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Meeting the Required Standards for Curtain Walling and Rainscreen Cladding

UL's head of Building Envelope, Gailord Nepp, provides guidance for UK housebuilders and housing associations to meet required safety and performance standards.

High rise buildings

December 17, 2021

It’s almost a daily occurrence: another news headline on the increasingly erratic weather patterns and climate-related problems affecting people around the globe. The United Kingdom (U.K.) has been at the receiving end of some of these headlines — including deadly heatwaves and flash flooding in the summer and prolonged rainfall, high wind speeds and destructive storms during the winter.

So how do our buildings stand up against such a battering? In particular, what protections are given to high-rise homes and multi-storey apartment blocks that stand full square against the elements?

A common feature of these new home developments is high-performance curtain walling and rainscreen cladding.

This is not a single building product but more of an interconnected system of components — typically a bearing wall, layer of insulation and external cladding material. This system works to deliver benefits, such as better thermal insulation, improved aesthetics, reduced weight loading and reduced maintenance.

Understandably, post-Grenfell, even just the word “cladding” can raise concerns among many housebuilders, housing associations and other housing providers. But we would be wrong to discard the benefits of these building systems altogether. Developers, their specification and construction teams and also the residents of these homes, all need the enhanced performance these systems can deliver when correctly designed, specified, installed and maintained.

The façade or exterior envelope of a building is the element that makes the internal space habitable and keeps residents sheltered from the weather. As such, the façade must withstand even the most adverse climatic conditions. Rainscreen cladding does what its name suggests: it forms an external barrier, a rear-ventilated cavity that prevents moisture from affecting the main structure and a shield from strong wind force. The air cavity system within the cladding helps reduce condensation and promote stabilized conditions that prevent the formation of cracks and other structural issues. Even at ground level, rainscreen cladding can protect against flooding at the base of the building and prevent that moisture from causing severe structural damage.

It is used for new builds but also the external refurbishment of existing housing blocks, helping those buildings look and perform like new when used correctly. This sort of refurbishment can make a lot of sense environmentally, avoiding the waste and carbon cost of demolition and rebuilding.

So how do we rebuild confidence in cladding? What matters most, of course, is their performance in use and how well they stand up to the worst that the British weather throws at them. But the best indicator of that is the proper testing and third-party certification process applied to these systems. At UL, we aim to support clients by providing exactly this building envelope and façade testing and certification that will help manufacturers and housebuilders meet all the relevant performance demands and rebuild industry and public confidence in rainscreen cladding and other curtain walling systems.

For new home developments, cladding systems must also meet the requirements of housing warranty and inspection bodies such as the National Housing Building Council (NHBC) and Premier Guarantee.

NHBC Standards Chapter 6.9 states categorically that curtain walling must ensure “adequate in-service performance.” So, in assessing the suitability of these systems, housebuilders need to take into account weather resistance and a multitude of other factors, including acoustic performance, thermal bridging condensation and air infiltration.

Compliance with the NHBC guidance is most usually demonstrated by carrying out a suite of technical tests prescribed by the Centre for Window and Cladding Technology (CWCT) along with the product or system certification.

This means testing systems both on and off-site. Air and water testing of the “prototype” curtain walling system should be carried out according to the CWCT standard test sequence A or B and must pass the test at a pressure of 600 Pa for static air, water and dynamic water testing. The panels tested must be of a similar size and configuration to those that will be used on the building. Wind resistance of 2400 Pa, serviceability and safety testing of 3600 PA must also be carried out in accordance with the CWCT Standard for Systemised Building Envelopes, along with tests to determine resistance to water penetration, including joints and interfaces that will be permanently closed and watertight. Finally, a representative sample of the finished installation must be hose or spraybar tested — in effect, subject to water pressure of up to 220 kPa at the nozzle inlet producing 22 ± 2 l/min water flow to assess the performance of permanently sealed joints.

These requirements may sound complex, but with the help of an expert third-party testing provider like UL, manufacturers can readily produce the necessary documentation and certification for their products.

Only a handful of test and certification organizations offer appropriate services, and among 11 leading global TIC and EHS brands, UL Solutions rates number one globally on the important customer decision factors of "Organization I can trust," "Relevant technical expertise" and "Certification or endorsement is important for my customers" according to an independent brand survey conducted in 2021-22 by Presciant with 2,124 TIC and EHS decision-makers spanning 13 countries. Since 2017, we have been working closely with NHBC to ensure that UL International (UK) Ltd., a UKAS accredited testing laboratory (No. 5772) and a UKAS accredited product certification body (No. 4705) can be used to demonstrate compliance with NHBC Standards.

UL’s third-party testing and third-party certification is available throughout the U.K. and Europe to organizations engaged in fabricating curtain walling, brick slip cladding and rainscreen systems. The certification process includes an evaluation covering products and their performance, as well as an assessment of the manufacturer’s processes and factory production control systems. 

In UL’s accredited testing laboratories, we can simulate wind-driven rains, hurricane and tornado winds, as well as hot and cold extreme temperatures to assess and verify building envelope products and systems before, during and after construction. In the U.K., much of this testing takes place at UL’s testing facility in Telford.

Acquiring UL certification allows manufacturers and their customers to have confidence in the quality and increased levels of safety for rainscreen products, which leads to clear benefits for end-users. For manufacturers outside the U.K. who are looking to break into the U.K. market, the UL certification offers an opportunity to compete with local manufacturers and helps them align with local performance and business expectations. Indeed, our testing and certification facilities allow our customers to strengthen their market position, which is especially important for overseas companies seeking a competitive advantage to help them penetrate the U.K. market successfully.

Once a company achieves certification, they will have access to project and tender lists where they need to show that their products and procedures have been independently assessed for performance and quality by a third party. Certification also highlights their dedication to excellence and performance, which is always a valuable benefit in this competitive market sector.

Learn more about UL’s building envelope testing, certification and advisory services or contact UL’s Built Environment team in the UK at

Article revised on Aug. 31, 2022 with information about UKAS.