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 Susanne Theumer is availabe in German in the proceedings, which can be accessed here.

We are all familiar with this kind of situation: you need a new heating system, new windows or even a complete renovation. But overpriced air-to-water heat pumps, increased costs coupled with a shortage of skilled workers, the fear of exposure to noise, dirt and the effort involved in installing a ventilation system, and an unclear political framework pose major challenges. In addition, those willing to renovate are overwhelmed with an abundance of sometimes controversial recommendations, so that a lot of time has to be invested if one wants to obtain comprehensive information. What to do? Looking at the renovation rate that continues to be low, it is evident that most people are holding off, even though energy improvements are urgently necessary for climate protection.

Session 62 of the Research Group for Cost-Effective Passive Houses will therefore deal with the cost-effective renovation of residential buildings. The overall cost, including the costs resulting from inaction will constitute the benchmark. The research group will identify options for action that are affordable and good for both the occupants and the climate. In particular, the topics of heat pumps and the integration of ventilation systems with heat recovery will be examined in detail. This is because the better insulated the building envelope is and the more heat is recovered by the ventilation system, the lower the flow temperatures required and the smaller and more cost-effective the heat pump will be.

Occupants benefit immediately from the improved thermal comfort resulting from energy efficiency measures for the building envelope and have very low running costs (heating, ventilation, hot water, maintenance) in the long run. Furthermore, they can preserve and increase the value of their property. User satisfaction and a healthy living environment cannot be overestimated.

Aiming for “EnerPHit”, i.e. retrofitting with Passive House components, as the renovation goal has been and is advantageous in every respect. It is the right thing to do in terms of ecology AND economy, because although the initial investment is higher, in the long term one can achieve maximum savings in this way and become less dependent on future energy prices due to the extremely low energy consumption. In November 2024, the Passive House Institute (PHI) published a three-page position paper entitled “The existing building stock in the context of climate protection” [Positionspapier_2024]. It contains the key arguments in favour of EnerPHit renovations. We have compiled the various EnerPHit approaches (complete renovation, component by component, even especially cost-effective DIY projects, etc.) in the EnerPHit brochure “EnerPHit – Renovating with Passive House components – Solutions for fast, cheap and reliable deep retrofits” (basics as well as examples of buildings and details) [EnerPHit brochure_PHI_2024].

An EnerPHit building renovation requires a plan, and of course, funding is also welcome. Certified Passive House planners are available to provide advice and assistance, from inception to optimisation and cost-effective implementation. In addition, the building certification services offered by the Passive House Institute ensure that the quality is right. The rule is to go ahead with it, but not with “just anything” – rather, the right thing for your wallet AND for climate protection.

This is possible and has already been implemented many times, as demonstrated by the building renovations to be presented today. However, “go ahead with it” is also the rule for experts in the sense of “making it as easy as possible for the customer”: let us clearly formulate the goals and solutions for achieving these.

For implementation, there is no one solution but rather, a wide range of options, all of which lead to the same goal sooner or later, that will now be presented with a focus on renovation of residential buildings. For example, in [AkkP58] it was shown that it is not only insulation materials based on renewable raw materials that result in a building that is sustainable over its life cycle, but that above all it is essential to apply sufficient insulation. A U-value of 0.15 W/(m²a) for walls and 0.85 W/(m²a) for windows has proven to be a good target value. These characteristic values are reflected in the EnerPHit component method, see [Building criteria].

Take advantage of opportunities and do it right! That's the coupling principle: whenever a measure is already due in any case, combine it with a future-oriented thermal protection measure. For building renovation, the motto is: “If you do it, do it right!” – namely, in a cost-optimised manner using Passive House components.

In this way, we can achieve affordable building renovations with good air quality and thermal comfort that reduce the impact on our environment to a tolerable level and thus effectively counteract the climate crisis.

For a building retrofit to be successful, one has to have a definition of the goals, expert support throughout the entire process from the initial idea onwards, a reliable planning tool, highly efficient components, competent implementation and independent verification. In place of external expert support, one can take the time to develop one's own expertise and take the renovation into one's own hands. Competent implementation can even be carried out in a do-it-yourself manner to a large extent [NOW!], [Passipedia_Renovation].

Owners of existing buildings need expert advice with farsightedness. The solution with EnerPHit is recommended as it is highly energy-efficient and economically optimised over the lifecycle of the building, taking into account the different lifespans of the components. The consultant will assess the individual starting situation and will provide a basis for decision-making. For this, a tool will be used by the consultant that realistically calculates the energy balance and compares the appropriate options. The consultant will recommend appropriate renovation steps and advise on the linking of measures (e.g. window replacement combined with the installation of a ventilation system with heat recovery).

Components that deliver what they promise are necessary for the implementation. In addition, competent skilled workers or personal contribution are necessary to ensure correct execution of the work, and independent quality assurance is required (Figure 1) so that a result is achieved that is satisfactory in every respect.

Figure 1: The recipe for a successful, cost-effective building retrofit

State funding or regional subsidies can serve as incentives. The Passive House Institute provides specific recommendations for implementation in this respect [Passipedia_Kommunen]. Subsidies should not make renovation more expensive, nor should building renovations be carried out simply because they are subsidised. Instead, one should do what makes sense and is necessary with regard to the specific building and the individual starting situation. As Wolfgang Feist puts it: “What matters is: the most cost-effective approach is doing what makes sense. And EnerPHit is exactly that, a method for finding the most cost-effective solution.”

