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 Wolfgang Feist, Witta Ebel is availabe in German in the proceedings, which can be accessed here.

The iPHA Fact sheet can be found here - Cost effective renovation of residential buildings: If you do it, do it right!

An energy-efficient building renovation using Passive House components (EnerPHit ) is the Passive House solution for existing buildings. The EnerPHitconcept is able to respond to the individual characteristics of each renovation project. There are many ways in which improvements can be implemented in practice – but one basic principle unites them all: “If you do it, do it right.” Regardless of the reason for renovating an existing building, every single measure is designed to achieve an energy-efficient and economically optimised solution.

The PHI has regularly addressed this topic within the framework of the Research Group on Cost-effective Passive Houses and, together with partners, has also carried out several pioneering EU projects on the development and practical demonstration of the EnerPHit concept:

• EuroPHit: demonstrating step-by-step implementation
• outPHit: demonstrating execution of work using prefabricated components as far as possible
• Sinfonia: large-scale demonstration in an urban context (energy concepts with actual implementation in Innsbruck and Bolzano)

Comprehensive reports and online resources, planning details and documentation are available for these EU-funded projects [EuroPHit], [outPHit], [Sinfonia].The concept of the EnerPHit Retrofit Plan (ESP)¹⁾ developed in EuroPHit has proven itself in these projects. At the same time, planning aids and implementation tools have been created to facilitate practical implementation. At the heart of this is the PHPP (Passive House Planning Package [PHPP]), which can also be applied for existing buildings and to renovation processes in particular [Theumer 2025].

The new online version of the tried and tested energy consulting programme ENBIL offers a particularly attractive start. It allows existing buildings to be characterised in an intuitive and interactive way – based on this data, a PHPP data set is quickly and easily generated, which can then be used by an energy consultant or planner. ENBIL users can also quickly obtain an overview of potentially available measures for their building and access numerous predefined retrofitting steps and assess their economic viability. This online service is available free of charge [ENBIL].

With the “Efficiency Now!” initiative, the PHI has put together exemplary renovation measures for buildings. In many cases, these can even be carried out by homeowners themselves; however, prior consultation with a specialist is always helpful. ENBIL (see last section) can help you select measures for your own building. You can access an overview of the documentation on Passipedia explaining the measures, which can be downloaded free of charge as PDF files (German) or in the overview here.

In this context, you will also find important information on compliance with general principles of basic principles, e.g.

• The principle “if you do it, do it right”: Better (thicker!) thermal insulation improves energy savings as well as thermal comfort and quality in terms of building physics. Contrary to widespread preconceptions, the cost-effectiveness of thermal insulation measures (up to the EnerPHit level, which is the optimum) is improved when they are implemented with lower U-values; the optimum range is that of the Passive House characteristic values.

• The “high interior surface temperature” rule: Thermal insulation, regardless of the building component, should always be at least good enough to prevent condensation on interior surfaces in winter. This not only saves energy costs, but also prevents structural damage caused by condensation.

• Particular attention must be given to protection against moisture in the case of interior insulation. There are a number of proven concepts for this Thermal protection using interior insulation, which are described in detail in the instructions. It is crucial to ensure that there is no indoor air flow behind the interior insulation. In Central Europe, protection aginst moistre is generally also improved due to exterior insulation (it is important to ensure that the external surface is not executed to be vapour-tight – with the building constructions and materials available today, this is not necessary e.g. ventilated roof construction or inverted roof)

It is recommended that Passive House or EnerPHit-certified constructions are always used. Manufacturers can gain access to advice and certification at Optimize components (German):  

In the article [Pfluger 2025] in this Protocol Volume it has been explained why adequate home ventilation is an indispensable measure for every high-quality building renovation.

Ventilation is important not only for improving indoor hygiene (indoor air quality), but also for preventing structural damage caused by excessive air humidity (often on surfaces in winter) or insufficient air humidity. This was clearly demonstrated by the problems encountered during the Covid pandemic.

The principle “if you do it, do it right” also applies to home ventilation. A ventilation system with heat recovery that is as efficient as possible should be retrofitted at the latest when windows are replaced and airtightness is increased.

