BLACKPRINT: Ms Körner, you are currently building an apartment block in Messel, Hesse, which is insulated with straw bales and where foam glass boards made of recycled glass replace the classic concrete floor slab. How did the project come about?

Susanne Körner: The project began with a very interested client who wanted to build sustainably and in a climate-friendly way from the outset. Messel gave us the opportunity to bring in our experience from twenty years of straw-bale and timber construction. As early as 2005, we had realised our first building with straw-bale insulation in Hesse, the “Bambushaus”. Over the years, we have learned a great deal about timber construction methods, straw as an insulating material and the integration of different materials.

BLACKPRINT: And the client was open to these experimental approaches right from the start?

Susanne Körner: Yes, absolutely. The clients themselves are farmers. For them, using straw was an obvious choice, as they work with it in their own operation and wanted to provide it themselves. They approached us because we knew this construction method and had already implemented it. At the same time, we have been working for years on concrete-free structures in order to reduce CO₂ emissions. When it comes to sustainability, concrete is not exactly among the front-runners; it has an enormous carbon footprint.

BLACKPRINT: How is the house constructed?

Susanne Körner: It is a multi-storey timber-frame structure in which the load-bearing construction is made of timber and the bracing is provided by wood-based panels. The external walls are insulated with pressed, regionally produced straw bales and take on the entire insulating function. A mineral render was applied on the outside to ensure moisture protection. The wall build-ups are vapour-permeable, so that moisture can be released to the outside – a key point in straw-bale construction. With wall thicknesses of 45 to 50 cm and a U-value of around 0.13 W/m²K, the house clearly exceeds the requirements of many conventional new builds. The annual primary energy demand and heating demand are well below the statutory requirements of the KfW Efficiency House standard.

BLACKPRINT: In addition to the load-bearing timber-and-straw construction, the foundation is also exceptional: instead of a classic concrete slab, ready-made foam glass boards are being used here for the first time…

Susanne Körner: Yes, indeed. In earlier projects we had already worked with foam glass gravel, a recycled, compression-resistant material that additionally has insulating properties. For the Messel project, we have now used ready-made foam glass boards for the first time; compared with classic concrete slabs, the CO₂ emissions are considerably reduced. The boards from Glapor are made of recycled glass, are compression-resistant, thermally insulating, insensitive to moisture and very durable. The building’s loads are transferred through them into the subsoil. The building in Messel is the first application of the Glapor boards as an alternative foundation in multi-storey residential construction, so the project is breaking new ground in several respects.

BLACKPRINT: And for the waterproofing over the foam glass boards, you used the EPDM membrane HERTALAN® EASY COVER...

Susanne Körner: Yes, I came across the HERTALAN® membrane from CARLISLE® CM Europe via Glapor. The membrane meets all relevant standards for structural waterproofing and is suitable for high water-exposure classes. Precisely in a vapour-permeable build with straw-bale insulation, this is crucial in order to preserve the insulating effect in the long term and not to jeopardise the CO₂-reduced material strategy. At the same time, it fits perfectly into the building’s sustainability concept, as it is long-lasting, low-maintenance and removable. Such details are decisive too: a sustainable construction only works if all the layers are coordinated with one another, both technically and ecologically.

BLACKPRINT: And how did the tradespeople react to the unusual materials?

Susanne Körner: At first, there was certainly some scepticism, especially regarding the use of the foam glass boards instead of a reinforced concrete slab. So far there are hardly any practical examples of this, and for tradespeople it is completely new territory. For the timber-frame straw-bale construction method, there are more empirical values and built examples, which we complemented with targeted training of the tradespeople at the carpentry workshop. Once the tradespeople had familiarised themselves, the implementation went very well. The straw bales are densely pressed and clad on both sides, so that the necessary fire protection is maintained.

BLACKPRINT: Sustainability is often reduced to a building’s operational energy. In your experience, how important is it to consider the carbon footprint as early as the construction phase?

Susanne Körner: Yes, that is indeed often underestimated. In the past, the focus was almost exclusively on operational energy, i.e. heating, electricity, technology. In the meantime, we know that 40 to 50 per cent of a new build’s emissions already arise in the manufacturing phase, through the production, transport and installation of the building materials. This is precisely where we come in: by reducing or avoiding concrete, steel and petroleum-based insulating materials, enormous savings can be achieved. A building’s primary energy demand drops significantly the more natural and renewable building materials are used. Messel shows what is possible when sustainability is integrated into the planning and the design process from the very start.

BLACKPRINT: Together with your practice partner Tilman Schäberle, you have been building with renewable raw materials for over twenty years now. How has your attitude towards this changed?

Susanne Körner: The fundamental attitude has remained the same; only the urgency has increased considerably. In the past, straw or timber construction was a niche topic, where one had to explain and convince a great deal. Today, many clients already come with concrete questions and ideas. At the same time, demands are rising due to standards, cost and time pressure. Today, sustainability has to be an integral part of the planning from the very start. Added on afterwards, it does not work.

