Table of Contents
Exemplary renovations
This article is based on a paper published in the 62nd volume of the Research Group for cost-effective Passive House buildings on the topic of “ Cost-Effective Refurbishments”. The original article by Laszlo Lepp is availabe in German in the proceedings, which can be accessed here.
Introduction
In Europe in particular, but also in other parts of the world, there is a very large housing stock dating from the 1950s to the 1970s; at the same time, many turn-of-the-century buildings built between 1900 and 1920 still exist today. Buildings are generally responsible for around 35-40% of the final energy consumption and one third of greenhouse gas emissions [GABC]. The vast majority of buildings are energy-intensive and inefficient from an energy perspective, and need to be made fit for the energy transition as quickly as possible.
There are many reasons for building renovation. The most important is certainly that components have reached the end of their life cycle and need to be replaced. However, new requirements for accessibility, space requirements and increased building density, etc. also contribute to the need for modernisation. In the author's view, building renovation must meet current and future (certainly even stricter) climate protection requirements in order to significantly reduce carbon emissions. The dilemma of mediocre quality is an important issue. Renovations with just an average energy-relevant quality do not contribute towards effective climate protection. However, most national building regulations do not currently require a better standard. Those wishing to carry out renovation are thus discouraged from a better standard of renovation work and more efficient climate protection. This is to the detriment of everyone. A poor quality of renovation is a missed opportunity for better climate protection, because the same building component certainly won't be replaced for the next ten or twenty years due to economic reasons. This leads to a lock-in effect, because the average or poor quality remains over a long period of time.
But one thing is clear: in order to achieve climate protection goals – for our own survival – we must reduce the energy consumption of buildings to such an extent that they can be supplied entirely with renewable energies in the future. This is only possible with energy-efficient buildings, regardless of whether they are newly built or renovated. With the lock-in effect, the chance for a renewable energy supply is wasted.
SINFONIA
From 2014 to 2020, the EU project SINFONIA (abbreviation of the project title: Smart INitiative of cities Fully cOmmitted to iNvest In Advanced large-scale energy solutions) promoted large-scale, integrated and scalable solutions for the energy supply of buildings in medium-sized European cities. The initiative focused on cooperation between the cities of Innsbruck in Austria and Bolzano in Italy. In each case, the aim was to reduce primary energy consumption by 40 to 50% and increase the share of renewable energies by 20% in each pioneering district. This was achieved through a comprehensive package of measures combining the modernisation of more than 100,000 m² of living space (2/3 in Innsbruck and 1/3 in Bolzano) with the optimisation of the electricity grid and solutions for district heating and cooling [SINFONIA].
From demonstration project to replication on a large scale
This project focused on the development and implementation of scalable solutions in the two pioneer cities. To achieve this, district typologies and corresponding renovation models were defined. These are intended to enable cities to easily assess their requirements and efficiently define their long-term renovation strategies. The selected buildings from the two districts in Innsbruck and Bolzano are typical existing buildings, most of which date from the same decade and reflect the challenges of energy-efficient renovation found in most European cities. To ensure scalability and transferability, these models and typologies were not only tested and validated with all stakeholders in the participating cities of Innsbruck and Bolzano, but were also observed by five so-called early adopter cities that actively participated in SINFONIA; these “early adopters” were the cities of Paphos in Cyprus, Seville in Spain, La Rochelle in France, Borås in Sweden and Rosenheim in Germany.
The two housing associations involved in Innsbruck are Neue Heimat Tirol (NHT) and Innsbrucker Immobiliengesellschaft (IIG).
Preventing the lock-in effect
Studies conducted as part of the SINFONIA project, in which districtPH was used to calculate energy balances for city districts, clearly show that the Passive House standard or the EnerPHit standard developed for energy-efficient renovations as a minimum standard help to achieve global climate protection goals.
With lower standards or a delayed implementation of these requirements, the necessary targets will not be achieved, or at least not within the available time frame: the greatest amount of CO₂ emissions will be saved over a period of 50 years if not just the renovation rate but also the quality of renovation is increased. Comprehensive energy-efficient renovation is a unique opportunity for effective climate protection. Quality is therefore a top priority.
