather membranes are currently exempt from being part of an A1 or A2-s1, d0 vertical wall system; they are required by law a minimum Euroclass rating of B,s3-d0. In Scotland, this requirement applies to external wall constructions over 11m.

Following the introduction of the Building Safety Act in 2022, the latest regulations are subject to ongoing review and the construction industry should plan for further regulatory change. Best practice recommends the use of breather membranes which have a Euroclass A2-s1,d0 fire safety rating in line with the requirements for cladding and other external wall materials.

Breather membranes play a key role within the building envelope. Their vapour permeability facilitates the release of excess moisture vapour into the exterior, helping to keep internal wall roofing structures dry and protect the condition of insulation layers. They can also serve a valuable purpose as a line of defence against water penetration during the construction stage and the lifetime of the completed building, but the importance of this function – and the level of performance required to fulfil it – can depend on the size, design and location of the building in question. A membrane which proved to be adequate for one project could prove to be wholly inadequate for another project.

The nuances of water protection from building membranes are often overlooked or misunderstood. It’s a common problem within the construction industry and more clarity on the subject would extend the lifespan of many future constructions, avoiding the need for expensive remediation work further down the line.

How is water resistance assessed in breather membranes?

A breather membrane’s ability to resist the passage of water through a vertical wall system is measured by its watertightness. The current code for Breather Membranes is BS EN 13859-2 which has superseded code BS4016 :1997.  BS EN 13859-2 lists three classifications in descending order of watertightness: W1, W2 and W3.

Which water tightness ratings are suitable for breather membranes?

W3 is not used for wall panels. To comply with BS 13859-2 a membrane should carry either a Class W1 or W2 resistance to water penetration. The Structural Timber Association (STA) issued an Advice Note which states that a membrane with a W2 classification is suitable for use in the majority of applications where continuous rainscreen cladding is used without gaps such as masonry or render board. The manufacturer should confirm that a Class W2 membrane achieves a minimum of zero leakage in the aged condition to BS 13859-2.

However, the level of acceptance for W2 under BS EN 13859-2 is often subject to confusion as the test and water leakage acceptance rates differ from the testing used for the superseded BS 4016. Having reviewed the differing EN and BS test methods, the STA recommends that breather membranes achieve zero leakage under the EN test as a minimum value.

During the service life of the building wall, STA recommends Class W1 to BS EN 13859-2 for all projects and cladding types specifically for projects with open jointed façade cladding or leaky cladding and Class W2 to BS EN 13859-2 for projects with continuous rainscreen cladding types.

With its higher test credentials, a W1 membrane provides better protection of sheathing / frame materials during the construction process than a W2 membrane.

What test criteria apply to W1 and W2?

The below table from the STA Advice Note provides the minimum test criteria

An important element of the test criteria is the performance of the membrane in aged condition.

Which locations require W1 resistance?

When determining a suitable membrane classification for a building, one of the most commonly overlooked factors is the local conditions.

Class W1 is recommended as good practice when a building occupies a location that is known to be exposed to high winds and driving, persistent rain. These locations are typically found on high-altitude exposed sites and westerly coastal sites in England, Ireland, Wales and Scotland. Cliff or sea fronting locations are particularly exposed. In such cases, a membrane that achieves W1 aged water penetration resistance is recommended to be used with full rainscreen cladding, in conformance to BS 13859-2.

BS 8104 recommends the use of the local spell index method to assess the resistance of a wall to rain penetration. The exposure categories in terms of wall spell indices are given in the table below using the local spell index. They should not be regarded as exact as local circumstances or experience may require adjustments to be made. When an assessment produces an intermediate index, the designer should utilise local knowledge, topography and experience to determine the most appropriate exposure index.

Figure 1 from NHBC Standards 6.1.16: Exposure  shows the below map indicating levels of exposure across the UK in the 4 categories outlined above.

In general, any area within 30 miles of a west facing coastline is considered to be in a severe exposure area. However, localised effects on any site must also be taken into consideration. Exposure gradings and local knowledge should be referred to when discussing design requirements. Further reference to BS 8104 is advisable to ascertain localised effects.

Should you take into account the height of a building?

NHBC Standards – Section 6.1.6 references how site-specific calculations can be made using BS 8104 ‘Code of practice for assessing exposure of walls to wind-driven rain’ which takes into account wall heights.

This document is available online: https://knowledge.bsigroup.com/products/code-of-practice-for-assessing-exposure-of-walls-to-wind-driven-rain/standard

UV resistance is also an important consideration in certain buildings. Most breather membranes are manufactured from polyolefins which can be affected by long-term exposure to UV. Open jointed façade cladding, with gaps that expose the membrane and the passage of ultraviolet light and weather, require a Class W1 resistance to water penetration and confirmation that the membrane has been tested to a minimum of 5000 hours of accelerated UV ageing with an energy minimum of 812MJ/m2 to BS 13859-2.