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The Lilienhof on the Munich Au located to the right of the Isar is a typical residential complex from the time of reconstruction in the 1950s: undemanding architecture with small apartments, because bombed-out and displaced people quickly needed a place to stay.

For six decades the houses were kept in good condition by the municipal housing association GWG, but not fundamentally modernized.

Then came the general renovation in 2009.

A ten centimeter thick wall insulation made of Resol rigid foam, insulated cellar ceilings and triple-glazed insulating windows drastically reduced the heat requirement.

Individual heaters were thrown out and replaced by a central heating system that draws its energy from a gas engine heat pump with a gas condensing boiler and a solar thermal system on the roof.

The four 1950s buildings with the shocking primary energy requirement of 311 kilowatt hours per year and square meter were transformed within five years into low-energy houses with almost 50 kilowatt hours of primary energy requirement per square meter and year.

"If you include the electricity from the photovoltaic system on the roof, we have practically achieved CO2 neutrality there," explains Gerda Peter, managing director of the GWG.

The renovation rate must be at least doubled

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There are thousands of facilities like the Lilienhof in German cities.

It will be decided in them whether the climate protection plans will be more than beautiful declarations of intent that dissolve more and more into warm air as the day of truth approaches.

This is for the German building stock in the year 2050. Then all buildings should be "almost climate-neutral".

At the moment there are 21.7 million buildings, which represent a good third of the final energy consumption in the Federal Republic of Germany and as of today are definitely not on the way towards climate neutrality.

The German Energy Agency (dena) calculated in its building report 2019 that the German building sector will emit up to 28 million tonnes of carbon dioxide more by 2030 than it should according to government plans.

"In order to achieve climate neutrality in 2050, we have to at least double the renovation rate," urges Harald Garrecht, director of the Institute for Construction Materials at the University of Stuttgart.

So far, this rate has been stagnating at around one percent per year, and there are reasons for that.

Climate neutrality has its price.

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In the case of the Lilienhof, specifically 27 million euros, which, however, included an underground car park and the addition of one floor to the houses so that the living space increased by around a third.

Nevertheless, renovation costs of around 3000 euros per square meter come up.

Model project without a successor

In addition, there were considerable start-up difficulties with the complex system technology of the specially developed gas engine heat pump.

"Now it is running smoothly, but we had a lot of maintenance work," says Gerda Peter.

And so for the Munich housing association, despite all the pride in the climate neutrality achieved, the Lilienhof remains a model project without a successor for the time being.

Climate protection in existing housing construction will therefore be a few sizes smaller than in the Munich flagship estate.

In the Mariendorf residential park in Berlin from the 1970s, for example, the goals were set significantly lower.

There, the state-owned Gewobag has reduced the heating requirement in the settlement, which comprises a good 800 apartments, by almost half using conventional means: With thermal insulation and a modernized heating center with combined heat and power units and gas condensing boilers for peak loads.

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In addition, two buildings were fitted with facade solar collectors, which supply electricity to the apartments in the settlement and to currently seven charging stations for electric cars.

What is not needed flows into battery storage.

The complete package cost a good 1000 euros per square meter of living space.

But there are only the consumption values ​​of a medium-quality energy-efficient house.

Concern for the monument protection

Listed residential buildings are a particular challenge, as climate protection often collides with concerns about historical heritage.

"I can develop a new building as a passive house, it may still work in the existing building, even if the building looks different," says civil engineer Harald Garrecht, "but with the preservation of monuments we actually want to preserve our culture."

Much of what is possible, such as wall insulation, in normal existing buildings is ruled out in the case of a monument.

Climate neutrality can only be achieved with a holistic approach that goes beyond the individual building.

“I don't focus on the insulation, but try to improve the building so that I can cover the energy needs with renewables from the surrounding area,” says Garrecht.

In the center he sees the heating: "All the energy I harvest in the object should go into the warmth."

On the Margarethenhöhe in Essen, the Stuttgart professor is realizing this vision together with the owner and preservationists in a model test.

This first German garden city was donated by Margarethe Krupp in 1906 and is still owned by the foundation of the same name today.

A handful of the around 900 listed buildings are being renovated in a climate-friendly way in a model project.

The first German garden city, the Margaretenhöhe in Essen, was donated by Margarethe Krupp in 1906

Source: Margarethe Krupp Foundation

In addition to highly insulated windows, this means above all the installation of high-performance interior insulation made from Aeerogel panels, low-temperature heating in floors or walls and intelligent building services.

This is not yet an efficiency house, "but it meets the requirements of the monument protection and is so good that we can get by with the available renewable energies as far as possible."

It won't work without funding

Some of these come from specially developed roof tiles, which are visually indistinguishable from conventional ones, but are equipped with solar power and solar heating modules.

Most of the heating energy, however, is supplied by heat pumps that are installed in the extensive gardens of the settlement.

The key to successful climate protection in existing buildings should therefore lie in networking.

"We want to get away from the exclusive consideration of a single building and instead think as a whole of the chain of CO2 savings in the quarter," says GWG managing director Gerda Peter.

One thing is clear: Trimming Germany's building stock to climate neutrality is an expensive undertaking, even with inexpensive solutions.

“We need clever concepts for financing.

It will not be possible without appropriate funding. "