Interview with Architect Kay Künzel on the ‘Energiesprong’ Principle

// As interesting as new construction projects with minimised energy consumption may be, in practice only around one to two percent of existing buildings are newly constructed each year, which means that for every new building there are fifty to one hundred old buildings. By far the greatest contribution to the desired energy transition will therefore have to be made by the energy-efficient refurbishment of the existing stock of old buildings. The ‘Energiesprong’ programme successfully developed in the Netherlands offers a promising approach to this, providing for the serial refurbishment of terraced housing estates or apartment blocks with modular prefabricated ‘plug & play façades’. To ensure cost-effective realisation, the various interventions are based on production techniques from the automotive industry, among others.

BLACKPRINT: Mr Künzel, the ‘Energiesprong’ programme has been modernising the energy efficiency of entire terraced housing estates in the Netherlands for years with the help of prefabricated ‘plug & play façades’. Heat pumps, heat recovery, prefabricated roof modules and photovoltaic elements are also used to achieve the desired zero-energy house standard. The concept is now becoming increasingly popular in mainland Europe: More than 11,000 residential units in Germany are currently being planned and prepared for refurbishment according to the Energiesprong principle. What convinced you to take that step?

Kay Künzel: For me, the main challenge is that we have a huge building stock that simply has to be tackled keeping the environment and the ecosystem in mind. The Energiesprong principle acknowledged by the German Energy Agency (dena) offers far-reaching opportunities for this. Another motivation for me is that I’ve been working with prefabricated timber elements for years. I’m drawn to the material - not only because of its versatility in façade and interior design, but also due to the wide range of natural insulation options it offers. The fields of architecture and construction are challenging enough. So if I have a key in my hand that simplifies a lot of things in the process and gives me a construction kit of serially recurring and multipliable detailed solutions, then that gives me square metres on the wall.

BLACKPRINT: In which project did you first implement the Energiesprong principle?

Kay Künzel: We really implemented the principle holistically in the renovation of a seven-storey apartment block from the 1960s with a total of 27 units in Frankfurt. We modernised the original building not only visually, but above all in terms of energy efficiency with the help of serially prefabricated timber façades with modern bay windows. The newly installed timber frame construction elements were planned digitally beforehand and then completely prefabricated in the factory, insulated with cellulose and fitted with triple-glazed timber-aluminium windows and blinds. In combination with other measures such as the use of heat recovery and a PV system on the roof, we were able to reduce the flats' heating energy requirements by a factor of 40. Instead of the previously installed 120 kW gas heating system, we have now been able to install a small heat pump with a power consumption of 3.26 kW. To improve the microclimate, we have also greened the dormer windows and unsealed the courtyard areas. Rainwater and wastewater from the house are treated and stored in 25,000 litre cisterns. This is used to supply the heat pump with energy. As a result, we have achieved the passive house standard and even have a net energy surplus!

BLACKPRINT: Were there any special requirements for the project?

Kay Künzel: Yes, the big challenge was that we were dealing with a building that was largely inhabited and where we didn't want to have to give any notices for refurbishment. The refurbishment according to the Energiesprong principle provided us with an ideal tool that enabled us to have completely prefabricated elements delivered to the construction site and assembled floor by floor. The entire construction and planning process was implemented in line with BIM, from measuring to the CNC machine. This process makes it possible to work precisely with all the peculiarities of the existing building. The prefabrication of the elements in the hall resembled a Lego game in which all components were individually cut, numbered, produced, stacked and neatly documented. The result was the finished façade modules with the integrated balcony extensions, with all windows, window sills and fall protection, with the corresponding shading elements, electrical cables and other installations.

BLACKPRINT: The residential value of the flats is likely to have improved significantly after the conversion...

Kay Künzel: Absolutely and noticeably! The building dates back to the 1960s, so it had a good 60 years under its belt. Accordingly, there was a need for renovation in every nook and corner of the building. The façade itself was still relatively unproblematic because it was still functional without insulation. However, the building services and the entire interior were no longer up to date. As a result, the entire electricity and water supply throughout the building had to be replaced. We also integrated a lift, added bay windows to the individual flats and completely renovated the restaurant area on the ground floor. Added to this are the interior design measures, i.e. the installation of new floors, toilets and shower facilities. The residents experience a quantum leap from ‘before to now’!

BLACKPRINT: That sounds very positive. But what about the economic side? Is it possible to quantify how long it will take to recoup the investment costs?

Kay Künzel:  As a starting point we have a house that was barely habitable, the first thing to do is to generate a rent at all. Despite the absolutely high level of investment, the return on investment is less than 10 years! There is also the advantage that the high quality that has been realised will last much longer. As a result, I have a building that will hardly require any refurbishment in the next 30, 40 or 50 years. Such projects are therefore not only future-proof and therefore truly sustainable, they are also much more economical overall.

BLACKPRINT: And can you also say what the rent is like?

Kay Künzel: This project also followed a socially responsible, partnership-based approach. Alongside heating costs, tenants face a range of other charges - from cable connection fees to shared internet. By addressing these collectively, we were able to free up budget for meaningful investment. We've introduced a warm rent model, where services are bundled into a fixed rental rate. The project also includes extended living spaces, natural building materials with healthy interior finishes, enhanced sound insulation, effective shading, and passive cooling - thanks in part to extensive greenery. As a result, residents now enjoy significantly improved living comfort, while still paying only a modest increase under the warm rent model. Rents were adjusted in a socially fair way, with personal circumstances taken into account. In the end, the result is a win-win for everyone involved.

BLACKPRINT: : A complete success, then. It's all the more incomprehensible that the principle of modular renovation hasn't yet become more widely accepted....

Kay Künzel:  Well, there's always resistance. But fundamentally, awareness of modular construction processes is growing in the industry and among specialist companies. Especially now that the new construction market has collapsed. And of course, processes have to adapt and become established on the part of all those involved. All of this takes time. The Enegiesprong concept offers an open principle here, allowing everyone to consider their contribution to a serial process. And it's not really the case that we in Germany have no experience with modular, serial construction. Look, for example, at the recurring model of prefabricated buildings or the prefabricated housing industry, which offers single-family homes from catalogues. These are all optimized construction processes for a very specific need.

BLACKPRINT: Do you have the impression that serial renovation based on the Energiesprong principle will continue to prevail?

Kay Künzel: Ultimately it's always up to the stakeholders involved! And at the end of the day, we have enough homes for 80 million people in this country. Now we just have to see that this stock is maintained and improved. On the other hand, up to 10,000 euros per square metre are currently being asked for new builds. And this is also because no one will budge from their short-term orientated yield expectations. Instead of reverting to the Viennese model, in which the city uses its pre-emptive rights to ensure affordable rents, new construction in our affordable rents, new construction here is largely left to the free market. We have just shown that things can be done differently with a subsidised housing project near Aachen, where we have created affordable living space to the passive house standard using timber construction and serial approaches. And people can live there for seven euros. It's quite possible if you want it!

BLACKPRINT: Another question: you are a member of Green X Future Building Solutions. Does this network of building stakeholders also come into play in the realisation of serial renovations?

Kay Künzel: Green X Future Building Solutions is a growing network, an association not only of engineers and architects, but also of material suppliers, manufacturers, timber construction companies, and system suppliers. It's been a good twenty years since this movement was founded, motivated by the desire to build in a much more sustainable, energy-efficient, and ecological way. And this network still exists, but has since grown significantly. And it's always great to see how large these industry partners and architectural firms have become. And that today we're dealing with true professionals!

BLACKPRINT: Mr. Künzel, thank you for the interview!

The interview was conducted by Robert Uhde.