BLACKPRINT: The client was very closely involved, particularly when it came to harvesting the straw.

Susanne Körner: The client already knew the material from personal experience. Together, we harvested the straw and processed it directly. This enabled short distances, high quality and very good logistics. We had three different batches from the region. Length, density and dryness therefore had to be checked precisely.

BLACKPRINT: Were there particular challenges with the approval?

Susanne Körner: The straw bales are checked via a CE marking recognised under building regulations in accordance with the LBO (the state building code). Once that is in place, they can be used in accordance with the requirements of the building authority approval. For the foam glass boards, the building authority approval was already available; the proof of load-bearing capacity had to be provided. Both worked without any problems.

BLACKPRINT: Do you see the Messel project as a model that can be transferred to larger buildings?

Susanne Körner: Yes, absolutely. The modular construction method is outstandingly scalable. The location, subsoil and permit naturally have to be examined in each case, but the basic principle is transferable. It is precisely such solutions that we need: functional, sustainable and replicable.

BLACKPRINT: Let us talk about another innovative project by your practice, the office and operations building for the Michelstadt wastewater association, completed in 2025. What was the idea behind it?

Susanne Körner: The office and operations building for the Michelstadt wastewater association is the first straw-bale-insulated public building in Hesse. It shows that resource-efficient, regional building is not a niche idea but already works today in the regular everyday operations of institutions. At the same time, the building illustrates how sustainability can be implemented in a public building.

BLACKPRINT: That requires a great deal of openness on the part of the client…

Susanne Körner: Yes, the association plays a pioneering role in the region, among other things through its innovative facility for phosphate recovery from wastewater. The client wanted the new building to reflect this attitude too, that is, to be implemented in a functional, ecological and resource-efficient way. From the outset, it was therefore clear that the sustainability goals should go beyond purely technical energy or efficiency thinking. That is why we deliberately opted for regional building materials, straw-bale insulation and local construction timber.

BLACKPRINT: What did the structural implementation look like specifically?

Susanne Körner: We opted for a classic timber-stud frame construction combined with straw-bale insulation. As a result, this leads to a negative CO₂ balance. The construction timber for the roof, walls, internal walls and floor slabs, and the timber for the façade, panelling and internal wall cladding, comes almost entirely from the Michelstadt forest; in total, around 130 m³ of spruce and Douglas fir were processed, while about 1,200 straw bales from the neighbouring village provide the insulation. The prefabrication of the timber structure, including the straw-bale insulation, took place in a workshop hall belonging to the wastewater association. This not only shortens the construction time but also strengthens regional value creation. The façade surfaces of glazed Douglas fir boards were installed at a slight incline in order to protect them from the effects of the weather and to increase their service life. At the same time, the vapour-permeable wall build-ups and clay building boards with clay plaster ensure good moisture management.

BLACKPRINT: And in the interior…

Susanne Körner: In the interior, we rely on natural materials such as clay plasters, offcut timber, goat-hair carpets and rubber floors. The energy supply is provided by a combined heat and power unit in combination with a ventilation system and natural recooling via a ground heat exchanger. In this way, energy consumption remains permanently low, while a healthy indoor climate and high user comfort are ensured.

BLACKPRINT: Another special project by your practice is the Bambushaus in Darmstadt, built as long as twenty years ago. What makes this building unique?

Susanne Körner: In 2005, the Bambushaus was the first building in Germany with a load-bearing bamboo structure, and to this day it is still the only one in Germany. In total, 33 bamboo columns support the structure; at the time, that was a completely new approach. For us, bamboo is fascinating, because it grows extremely quickly, is very strong and light, and binds CO₂. The walls are made of pressed straw bales with clay plaster, the insulation of hemp, and the roof is planted. This creates a building that relies entirely on renewable raw materials and at the same time develops a clear, aesthetic identity.

BLACKPRINT: Were there particular challenges in the implementation?

Susanne Körner: Yes, at that time bamboo was not yet approved by the building authorities in Germany, so we had to prove the construction precisely. The connection technology came from a specialised supplier. For the tradespeople, too, the combination of bamboo and straw bales was new; training and close collaboration were therefore necessary. However, once the materials and their properties were understood, the implementation ran smoothly.

BLACKPRINT: What role does aesthetic design play in your projects?

Susanne Körner: A very central one. It is important to us that nature-connected building and design quality go hand in hand. The timber façade of Douglas fir boards of varying widths, the nature-based interior materials and the extraordinary wall design – all of this conveys a sense of value and identity to users and visitors. Aesthetics and function complement one another; sustainable materials thus become visible and tangible.

BLACKPRINT: What feedback have you received from the users of your buildings?

Susanne Körner: The feedback is predominantly positive. Thanks to natural materials such as timber, straw, clay or rubber flooring, the rooms feel warm and inviting, and many users appreciate the special atmosphere that these materials create, in residential, office and administrative buildings alike. Sustainability thus becomes directly tangible, without being reduced merely to technology or energy consumption.

BLACKPRINT:  Ms Körner, thank you very much for this conversation!