Old oak window frames at Wynendaele Castle have been fitted with new, heat-insulating glazing

Respectful restoration of Flemish cultural heritage with FINEO vacuum insulation glass

‘Wynendaele Castle is a magical place,’ enthuses Jérôme Matthieu de Wynendaele, the castle’s owner. ‘FINEO is a magical product,’ adds Christophe Vanassche, project consultant at RenoWindow, a Belgian company specialising in sustainable window renovation. The energy-efficient restoration of the historic glazing at the idyllic, moated castle in West Flanders therefore brought together two partners that could not have been more perfectly matched.

Wynendaele Castle was originally built as a wooden castle complex in the late 11th century. In the 13th century, it was replaced by a walled circular castle. In the 12th and 13th centuries, the castle was the seat of the Counts of Flanders and later became the setting for many significant historical events, among them the fatal riding accident of the 25-year-old Mary of Burgundy in the surrounding woods. In the 1870s, the Matthieu family — who have owned the estate continuously since 1833 — remodelled the castle extensively in the neo-Gothic style, giving it its current appearance with medieval-style towers, battlements, stepped gables, and arched doorways.

After moving to West Flanders in 2021, Jérôme Matthieu de Wynendaele was faced with major challenges. Saving energy was, of course, the main issue. How could the spirit of a 2,000-square-metre, 1,000-year-old listed castle be preserved while reducing energy consumption? The castle's owner is convinced that the only solution lies in FINEO's highly thermally insulating vacuum glazing.

'The Flemish Heritage Authority asked us to preserve the existing window frames wherever possible. That's why we needed heat-insulating glass that was just as thin as single glazing,' explains the castle owner. FINEO vacuum insulation glass was chosen, as it combines modern energy efficiency with a minimal installation height.

The specialists at RenoWindow recognise the clear advantages of renovating historic windows using vacuum glass. Rather than complete reconstruction, the process focuses on upgrading the existing frames and replacing the glass to improve energy efficiency. This approach leaves reveals and connections to the masonry untouched — a significant advantage over complete replacement, where historically accurate windows often require customisation and manual fabrication.

Traditionally installed, state-of-the-art customization

Wynendaele is equipped with 203 windows in around 30 unique designs. Before replacing the glazing, RenoWindow measured all existing glass panes with millimetre precision. Based on this data, AGC Glass Europe produced around 60 different glass formats and thicknesses, ranging from 7.7 to 11.7 millimetres on the ultra-modern FINEO production line in Lodelinsart, Belgium – each one custom-made precisely to the respective installation situation. Due to strict requirements of the preservation authorities, the glass units were manufactures in small dimensions and inserted into the existing glazing bar frames to preserve the original subdivision. Every window frame was, of course, thoroughly inspected for damage and precisely measured. The seals and fittings were also checked, with any necessary replacements carried out.

Replacing the glass took around two weeks and was carried out in the traditional way. After milling and cleaning the existing rebates, classic linseed oil putty was used to insert the glass. Finally, the painted surface was professionally restored. In only a few cases was it necessary to remove the window sashes to enable more efficient milling. ‘Our aim is always to preserve the original aesthetics of the frames and not to use any additional materials,’ explains Vanassche. ‘Thanks to the insulation value and thickness, we can offer glass replacement while maintaining the frame's original appearance 100 per cent.’

A vacuum space just 0.1 millimetres thick provides thermal insulation

Stefan Lips, Sales Manager Europe for FINEO by AGC', explains: ‘The functional centrepiece of FINEO vacuum glazing is a 0.1-millimetre-thick vacuum space between two panes of glass that ensures high energy efficiency with an extremely slim design.’ This cavity virtually eliminates heat conduction and convection, providing highly efficient thermal insulation. Unlike conventional insulating glazing, which is filled with inert gases, FINEO's insulating effect is achieved solely by the vacuum.

Despite having a total glass thickness of just 6.7 to 19.7 millimetres, FINEO achieves thermal insulation values comparable to those of triple glazing. With a Ug value of 0.7 W/(m²K), it is a considerable improvement on the single glazing typically found in older buildings, which has a Ug value of around 5.8 W/(m²K).

'This project was special for our team. Working in this historic moated castle was technically challenging and personally enriching. It's not every day that we're asked not to disturb the tower spirit while working,’ says Christophe Vannasche, smiling. Meticulous planning of the construction site logistics was required to avoid disrupting the castle's operations. Due to the surrounding moat, many windows were difficult to access, so a squirrel scaffold was used.

Sustainability encompasses more than just energy efficiency

In addition to its thermal properties, FINEO is characterised by increased light transmission. Compared to triple glazing, around 15 per cent more daylight enters the interior, which contributes to the quality of the space, particularly in living and working areas. The vacuum glass also provides the same high level of sound insulation as modern triple glazing — the glass used in Wynendaele achieves values of up to 36 dB. FINEO is also flexible in its application: the standard glass can be customised to meet different requirements, offering additional functions such as solar control, sound insulation and safety glass. For use in listed buildings, FINEO Heritage is available, combining Fourcault glass to preserve the historical appearance of the original glazing.

‘FINEO impressively combines high performance with sustainability, setting new standards in window technology,’ says Clément Lemoine, Head of Product Management for FINEO by AGC. ‘Our laboratory tests prove that FINEO retains its insulation and soundproofing properties for over 60 years, providing our customers with long-term peace of mind.’

FINEO's high quality and reliability are confirmed by various certifications and approvals that recognise it as a pioneer in the glass industry. For instance, the vacuum insulation glass is 100 per cent recyclable and has Cradle to Cradle certification, which verifies the sustainable nature of the material cycle and the environmentally friendly production process.

In November 2024, AGC's FINEO was the first manufacturer to receive the CE mark for its vacuum insulating glass, an internationally recognised seal of quality guaranteeing building owners and planners safety, reliability, and high product standards.