The wide range of available Passive House certified ventilation units can also be used for the renovation of existing buildings.

In his presentation, Rainer Pfluger presented a bunch of concepts for achieving cost-effective ventilation solutions, especially for renovation projects:

• “The best duct is: no duct” or: keep the duct lengths as short as possible. By utilising fluid dynamics (e.g. the Coanda effect), an air inlet can be installed with a short supply air duct below the ceiling at any point in the room – wherever access is possible at a low cost.
• “Directed air flow”, also known as “cascade ventilation”, allows for short supply air and extract air ducts as well as sensible multiple use of the air flows. The air flows from the supply air rooms through the corridors into the extract air rooms. With extended cascade ventilation, living areas can also become air transfer zones if the floor layouts are appropriate.

Figure 1: Vertical section of door lintel with air transfer vents [Ebök]

• This requires passive air transfer vents that connect supply air rooms with exhaust air rooms. There are numerous tried-and-tested components available on the market today for this purpose.
• For ventilation units with heat recovery, it is advisable to look for PHI certification, which tests for low noise levels, good air quality, a good level of airtightness, low power consumption and high heat supply efficiency. These devices are available as large units for centralised building solutions, and as smaller units for home ventilation, and also as room ventilation units for a single exterior wall opening in a room.

Balanced air flow is important for energy-efficient operation: this can now be achieved in all device classes with modern measurement and control technology – an important step that the PHI will be pushing for in future new certifications.

Around 90% of ventilation heat losses in renovated buildings can be prevented through high quality heat recovery: this significantly reduces the winter heating load, due to which the required output of heat pumps and heat transfer systems (and thus also the investment costs) can be lowered. In this way, excessive strain on the power grid during cold periods can be avoided, and implementation of the energy transition can be significantly facilitated Of course, the same applies to improved thermal insulation of the building envelope..

The article [Schnieders 2025] discusses future-oriented solutions for heating in renovated existing buildings. An analysis of the energy-related boundary conditions initially shows that in future, heating systems will be either

• sustainablysupplied district heating – or
• high performance heat pumps

This is mainly due to the expected increased costs for fossil fuels. The PHPP analysis (based on the PEr method [PEr 2020]) had already shown this result a good ten years ago: replacing the fossil fuels still predominantly used today with sustainably produced fuels on a large scale would involve significantly higher costs – biomass availability is far from sufficient and the high losses associated with power-to-gas (P2G) conversion preclude the use of such synthetic fuels for heating buildings, except in very exceptional cases; the fuels produced in this way will hardly be enough even for the priority application areas (aviation and backup power plants during periods of low energy availability in winter).

Heat pumps used in new Passive House constructions have already proven their worth – today, almost all newly constructed Passive House buildings are equipped with heat pumps. Due to the low heating load (around 10 W/m²), the flow temperature can be kept low, enabling the use of cost-effective solutions for the heat distribution system.

EnerPHit renovated buildings also have low heating loads of around 20 W/m². Thus with renewal of the heating system it is possible to continue using the existing heat distribution and heat emission systems (usually radiators) and achieve a low flow temperature (important for the efficient operation of heat pumps). There are a number of alternative solutions for hot water generation – here too, continued use of an existing distribution system is usually the easiest option, even if it is not the most economical solution in all cases. The efficiency of a solution with a heat pump for a central domestic hot water system with 60°C (legionella protection) is naturally associated with lower COP values compared to coverage of the heating energy demand with significantly lower necessary temperatures. An interesting option is to switch to a 2-pipe system with thermal instantaneous water heaters in the apartments, but this involves some installation effort. Heat recovery from shower water is now possible with a range of PHI-certified heat exchangers, and systems suitable for renovation projects are also available.

Decentralised electric instantaneous water heaters usually also lead to a lot of construction work in the apartments since a new three-phase power line usually has to be laid in the bathrooms; decentralised electrically heated small storage tanks increase the electricity demand even further. An interesting option is the use of electric compact hot water heat pumps with water reserves of between 80 and 250 litres, which can be installed directly in bathrooms. Due to the short pipe lengths, small water storage capacity and regular water renewal, these heat pumps can be operated at lower temperatures – their efficiency is therefore no lower than that of central heat pumps, and there are no heat losses from central storage tanks and the distribution network. Various solutions of this type were discussed in the article [Feist 2025]. A number of pilot projects have already been implemented and PHI measurement campaigns are in progress to determine the practicability and efficiency during operation in practice.