A poor quality of renovation means that the opportunity for good climate protection in coming decades will be missed. Higher renovation rates of a lower quality will lead to lower emissions in the first ten years. However, the cumulative emissions over 50 years will be higher with a low quality of renovation than if the implemented renovation measures were of a high quality standard from the outset. The buildings would remain at an average level of efficiency for decades. In order to avoid this lock-in effect, it is more effective in the long term to implement energy-efficient renovation in a step-by-step manner and to strive for the highest quality with each individual measure.
Reasons for step-by-step retrofit
Renovation to the EnerPHit standard can be carried out in one go or in several steps. There are many reasons for step-by-step renovation. Financial reasons can play a role, as can technical reasons (urgent renovation of one component while other components are not yet at the end of their life cycle) or legal reasons (tenancy law or other legal requirements that prevent extensive renovation).
All residential buildings renovated as part of the SINFONIA project are subsidised social housing, where local authorities provide affordable housing for people on low incomes. Available homes are allocated to registered persons in a specific order. The rights of these tenants are protected by tenancy law, which is very strict in Austria and offers residents a high level of protection. On the one hand, this makes good sense, but on the other hand, it can make it considerably more difficult or even impossible to make changes to the building, such as comprehensive thermal renovation. In Austria, depending on the measure, a certain percentage of the residents must consent. General measures require the consent of at least three quarters of the residents; in the case of changes to the heating system and/or hot water supply, all residents must agree. The installation of mechanical comfort ventilation systems in homes requires the consent of each individual resident, otherwise installation is not possible. Because the apartments in the SINFONIA project were renovated while occupied, there were many reservations on the part of the residents and in some cases less acceptance, particularly with regard to the retrofitting of a mechanical ventilation system.1)
EnerPHit retrofit plan (ERP)
In 2015, the Passive House Institute introduced a certification system for step-by-step retrofit on the EnerPHit standard. A step-by-step renovation requires the creation of a comprehensive renovation plan in which several renovation steps are carefully planned. The plan documents, for example, the location of the airtightness layer and the insulation layer, as well as schematic representations of the component connections. Only with such a plan will a step-by-step retrofit ultimately result in an energy-efficient and comfortable renovated building. An online certification platform facilitates the exchange and organisation of documents. This is particularly important as the renovation steps can take several years to complete. The first step of the renovation is pre-certified and the EnerPHit certificate is only issued after the final step, when all EnerPHit criteria have been met. Each individual retrofit step is calculated in the Passive House Planning Package (PHPP).
The schedule in the ERP ensures that the current status of the individual components is recorded and graphically displayed, showing when repair work needs to be carried out and when the component needs to be replaced. This approach provides an overview and bundles components that coincide in time into packages of measures.
It is possible to record the costs with the ERP. The investment costs, subsidy costs, annual maintenance costs and economic constraints are entered for the specific renovation measures. The sum of the annualised capital costs and the annual energy-related costs can be compared measure by measure.
The front page of the EnerPhit Retrofit Plan (ERP) for the IN40 apartment building in Innsbruck shows the interim results of the individual renovation steps. The ERP is a detailed document of up to 25 pages that contains all the information on the desired overall EnerPHit result and specifies how the individual steps are to be implemented. The ERP also contains information about the thermally relevant components, window quality, the ventilation concept, any potential photovoltaic system, etc. It also highlights the relationships between the individual renovation measures.
In the case of the IN40 apartment building in Innsbruck, two-thirds of the windows were replaced in the first step, and a quarter of the apartments were fitted with a mechanical ventilation system. In the second step, replacement of the remaining windows is planned. In the third and final step, the remaining apartments will also be fitted with a mechanical ventilation system with heat recovery, and the EnerPHit standard will thus be achieved.
Renovation examples (selection)
Various measures were implemented during the renovation of numerous buildings in Innsbruck. Of course, the most important measures were exterior wall insulation, window replacement, thermal bridge reduction, improvement of airtightness and the integration of ventilation with heat recovery. Due to the legal framework in force at the time of the renovations, heating systems and hot water systems could not be changed. For switching from decentralised systems to a centralised system it would have been necessary to obtain hundred percent consent from the tenants, which it is practically impossible to achieve. Apartment-based systems are the responsibility of the tenants without the property developers having any decision-making authority. However, the thermal building envelope of the modernised buildings has been optimised in a future-proof way, conversion of the building services systems can be carried out at any time and cost-effective heat generation systems can be used due to the greatly reduced heating loads.
