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Case Studies

Gentoo Low U-value Windows

Gentoo Low U-value Windows

Case Study: Code 6, zero carbon


As part of achieving higher levels of the Code for Sustainable Homes (CSH) and BREEAM (Building Research Establishment Environmental Assessment Method), building designers are maximising thermal efficiencies on wider cavity widths by positioning the window directly over the cavity.  

Historically, the window loading had to sit partly over a structural inner or outer leaf as cavity closers were not designed to take the weight of a window, or be used structurally for fixing windows.

Maximising thermal performance in these applications relies on cavities up to 300mm wide and being able to source windows with a U value of 0.8W/m²K, the requirement for Code Level 6, zero carbon developments.


Gentoo Homes required a cavity closer and window solution that would help it meet Code Level 6 requirements on two properties in Durham.

Weighing up to 60kg each, the challenge posed was to devise a structural cavity closer that would allow the 0.8W/m²K U-value triple-glazed windows to sit over the 300mm cavity.

The walls had an overall thickness of 502mm with 300mm EPS bead filled cavity, which achieved a U-value of 0.10W/m²K.

The windows had to achieve Secured by Design standard.


Eurocell worked with Gentoo to develop a structural cavity closer based on its Cavalok BigBlok system.

This utilised new structural thermal materials, enabling the zero-carbon

windows to be seated and fixed directly onto the main body of the closer, which eliminated the need for strapping back over large spans to inner structures.

In what represents a completely zero-carbon solution, Eurocell Eurologik windows, which achieve a U-value of just 0.8 W/m²K, were specified alongside the cavity closer.

The architects at Gentoo orientated the windows to maximise use of solar energy transmittance, in line with Passivhaus principles. The 70 per cent light transmittance of the glazing enabled credits under the CSH to be gained for daylighting.

Client feedback

“We were delighted with the structural cavity closer and zero-carbon window solution from Eurocell,” said Verity Moss, Project Architect at Gentoo.

“One of the first challenges we had to overcome was supporting the triple glazed Eurologik 0.8W/m²k U-value window over the 300mm cavity. The only way this could be done was to make the cavity closer structural, and we were delighted that Eurocell was able to work with us to develop a solution based on its proven Cavalok BigBlok system.”


The Eurocell windows and Cavalok cavity closers play an important part in helping the three-bedroom Gentoo properties in Sunderland achieve Code Level 6 requirements, along with Secured by Design standards.

Initially developed as a bespoke solution for this project, the Cavalok BigBlok structural cavity closer, which is the UK’s only BBA-approved 100 per cent post-consumer recycled PVC-U cavity closer, is now offered as part of the range.

The Eurologik 0.8 U-value window continues to make use of post-consumer recycled PVC-U to create more affordable energy rated windows.

Eurocell has invested £3m in enlarging its UK-based post-consumer PVC-U recycling operation. This has created the largest and most advanced facility of its kind in the UK with the capacity to process up to 12,000 end-of-life PVC-U window frames per week.

This post-consumer recycled material is used by Eurocell to produce window profiles, PVC-U Thermal Inserts, cavity closers and decking products. Eurocell can now recycle around 20,000 tonnes of PVC-U each year. By recycling in the UK, Eurocell provides specifiers looking to minimise the carbon footprint of their development with additional benefits, because its products are ‘Made and recycled in Derbyshire’.

The Details

Gentoo Low U-value Windows
Location North East

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