BLACKPRINT: Mr Rebbereh, at first glance your Green Steel Home does not look like a typical steel structure. How did the idea come about?

Thorsten Rebbereh: In addition to architecture, I have other business areas — among them a food company that has grown significantly. To support that growth, I needed to build a new logistics hall — in steel frame construction, because that's the fastest and most cost-effective way to create volume. While that was going on, I had a phone call with my daughter in Hamburg, who told me about the far too high construction costs in the city and that, even as double earners, they couldn't afford owning property there. That made me think. In the past you could build a house on a normal income; today you cannot, despite historically low interest rates. In my view the reason lies in the processes: we still build single-family houses today much like 100 years ago, stone on stone, with many trades following one after the other. In industry there is assembly-line work, just-in-time and process optimisation; in construction there is hardly anything like that so far. And that's when I had the idea to simply transfer the principles of industrial construction to residential construction.

BLACKPRINT: What are the advantages compared with conventionally built single-family houses?

Thorsten Rebbereh: The biggest advantages are speed and precision. The components arrive just in time at the construction site; everything is prefabricated like an IKEA kit system. The steel frame is up in two days, the entire building envelope in about 14 days, and practically without waste. Interior finishing takes a few weeks, so the house is completely ready to move into after about three months. In a conventional shell construction the same process often takes a year.

Das Stahlskelett des Green Steel Home lässt sich in The steel skeleton of the Green Steel Home can be erected in two days, and after about three months, the house is completely ready for occupancy.

BLACKPRINT: What convinced you to actually pursue the idea?

Thorsten Rebbereh: After the conversation with my daughter got me thinking about construction costs, I really wanted to implement the idea. That's where my entrepreneurial spirit came into play! In addition, Salzgitter AG is practically on my doorstep. Through my network I was able to make contacts there, originally only with the aim of buying steel more cheaply. And then the whole thing developed on its own: Salzgitter AG is currently intensively involved with the topic of "green steel" under the SALCOS® branding. So I almost automatically slipped into the sustainability track.

BLACKPRINT: Steel and sustainability? You have to explain that ...

Actually I only wanted to build an inexpensive house. Only afterwards, and through cooperation with Salzgitter AG, did I realise that steel is practically the only building material in construction that we can actually recycle without loss of quality. A wooden element eventually becomes a chipboard panel, maybe an IKEA cabinet — and then it's over. A steel beam, on the other hand, remains a steel beam. Even if I melt it down, it becomes steel of the same quality again. For that reason I think steel has a far too bad reputation. The material is always associated with a very high CO₂ footprint. But if you break everything down per square metre of floor area and then use as much green steel as possible, the CO₂ footprint of steel is ultimately unbeatable — even compared with conventional construction and even compared with timber-frame construction. And here, in the understanding of buildings as material banks, lies the real game changer for sustainable building. In the past we mostly built conventionally. Many materials can ultimately only be downcycled. Masonry might then become a subbase for a cycle path, but that's not real recycling to me. Really sustainable is when we use building materials in a way that keeps them separate by type and when, after deconstruction, they can be reused directly.

BLACKPRINT: You implemented the Green Steel Home with green steel?

Yes, production runs 100 percent on green electricity, produced here via the so‑called "electric arc furnace route" — for that I even obtained a certificate after overcoming some bureaucratic hurdles. The topic is still in its infancy, though: if you have a house certified today according to QNG, you still can't include green steel in the balance — green steel is still counted as grey steel. Because the CO₂ quantities are so small, however, you still achieve certification. One tonne of steel may have a footprint of around 1.8 tonnes of CO₂, which sounds high at first. But in building I need very little steel — in my case just 10.4 tonnes for around 150 square metres of living space. Compared with masonry, reinforced concrete slabs or even wood, I need considerably less material overall.

BLACKPRINT: Back to the construction: you rely on a bolted structure without load-bearing interior walls in your Green Steel Home. What possibilities does that open up for later conversions?

Thorsten Rebbereh: The free‑spanning steel structure enables maximum flexibility in the floor plan. Walls can be placed almost anywhere, which opens up entirely new possibilities when life circumstances change. I find the gallery above the dining and living area particularly exciting: here I could insert a ceiling and thus create two additional rooms without compromising daylight. This kind of flexibility is not found in conventional solid constructions.

BLACKPRINT: For sealing the flat roof you chose the HERTALAN® EASY COVER EPDM membrane from CARLISLE® CM Europe. How did that come about and what advantages do the membranes offer?