Balcony
Part of the heat loss in existing buildings is due to thermal bridges. Balconies are particularly susceptible to this. In the past, balconies were usually constructed as cantilevered concrete slabs or with mounting brackets without thermal separation from the building envelope. There are two common ways to avoid these significant heat losses after renovation: either the balcony slab is wrapped in thermal insulation all around, or the old balcony is removed and replaced with a new, thermal bridge-free construction. The best solution in terms of energy efficiency would be to close up open balconies using glazing, but changes in use (heated space instead of open balcony) usually do not conform to building regulations and, based on experience, the vast majority of tenants do not want to do without their open balconies.
The first option, wrapping the balcony slab all around with thermal insulation, usually isn't feasible due to the shortage of space in the floor build-up. In addition, subsequent insulation of the exterior wall reduces the balcony area and leads to less acceptance among residents. In most cases in the SINFONIA projects, the second solution therefore proved to be the most appropriate. An example of this is the IN40 apartment building by Neue Heimat Tirol in Innsbruck, where the balcony construction was removed entirely. The newly built, almost free-standing balcony construction was implemented with just a few thermal bridge-free connections to the thermal envelope.
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For the IN22/23 project by Neue Heimat Tirol, the new freestanding balconies were significantly enlarged in order to provide significant added value to residents. The outside area of the homes was improved, with the balconies serving as fully usable outdoor spaces. This measure resulted in increased acceptance and agreement by tenants.
Basement ceiling
Many of the basements in the SINFONIA buildings have a low ceiling height. In many cases, the ceiling height is between 1.90 and 2.00 metres, and in some places it is even less than 1.80 metres. The basement ceiling is often a smooth concrete surface, and in some cases it is a ribbed ceiling construction.
Basement ceilings with a room height of less than 2.10 m in the basement could not be thermally improved for legal reasons in compliance with the prescribed minimum room height. In such cases, the EnerPHit criteria provide exemptions – as is also the case for listed buildings or components, for example – so that these restrictive framework conditions do not fundamentally stand in the way of energy-efficient renovation.
Mechanical ventilation with heat recovery
Experience has shown that retrofitting a mechanical ventilation system with heat recovery during renovation work while the building is inhabited is the biggest challenge. The construction work constitutes an invasion of privacy, causes the longest disruption to residents and creates the most dust in their homes. In addition, Austrian law requires the consent of each individual resident for entering the rented property in order to carry out the planned renovation measures.
Both property development companies in Innsbruck (NHT and IIG) tried to optimise the installation effort and the order of further construction work (building contractor, ventilation technician, electrician, drywaller, painter, etc.). Temporarily vacant, unoccupied apartments were used to minimise the construction time and dust exposure. These model apartments made it possible to reduce the duration of construction work in the occupied apartments from several weeks to just five working days per apartment.
Various concepts were implemented in the SINFONIA project. Depending on the floor plan design, the number of approvals, technical feasibility, and much more, it was decided whether mechanical ventilation would be installed on an apartment-by-apartment or building-by-building basis and where the distribution pipes could be routed in the existing building.
Ventilation system in each apartment
- One ventilation unit per residential unit
- Horizontally positioned ceiling unit installed in the bathroom
- Outside air (AUL) and exhaust air (FOL) via the façade, distribution of supply air (ZUL) and exhaust air (ABL) in the suspended ceiling in the corridor
- Fire protection was not a problem
- Maintenance and filter replacement necessary in every apartment
Ventilation system for each building, distribution indoors
- One ventilation unit per building
- Installation usually in the attic area
- Outside air and exhaust air ducts through the roof, vertical distribution of supply air and extract air ducts in the stairwell on the inside
- Fire dampers between stairwell and apartment
- No maintenance or filter replacement inside the apartment necessary
The following images show how a central ventilation system with vertical air ducts can be implemented in a space-saving manner. Triangular openings in the stair landings allow for a vertical distribution system for supply and extract air without any restriction of the minimum stair width (1.20 m circle) required for the emergency escape route.
In the corridor area, the distribution box of the apartment is installed suspended from the ceiling and later disappears into the suspended plasterboard ceiling. Short ducts and valves above the room doors ensure a simple, efficient distribution system within the apartment.
