Current condition

The glazed curtain wall of the office building at 15 Wetstraat in Brussels is approximately twenty years old. As a result, various components of the air and water sealing are gradually reaching the end of their expected service life. The client therefore wished to know which measures were necessary and appropriate to restore the façade’s performance to the required standard – and to what extent a renovation would need to go to achieve this.

Bureau Bouwtechniek was commissioned to carry out a technical assessment of the existing façade and to compare various renovation strategies. In doing so, we looked not only at the condition of the materials, but also at thermal performance, air and water tightness, energy consumption, feasibility and cost. This provided the client with a well-founded basis for deciding where intervention is needed, how thorough it needs to be and what investment is required.

Analysis of the existing condition

The façade is not a uniform whole. We mapped out seven different façade zones and their specific construction and combined an analysis of the as-built documents with on-site investigations. Infrared thermography revealed, amongst other things, heat losses at the joints between façade elements, joinery, opening sections and other structural junctions.

The analysis revealed that it was not so much the glass alone that constituted the weak link. Above all, airtightness still offered significant potential for improvement. Around the openings, seals had deteriorated and warped, and in places did not provide a sufficient seal. Connections between different façade elements – such as the roof and façade, the floor and façade – and complex corners also emerged as critical points.

The calculations highlighted the significance of this: in the existing condition, approximately 39 per cent of heat loss through the façade was attributed to air infiltration, compared with 61 per cent due to transmission losses through the façade elements themselves.

Renovation scenarios

Rather than immediately proposing a single solution, we developed three scenarios. For each scenario, we examined not only the theoretical improvement, but also how the works could actually be carried out in a building that remains in use. Accessibility, working from inside or outside, phasing, safety, disruption to users, quality control and cost were all taken into account.

The first focuses on the maintenance and repair of the existing opening elements and seals.
The second goes further and tackles the airtightness of the façade and its critical junctions more systematically, whilst retaining the existing glazing.
In the third scenario, the glazing is also replaced with higher-performance glass.

This yielded a striking result. According to the calculations, a thorough improvement in airtightness without replacing the glazing can reduce the heating energy consumption associated with the façade by approximately 29%. Complete replacement of the glazing increases this potential to approximately 41%, but requires a much larger investment and also entails greater material use, longer construction time and a higher embedded environmental impact.

Conclusion: do not carry out more renovation work than is necessary

Based on the comparison, the study identified the second scenario as the preferred strategy: retaining the existing glazing, but making targeted investments in the airtightness of the façade. According to the study, this combines a significant improvement in energy performance with a lower investment and environmental impact than a complete replacement of the glazing. Furthermore, the work is less invasive and does not require the building to be vacated for an extended period.