Sustainable extension inside and out

The private school Feldballe Friskole wanted to create more space with an extension which had a positive climate footprint. The result is a 250-square-metre building with straw in the walls, a wooden roof and materials that do not contain any hazardous chemicals. The firm of architects Henning Larsen designed the extension, for whom the local school has been a sustainable ‘experiment’ which they hope to scale up in other projects.

The firm of architects Henning Larsen has an impressive number of international projects under its belt

Seoul Valley in South Korea, Cockle Bay Park in Sydney and the Harpa concert hall and conference centre in Reykjavik, to name but a few. In Denmark, the company is behind the striking Moesgaard Museum and the modern inner-city school Frederiksbjerg Skole in Aarhus. And now it can add an extension to Feldballe Friskole in Rønde on Djursland to its portfolio.

Even though it is on a completely different scale to the company’s usual projects, Henning Larsen decided to come on board given their interest in sustainable building. Magnus Reffs Kramhøft, an architect at Henning Larsen, was responsible for the project of designing the extension, which was completed in autumn 2019. He says:

“We chose to participate in the project as we are very interested in and devote considerable resources to sustainability. In recent years, we’ve taken a radical look at the materials we usually use for our projects, and have tried to think along more sustainable lines. Feldballe Friskole gave us the opportunity to test things out on a smaller scale, but our hope is that the experiences can be scaled up to whole and bigger buildings in future.”

Feldballe Friskole wanted an extension that could convey what sustainability is all about to its pupils, and an extension with a healthy indoor climate, a beautiful architectural design and, not least, a positive climate footprint. Therefore, the team had to rethink every single building part, walls, roof and foundations, and try to come up with new and sustainable solutions.

Straw in the walls

Straw has been used to insulate the walls. It is a residual product from agriculture which is usually ploughed back into the ground or burned – but now a new application has been found for it.

“The most sustainable thing, of course, is not to build anything at all. However, when that is not an option, straw is a very sustainable choice because it is a fast-growing residual product. In the past, straw bales were stacked on top of one another for insulation. This system has now been industrialised, and can thus easily be integrated into modern construction processes,” says Magnus Reffs Kramhøft.

Precisely dimensioned boxes are filled with straw and can be used a bit like large LEGO bricks. They are generated according to a 3D model, and produced to accurately and perfectly match each individual project.

“Straw insulation might sound slightly hippyish. And it’s no help that teachers and the industry in general have been preaching the fact that wood and straw can rot, and that one should therefore use inorganic materials. But straw insulation doesn’t rot if you build properly and take the right precautions. Even concrete constructions can provide the right conditions for mould fungus to thrive if they are not built correctly. If a building is properly designed and built, organic materials can last for many, many years,” says Magnus Reffs Kramhøft.

Wooden roof

The roof of Feldballe Friskole’s new extension is built from wood, through and through.

“An exciting conclusion from building the roof is that we have managed to construct it without a vapour barrier, but instead achieved a vapour barrier effect using OSB boards and taped joints. The same boards hold the insulation in place, and also function as structural construction elements by providing a disc effect, thus performing three functions,” explains Magnus Reffs Kramhøft.

Troldtekt acoustic panels are installed on the ceilings in the rooms on woodfibre boards, which have the same acoustic properties as mineral wool.  The roofing is modified wood with standing seam boards and drainage channels. The boards can easily be removed and replaced – and thus support the basic approach of designing a structure that can subsequently be dismantled and reused in future. 

The indoor climate has been an important focus area. Therefore, natural materials with the least possible degassing were selected. Clay plaster in particular is extremely effective in this respect, as it does not degas, but on the other hand absorbs and releases moisture and thus contributes to stabilising the indoor climate.

Troldtekt panels have very good fireproofing properties, while also ensuring good acoustics.

>> See more about the school here


Ready for more sustainable buildings

As to the question of how realistic it is to upscale the building principles involving, among other things, straw insulation and wooden roofs to larger projects, Magnus Reffs Kramhøft says:

“It’s certainly realistic. All it takes is a willingness and know-how from the various players in the industry. In general, having to implement new methods in our processes can act as a barrier. However, we all need to do what we can to help minimise greenhouse gases from the construction industry.” 

“Taking such a radical approach to a project as we have done here is a big step. There’s probably no single right answer. This project is just one of many possible avenues that you can take. For the company, it has been a good learning experience, which we will try to integrate bit by bit in new and larger projects in order to contribute to the transformation of the construction industry. It has certainly opened the doors to a new way of thinking and building.”