Ventilation system in each building, distribution indoors and outside
- One ventilation unit per building
- Installation usually in the attic area
- Outside air and exhaust air ducts through the roof, vertical distribution of supply air in the stairwell on the inside and of extract air outside on the façade
- Fire dampers between stairwell and apartment or FLI-VE 2) for extract air duct in the exterior wall
- No maintenance or filter replacement inside the apartment necessary
In the project described here, the corridor area was used as a fresh air reservoir. The supply air comes directly into the corridor from the vertical distribution system in the stairwell. According to the original plans, the supply air would be introduced into the individual rooms from the corridor via active air transfer devices. This solution was actually tested and measured in an empty model apartment. However, a solution without active air transfer was implemented instead, using natural air exchange via open doors. Feedback and experiences have been requested from the property development company and were not yet available at the time of writing.
Ventilation system in each building, distribution outside
- One ventilation unit per building
- Installation usually in the attic area
- Outside air and exhaust air ducts routed through the roof, vertical distribution of supply air and extract air outside on the façade
- FLI-VE for supply air and extract air ducts in the exterior wall
- No maintenance or filter replacement inside the apartment necessary
This solution received the highest approval rating in the SINFONIA project beforehand and also the highest satisfaction rating after implementation. Only one core-drilled hole was required in the exterior wall in each room, which greatly reduced noise and dust exposure inside the apartments.
The existing floor plan (supply air rooms in the south and extract air rooms in the north) as well as an existing insulation layer of the exterior wall (including adhesive and plaster: 80 mm) proved advantageous. The façade-mounted ductwork was installed in slots within this 80 mm layer. The new 200 mm thick insulation could thus be installed over the ventilation ducts across the entire surface without cut-outs.
Prepared for, but not yet connected
An important aspect of step-by-step retrofit is that subsequent measures are taken into account when the preceding measures are implemented, so that consequential costs can be minimised or avoided, or so that already incurred costs do not have to be expended again.
During the retrofitting of the mechanical ventilation systems in the SINFONIA project, care was always taken to ensure that the necessary construction work was carried out as far as possible, even if certain residential units did not want ventilation. If the tenants of these apartments do express a desire to have a mechanical ventilation system installed at a later date, or in the event of a change of tenants, these apartments can be easily connected – in the case of IN22/23, for example, to the central system – because all the necessary preparations were made during the implementation of the first step.
Shown in the following illustration: distribution ducts, silencers, dimensioning, etc. are prepared; only the connections need to be connected to each other in order to retrofit the missing apartment in this stairwell.
Measurement results
In the numerous renovated SINFONIA buildings in Innsbruck, the University of Innsbruck (Energy-Efficient Construction Unit) carried out measurements relating to consumption and thermal comfort in the renovated buildings, and the consumption data at building level was evaluated by the Passive House Institute [MONITORING]. The consumption data for heating reflects the stage of renovation achieved within SINFONIA. Usually, both the calculations and the measurements are carried out for the entire building. However, the situation in these neighbourhoods proved to be considerably more complicated. Changes were consistently made to many of the old buildings, also to individual apartments. Various systems and energy sources (electricity, gas, district heating, wood, coal) are used for heat supply, and as a rule there are no total meters for the buildings since the supply is predominantly decentralised. The central district heating supply for all apartments could only be evaluated in one of the residential buildings. In the other cases, individual apartments were measured instead of the entire building, for which the consent of the respective residents had to be obtained. The results from these samples were extrapolated to the corresponding buildings and the associated statistical errors were determined. For this analysis, the expectancy range in which 95% of the measurement results will lie statistically (confidence interval) is relatively large due to the above-mentioned limitations.
The monitoring data for the more than 29,300 m² examined shows that, on average, the retrofits reduced the heating energy consumption values for useful heat to just 31.8 kWh/(m²a) (reference: treated floor area “TFA”). An overview of the measurement results for eight residential buildings and two schools is shown in Figure 16. Since not all of the projects had been fully renovated in the first stage, particularly with regard to windows and ventilation technology with heat recovery, the results vary considerably between 10 and 58 kWh/(m²a). These results are the unchanged measured values, without any calculated correction of the consumption values due to deviating room temperatures. For this reason, the measured values correspond to the actual room temperatures (“real” °C). The project with the highest consumption (BEST19) has a particularly large confidence interval, which is due to the small number of apartments measured. For this project it must be considered that far ventilation technology with heat recovery had not been implemented in the monitored apartments so far.
