Five thoughts on heritage buildings versus net-zero

Reusing heritage buildings avoids unnecessary carbon emissions and preserves the fabric of our towns and cities. We just need to reimagine them for a net-zero future

July 2022

Words by Malene Holmsgaard, Martin Sing and Chessa Stevens

“There is a tension at the heart of heritage work. Regulations for listed buildings tend to restrict changes, making it difficult to reduce their energy use. But they must adhere to modern functional and energy-efficiency requirements as far as possible in order to remain viable and useful”

1

Climate change threatens the past as well as the future

Conserving our global heritage and tackling climate change share a common ethos. Heritage conservation principles teach us the value of preserving and protecting historic buildings for future generations, while sustainability teaches us the value of preserving and protecting all resources for future generations. Historic buildings can be seen as a resource too: by conserving and updating them, we can avoid expending the embodied carbon inherent in new development. There is a tension at the heart of heritage work, however. Regulations for listed buildings tend to restrict changes, making it difficult to reduce their energy use. But they must adhere to modern functional and energy-efficiency requirements as far as possible in order to remain viable and useful. At the same time, climate change is making the need to protect our built heritage all the more urgent, with extreme weather placing new demands on ageing fabric and structures.

2

Treat existing buildings as a resource and use what’s there

Not all buildings can be upgraded to modern energy efficiency standards, but significant carbon savings can be achieved with bespoke solutions, particularly when the embodied carbon of the existing structure and the relatively short lifespan of new technologies is taken into account. WSP and the Technical University of Denmark have calculated that rather than replacing draughty wooden windows, insulation standards could be met by retaining and properly maintaining them, and installing secondary glazing. This will result in significant reductions in carbon, overall cost and resource consumption — especially since older windows have a lifespan of well over 100 years, compared to just 30 for thermal glazing. At Copenhagen City Hall, meanwhile, the new heating and ventilation strategy makes full use of the 120-year-old building’s original infrastructure, including brick ventilation ducts, roof pipes and basement corridors. In combination with a state-of-the-art building management system, this is expected to use 80% less energy.

3

Work with the structure, not against it

One of the core tenets of restoring any heritage asset is that we must first understand its significance, how it is made and how it performs compared to current requirements, before deciding on a solution — and each time the answer will be different. Preservation strategies may not always be large-scale or intensive. In Akaroa, New Zealand, the pre-1900 seawall was suffering from erosion and structural damage and under threat from more frequent storms and rising sea levels. Our team took a conservative approach that prioritized its historic value. The repair methodology focused on strengthening the existing walling and its foundations rather than making new interventions, slowing the rate of deterioration while maintaining the integrity of the structure and minimizing the loss of original stonework.

The major rehabilitation of the Centre Block at the Canadian parliament in Ottawa is showing how low-carbon strategies can be adopted without altering a building’s character. The net-zero design includes waste heat recovery combined with an underground geo-exchange field, which stores surplus heat during the summer for use in the winter, as well as rainwater harvesting. The project is led by CENTRUS, a joint venture between WSP and HOK, and is due to complete by 2031

4

Watch out for your own legacy

Maintaining existing materials correctly, using traditional products, supports a circular economy approach too. For example, wooden window frames will last longer if they are protected with appropriate coatings, but they will also be recyclable when they eventually need to be replaced — rather than having to go to landfill because they are contaminated with unrecyclable products such as modern paint. At the flagship Centre Block rehabilitation at the Canadian parliament in Ottawa, WSP is part of a team exploring how the existing structural steel elements might be reused as part of the new structure. This would eliminate the bulk of the embodied carbon that goes into recycling steel to create new elements. There is a perception that buying new materials is simpler and less costly than reusing existing resources. While this is mildly inaccurate in the short term, it’s grossly inaccurate over the longer term.

"There is a perception that buying new materials is simpler and less costly than reusing existing resources. While this is mildly inaccurate in the short term, it’s grossly inaccurate over the longer term"

5

Follow the science, not your gut

There is a long-standing position in the heritage community that insulating masonry walls can present more risks than benefits, but this may not always be the case. Advances in building science, analysis and hygrothermal modelling are bringing a more science-based approach and a better understanding of the forces at work. On the Centre Block rehabilitation, we were able to demonstrate that the walls could be insulated with marginal or negligible increase in the risks of condensation, mould and freeze-thaw damage, and that factors such as exterior wetting, poor water shedding, structural issues and original detailing of the assemblies would have a much greater impact. This is not a sweeping conclusion that we can apply to all heritage masonry walls. But it does show that by challenging our assumptions and basing decisions on analysis rather than gut instinct, we can come up with better, more sustainable and enduring solutions.

Malene Holmsgaard is director of building preservation at WSP in Denmark, Martin Sing is team lead for sustainability and energy at WSP in Canada, and Chessa Stevens is WSP’s national lead for built heritage in New Zealand

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