The comparison of the economic efficiency of different building technology solutions from [Schnieders 2025] and [Feist 2025] is summarised here.

It was found that in EnerPHit renovated buildings, heat pump solutions are consistently more economical compared to continued operation of the old fuel-powered system (BAU = business as usual). The central concepts of heat pumps hardly differ in terms of total life cycle costs, so a solution that optimally matches the individual case can be selected here. For buildings with many residential units (more than 10) connected to a single heat supply line, centralised and decentralised solutions are roughly equal in terms of costs. For buildings with few residential units on the other hand, the decentralised use of air-to-air heat pumps (also known as “split air conditioning units” or “split units” for short) is a good option for EnerPHit renovated buildings. These can be installed in stages, initially while the existing heating system continues to be in operation. As the renovation of the building progresses, these air-to-air heat pumps can cover an increasing share of the heating demand and also cover it completely once the modernisation is complete; this has already proven practicable in pilot projects.

The article [Lepp 2025] in this Protocol Volume documents EnerPHit renovation projects that have already been implemented.

The solutions implemented in the recently completed [Sinfonia] project in Innsbruck are particularly diverse: there are numerous examples here showing how frequently encuntered challenges were overcome:

• Preparation of component connections for measures to be carried out at a later date
• Attractive solutions for existing balconies: these include both radical approaches (demolition and new balcony) and the encasing and mitigation of thermal bridges through minimally invasive measures. The solution that is best suited to the specific situation that exists can be selected.
• A number of very different approaches for home ventilation were also implemented, all of which use highly efficient Passive House heat recovery systems. Acceptance of home ventilation is high among the residents.

In Innsbruck the work was mainly carried out according to the step-by-step implementation concept. However, there are also other examples (Arheilgen, Gießen, Nuremberg) where renovation of the entire building took place in a single conversion campaign based on prefabricated systems.

Within the framework of the Sinfonia project, the implemented building renovations were also evaluated using measurement technology. Figure 19 in [Lepp 2025] shows the summary results: in the final state, the EnerPHit renovation leads to 85% savings in heating energy.

In this Research Group (AkkP 62), we have shown that the challenges of the energy transition can be overcome also in the case of existing buildings. The EnerPHit Retrofit Plan offers cost-optimal customised solutions for existing buildings.

The decisive factor is to use the coupling principle (“if you do it, do it right”) to make cost-effective solutions possible: whenever a component is due for renewal, the energy efficiency of that component is also improved at the same time – optimised to an overall sustainable level.

In most cases, heat can then be supplied using heat pumps; alternatively, a connection to a heating network can be used where one exists.

Frequently, renewable energy generation with a PV system on the roof or façade can also be integrated into the retrofit plan. The PHPP takes this into account and allows optimisation also for this part of the solution, e.g. to the EnerPHit Plus standard, which in most cases leads to a net zero energy balance for buildings with less than three full storeys. However, electricity continues to be drawn from the grid in winter; the appropriate assessment method is PEr [PEr 2020], which takes into account the consequences of the non-simultaneity between demand and supply. Renovation to the EnerPHit standard will then result in only a minimal load on the grid.

The very low energy demand of EnerPHit buildings during operation makes low operating costs and optimised life cycle costs possible.

The very low peak loads that are also achieved (around 6 Wel/m² of living space for electricity with EnerPHit heat pump solutions) make it easier to achieve the energy transition objectives with an acceptable level of expenditure for seasonal energy storage and the electricity grids. The value for PEr (primary energy renewable) has proven to be a useful optimisation parameter.

Overall, it can be said that renovation according to EnerPHit guarantees a customised and optimised solution: the intention must be followed by action.

Refurbishment with Passive House components

Webinar: Heating with heat pumps in Passive House retrofits

Heating and cooling with split units in EnerPHit renovations