Developers in the UK are facing increasing demands to reach net zero goals which has led to a rise in structural timber construction. With growth in this area expected to continue in 2026 and beyond, it’s important that specifiers have access to transparent technical information to ensure products specified will provide the required performance. Here, John Mellor, Head of Marketing at building products manufacturer Glidevale Protect explores the area of watertightness classification and why this matters when it comes to wall construction membranes being used with cavity-facing structural timber wall panels.
Breather membranes serving a dual purpose
The breather membrane on a timber wall panel has two critical roles. Firstly, it allows water vapour to pass into the cavity to reduce interstitial condensation risk, while also acting as a temporary second line of defence against water penetration during construction.
The Structural Timber Association’s STA Advice Note 18 provides a useful guide on selecting external breather membranes for cavity-facing structural timber wall panels. While the guidance covers areas such as exposure, thermal performance and fixing requirements, one area that deserves particular attention is watertightness and the resistance to water penetration, especially the requirement for using a membrane either with Class W1 or W2 performance depending on the location of the project.
To ensure the correct specification of breather wall membranes it’s essential that manufacturers provide accurate and transparent technical information relating to the watertightness classification of their products, and that specifiers, designers and timber frame manufacturers clearly understand the difference to be able to choose the most appropriate solution.
Simply put, Class W2 is suitable for the majority of applications where continuous rainscreen cladding is used without gaps (for example, masonry or render board). Glidevale Protect’s Class W2 membranes have a proven track record, as well as offering confirmation of zero leakage in the aged condition to BS 13859-2. Its reflective membranes also feature technical ‘R value’ performance based on the worst-case scenario of aged thermal resistance values with these low surface emissivity values achieved using the correct fixing centres and incorporating printed logos.
Class W1 watertightness is recommended for very severely exposed sites such as high- altitude locations, cliff-top or sea-front sites, westerly coastal regions across England, Ireland, Wales and Scotland as well as projects in areas subject to frequent high winds and driving rain. Glidevale Protect offers two membranes suitable for these environments to help protect timber structures where exposure risk is highest.
Covering all bases – W1 and W2 products
Glidevale Protect’s range of Class W1 and W2 reflective and non-reflective construction wall membranes, all independently third party certified by BM TRADA, align with the requirements of STA Advice Note 18 for structural timber construction. The high-performance range includes:
Protect TF200 Thermo – a Class W2 (no leak) breather membrane providing protection to external walls during the build process. Featuring a highly reflective and durable surface to enhance the thermal performance of the final wall design. An Environmental Product Declaration (EPD) is also available for this product.
Protect Thermo Extreme – a robust, reflective external wall breather membrane which achieves a Class W1 water penetration resistance for suitability in severely exposed and high-altitude areas.
Protect TF200 – a water resistant breather membrane with Class W2 (no leak) water penetration resistance, available in a variety of standard colours and widths to suit factory installation. An Environmental Product Declaration (EPD) is also available for this product.
Protect TF200 Extreme – a non-reflective breather membrane ideal for buildings sited in severely exposed locations, where a Class W1 water penetration resistance is recommended.
In all structural timber applications, specifying Class W1 or W2 isn’t just about compliance, it’s also about safeguarding timber structures from real-world weather conditions. Glidevale Protect’s Technical team is on hand to support specifiers and designers regarding the appropriate type of construction membrane to use depending on specific site conditions.
To find out more, visit www.glidevaleprotect.com, call +44 (0)161 905 5700, or email [email protected]. Follow Glidevale Protect on LinkedIn.
