Chapter 14: Klasse
High-rise external wall design

What successful W1 testing means for the sector

As performance requirements for external walls continue to evolve, independently-verified testing is becoming increasingly important for architects, facade consultants and specifiers. Today's projects require confidence not only in individual products, but in how complete wall systems perform in relation to fire safety, moisture management, durability and buildability.

Against this backdrop, Klasse G-board has been successfully tested to W1 watertightness performance requirements as part of a tested external wall system. Combined with its Euroclass A1 non-combustible classification, the result provides valuable evidence to support more efficient and robust external wall design.

The significance of the testing is not that G-board is being presented as a membrane, but that it has demonstrated W1-level watertightness performance when assessed against the same demanding criteria used for weather-resistant membrane systems. For specifiers, this provides independently-verified evidence of the board's ability to resist water penetration and contribute to the overall moisture management strategy of the wall build-up.

Why W1 testing matters
Water ingress remains one of the greatest threats
to long-term building performance. If not properly controlled, it can contribute to material deterioration, reduced thermal efficiency, internal damage and
costly remediation.

For high-rise residential, student accommodation, hotels and other multi-storey developments, the ability to demonstrate robust weather resistance is, therefore, a critical consideration. Increasingly, project teams are

helping support key specification requirements for high-rise and higher-risk buildings.

Removing the need for a breather membrane
One of the most significant outcomes of the successful W1 testing is its impact on wall build-up design. Because G-board has been successfully W1 tested, the board itself can provide the required weather protection function within the wall construction, removing the need for a separate breather membrane.

For project teams, this can simplify installation, reduce material usage and minimise the number of interfaces that require detailing and inspection. Fewer components can also help improve quality control on site while supporting faster, more efficient construction programmes. Importantly, these benefits are supported by independently-verified testing data, giving specifiers confidence that simplified wall build-ups are backed by proven performance.

Proven on live projects
The practical benefits of G-board have already been demonstrated on major residential developments. Hill Group successfully used Klasse G-board on the St Ann's Project in Haringey, London.

Commenting on the board's performance, the contractor stated: "One of the key advantages of G-board is that it can be installed without a breather membrane, which on the St Ann's Project delivered significant cost savings and reduced installation time considerably.

“The fact that G-board has been successfully tested to W1 standards gives us additional confidence in its performance and suitability for demanding construction environments. Combined with its practical score-and-snap installation method, G-board offers an excellent balance of durability, reliability, and ease of use on site."

In addition to programme and labour savings, eliminating the need for a separate breather membrane can reduce material usage within the wall build-up, helping lower embodied carbon while supporting a more cost-efficient external wall solution.

From testing data to practical design benefits
The successful W1 testing of Klasse G-board demonstrates how independently-verified data can support better specification decisions and more efficient external wall systems.

For more information visit: www.klassegroup.co.uk

By helping eliminate the need for a separate breather membrane, G-board can reduce installation complexity, lower material usage and deliver measurable cost savings, while maintaining confidence in moisture management performance. Combined with its Euroclass A1 non-combustibility and weather exposure credentials, it offers a practical solution for high-rise residential developments where performance, compliance and buildability are all critical considerations.

As the industry continues to move towards evidence-based specification, tested performance will play an increasingly important role in delivering durable, compliant and buildable external wall solutions. The successful W1 testing of Klasse G-board provides a clear example of how technical validation can support smarter design decisions and improved project outcomes.

Want to learn more? Book Klasse's CPD, 'Specifying Compliant Through Wall Systems Used With External Facades', which explores the importance of W1 testing and includes guidance on the Building Safety Act and Gateway 2 requirements.