After processing, the measurement data is compared with the PHPP's preliminary calculations. In addition to the standard boundary conditions (with typical average usage, a winter indoor temperature of 20 °C and a climate data set corresponding to the long-term average in Innsbruck), a further variant uses the actual conditions at the building, as far as they are known, to enable a realistic comparison. All these calculations are determined using the PHPP as a realistic tool that correctly reflects the decisive parameters. This results in a comparative value for the heating demand that can be compared with the measured consumption, taking into account the remaining uncertainties. A comparison of the average consumption of all eight residential buildings and the two schools with the calculated values in the PHPP shows good correlation within the limits of accuracy of the measurement data mainly (Figure 17). With a well-maintained and complete PHPP calculation, reliable predictions can already be made during the planning phase. This confirms once again that the PHPP is a reliable planning tool that is also very well suited for renovation projects.
The PHPP was also used to calculate the heating demands of the existing buildings before renovation. The data for the existing buildings are calculations for uninsulated or only partially insulated buildings without ventilation technology, with old windows and poor airtightness. For this purpose, the calculations are based on standard conditions (Ti = 20 °C) for the long-term climate in Innsbruck and typical conditions of use. The area-weighted average value for all residential buildings is 140 kWh/(m²a). Compared to the measured value, this results in a saving of 77% in useful heat for heating the buildings.
To determine the magnitude of the absolute savings, the heated area of the 8 residential building projects and the two schools (29,351 m² of living space TFA) evaluated can be calculated using the difference between the measured consumption value and the calculated demand value of the unrenovated existing buildings. The savings achieved for these buildings amount to approximately 3,182,000 kWh per year (3.2 GWh/a).
Potential for complete renovation For the SINFONIA project, in the case of the retrofits in Innsbruck it was of interest to know the extent of the savings for the overall project once all renovations were completed. To this end, additional ventilation systems with heat recovery and thermally optimised windows are to be retrofitted. For the nine residential building projects and the two schools, savings of 85% compared to the existing buildings are expected. The average heating consumption will fall to around 21 kWh/(m²a). This would save around 3,950,000 kilowatt hours of heating per year (equivalent to 3.9 GWh/a). This means that the “nearly zero” heating energy consumption required by the EU will be achieved.
The documented results clearly show the enormous potential of energy efficiency in building renovation. This project proves that this potential can be realised not only in theory but also in practice. There is an urgent need to intensify efforts in this direction. The expertise and necessary products are available to enable replication on a massive scale. There should be no continued discussion of reservations; instead, possible measures should be implemented urgently throughout the EU and worldwide. With its proven successes, the SINFONIA project is an encouraging forerunner in this regard.
Quality assurance
Ultimately, the question arises: why do the buildings renovated as part of SINFONIA perform so well? Their good performance can be attributed to conscientious quality assurance. Two EnerPHit certificates and 15 ERP pre-certificates were issued for the SINFONIA buildings in Innsbruck. The Passive House Institute only issues these certificates if precisely defined criteria are met. This gives building owners the certainty that the desired and expected energy standard will actually be achieved – living comfort included.
Quality assurance is the key to success!
The preliminary results and findings of the EU-funded SINFONIA project have already been partially incorporated into the peer-reviewed paper entitled “Retrofit with Passive House components” in the Energy Efficiency Journal [ENERGY EFFICIENCY].
Acknowledgements
SINFONIA was funded by the European Union's Seventh Framework Programme for Research, Technological Development and Demonstration under Grant agreement No. 609019.
Literature & References
[GABC] Global Status Report. Global Alliance for Buildings and Construction, 2024. , accessed on 03.06.2025
[SINFONIA]Sinfonia | Low Carbon Cities for Better Living. Sinfonia, 2025. , accessed on 3 June 2025
[MONITORING] Comparison of measured and calculated energy demand and assessment of the results. Peper, Søren et al, 2021. , accessed on 2 July 2025
[ENERGY EFFICIENCY] Retrofit with Passive House components. Bastian Zeno et al., 2021., accessed on 2 July 2025