Thorsten Rebbereh: That was actually a lucky coincidence: through the cooperation with Salzgitter AG and their green steel, a consulting engineer from the company approached me and told me about a fully recyclable EPDM waterproofing system from CARLISLE® that is delivered to the site in one piece. Originally I had planned a conventional roof sealing, but then cancelled that implementation. The HERTALAN® EASY COVER system from CARLISLE® could be planned, manufactured and delivered within a week. Especially for small projects like my show house this is a huge advantage: the membrane is delivered like a whole "carpet" onto the roof and is simply laid out. Sizes up to 1,000 square metres in one piece are possible; there are no manually executed seams and no complicated and moreover dangerous welding works. That enables a completely different quality than when, for example, a roofer welds bitumen sheets with an open flame, with seams that are often not properly sealed and water can penetrate the waterproofing layer. In addition, the independent testing institute Süddeutsches Kunststoff‑Zentrum (SKZ) has certified a service life of over 70 years for HERTALAN® EPDM: that relieves the homeowner of the worry that flat roofs only have a short useful life.

BLACKPRINT: In implementing your Green Steel Home you integrated the approach of biophilic design. What is that about?

Thorsten Rebbereh: The approach dates back to the 1990s and goes back to the biologist E. O. Wilson, who formulated two central theses: first, that humans are genetically connected to nature, and second, that nature and technology need not be in conflict but can work together. Wilson also emphasised that interiors always need a connection to the outside world and that natural materials, patterns and haptics should be integrated. For the Green Steel Home I derived, among other things, the gallery level with 80 plants from that. The plants convert CO₂ into oxygen while the residents breathe, creating a natural cycle. On particularly gloomy days, smart‑home UV lamps — controllable by app or voice command — support plant growth, and an automatic watering system takes care of irrigation.

BLACKPRINT: How is the Green Steel Home heated?

Thorsten Rebbereh: The Green Steel Home has a decentralised, controlled residential ventilation system with heat recovery. In addition, an infrared carbon-area heating provides comfortable warmth, not through convection but through radiation. The system uses electricity generated by the photovoltaic system from the sun to produce a pleasant warmth that imitates the natural radiation effect of the sun. That way I bring the sun into the house and can greatly increase living comfort!

BLACKPRINT: And how is the quality of stay? For example, does it get very hot in summer so that an air conditioner becomes unavoidable?

Thorsten Rebbereh: That's a legitimate question, because many people think of overheating when they hear "steel construction". In fact, we paid very close attention to summer thermal protection. The house is highly insulated. We wanted to realise it without a heat pump, and we succeeded. Instead, we installed the infrared carbon-area heating. It runs very efficiently: it reacts extremely quickly, needs only a few minutes per hour to keep the set temperature of 20 °C, and keeps the indoor climate consistently pleasant. Also: the house is thermally highly insulated, so it does not heat up excessively in summer. A conventional air conditioner is therefore not necessary. The combination of insulated building envelope, area heating and photovoltaic system provides comfortable living quality with very low energy consumption. A particular clever idea is the use of the steel beams as a heat store: excess electricity from the PV can be fed directly into the beams, which store the heat and release it again with a time delay. This combines fast responsiveness (area heating) with storage capacity (steel beams) — similar to an old tile stove, only efficient and modern. The photovoltaic system covers as much self‑consumption as possible. Additionally, the system can be flexibly controlled, for example via electricity market prices. And even electric cars can be intelligently integrated in the future to use excess energy.

BLACKPRINT: A very innovative approach! To what extent do you see the Green Steel Home as a model for broad application, for example in terraced houses or serial housing solutions?

Thorsten Rebbereh: For terraced houses or serial housing solutions the concept would have to be adapted. My house is planned so that there are only ancillary rooms on one side, so you could do without windows on that side. Then it could be mirrored to build semi‑detached houses.

BLACKPRINT: And would it be conceivable on the other hand to convert no-longer-needed storage halls in steel construction into residential buildings?

Thorsten Rebbereh: In a very special individual case that might be conceivable, but as a rule I would rather advise against it. There are several reasons for that. The first point is building law: industrial halls are usually located in industrial zones, and enabling housing there is hardly feasible under planning law. There are also structural difficulties: halls usually have large building depths and lengths that are unsuitable for residential use. In multi‑storey housing, about 20 metres depth is the limit, otherwise adequate daylighting of the apartments is no longer possible. In a typical hall that becomes problematic quickly. In addition we then talk about several residential units, which in steel construction immediately raises questions about fire protection, sound insulation and thermal bridges. To ease the housing shortage, old industrial halls in steel construction are therefore not a real solution.

BLACKPRINT: Why has steel construction so far not prevailed in residential construction?

Thorsten Rebbereh: It already starts at universities — steel construction for residential buildings is practically taboo there. In addition there are objective issues such as fire protection, sound insulation and thermal bridges. In my Green Steel Home this works because the house is very compact and has a well‑insulated envelope. And because single‑family houses fall into building class 1, there are no major fire protection requirements. Therefore I can also leave the steel beams visible without complicated cladding. That makes things simpler, cheaper and also visually attractive in the industrial style. In result the house is up as a shell in three days and the costs are around €2,150 per square metre, well below the usual market prices of €3,000 to €3,800. In this way home ownership becomes achievable again for the average earner!