In Germany, elegibility for the subsidy must either be provided in advance by demonstrating an “Efficiency House” standard using a DIN V 18599 tool or, in the case of subsidies for individual measures, by complying with specified component qualities. EnerPHit certification can be subsidised to 50% of the costs through construction supervision if an energy efficiency expert is commissioned. An easier way is to deduct the costs for renovation measures and the consultancy/certification subsequently against tax – with the same required level of energy efficiency as for individual measures.

In Austria too, various subsidy programmes at the state level can be used to meet the higher investment costs.

Regardless of any subsidies, it is generally advisable to follow the principle: “if you do it, do it right”: if a component of an old building is already being “tackled” anyway, e.g. scaffolding is being erected, a roof is being repaired or a window is being replaced, then it is very important that this is always done in a truly energy-efficient manner. It is usually a very long time before such a component becomes “due” for renewal again. The cost difference between an “already due anyway” renovation and an EnerPHit measure is small and worthwhile. [AkkP_39; Planner's Handbook].

Figure 2: EnerPHit Retrofit Plan and first step of an EnerPHit building renovation, known as pre-certification: 20% energy savings achieved

First of all, the goal must be set: what can be achieved and what do I want to achieve? Nowadays, it is proven and well known that the “EnerPHit standard” is well suited as the goal for a building renovation as it is:

• a sustainable, future-proof standard - cost-optimised over the lifecycle - comfortable, free of structural damage, fresh air is always ensured, etc.

Energy-efficient renovation with Passive House technology in existing buildings is MORE than just energy efficiency; it is worth mentioning economic efficiency as an example: In the case of building renovation, the question inevitably arises as to what criteria should be used to determine the level of thermal protection for the building envelope. The recommendation is: achieve the economically optimum level of thermal protection for each component! Although every economic efficiency calculation includes an ABSOLUTE economic optimum, it is better to speak of an “optimal range” due to the usual uncertainties involved in such calculations (assumptions about interest rates and future energy prices). If an insulation thickness at the “upper” end of the economically optimal range is selected, then greater independence from future energy price increases can be achieved with approximately the same profit – while also contributing to reducing the risks posed by climate change. [AkkP_42; Passipedia_Wirtsch.]

Much depends on the initial situation: a housing association that wants to renovate apartment blocks while they are inhabited, in which there is a backlog of renovation work or where individual flats can only be renovated when the tenants move out, has different priorities to a homeowner who discovers that the roof is leaking. What both have in common is that the long-term energy-relevant goal of the renovation should be “EnerPHit”. But this goal can also be achieved in individual steps, apartment by apartment, or component by component. An airtight, thermally insulated envelope is the basis for a successful EnerPHit renovation, but even if this is sometimes not possible and the heating system has to be replaced first, it is still advisable to use Passive House components as used in new buildings.

Figure 3: EnerPHit Unit seal for a renovated flat

Step-by-step retrofit can be pre-certified with an EnerPHit Retrofit Plan (ERP) if energy savings of at least 20% are achieved, see Figure 2.

If a single apartment in an existing building has been renovated, the “EnerPHit Unit” certificate can be awarded, see Figure 3.

EnerPHit Unit: This is a variant of the EnerPHit component method. Individual modernised residential and commercial units in buildings with more than one unit can be certified. The unit in question must be modernised entirely using Passive House components. The rest of the building is not taken into account for certification.

Communicating this goal clearly and early on to all parties involved is important for successful renovation.

A specialist will assist in defining the goal, but the Passive House consultant will check more than just “yes” or “no” with regard to the EnerPHit standard. Among other things, they will determine what needs to be done to achieve the EnerPHit Plus standard and how the set goal can be achieved with simple, robust planning in the most cost-optimal way possible. Renewable energy, e.g. in the form of a PV system, can be added to the EnerPHit Classic proposal at any time, but the available unshaded area, its orientation and line routing (e.g. installation in a chimney that is no longer used for heating) can already be checked at an early stage.

Is the building a listed building? The same will then apply: renovation with Passive House components as best as possible – wherever this can be done [AkkP 32].

EnerPHit is a clearly defined modernisation standard that offers the flexibility needed for existing buildings. The specialist will decide how EnerPHit is verified depending on the individual situation, either via the component method or the energy demand method. Justified exceptions are allowed, e.g. due to stipulations by the responsible monument protection authority or due to lack of space (basement ceiling insulation with low room height); EnerPHit thus offers the flexibility necessary for the modernisation of existing buildings. The “EnerPHit Retrofit Plan” (see article by Laszlo Lepp) documents the interdependencies of the measures and the schedule.

If external insulation cannot or may not be installed on the building, internal insulation is recommended. Reference is made here to Research Group Protocol Volume No. 32 “Factor 4 also for sensitive old buildings: Passive House components + interior insulation” [AkkP 32]: 40-100 mm internal insulation is advisable (to achieve a U-value of 0.40, see component criteria). The specialist will also give attention to airtightness. Competent advice on building renovation is based not on (potentially changing) subsidies, but on solutions that are optimal for the client in the long term. The fact that improvements to the building envelope DIRECTLY lead to a reduction in operating costs is a major plus.

The PHI has been leading the way in energy-efficient building modernisation for decades. The PHI's advanced education programmes [Passipedia_Weiterb.] convey the findings from research on building renovation. The PHI works together with the energy consultant networks DEN and GIH, among others.

The Research Group Protocol Volume No. 24 “Use of Passive House technologies in the modernisation of old buildings” [AkkP24] had already suggested the use of components used in new Passive House construction for the renovation of existing buildings. In 2012, the EnerPHit planner's guide was published, which deals with the EnerPHit standard in a detailed and comprehensive manner. In the EU project EuroPHit, which ran from 2013 to 2016, the PHPP was upgraded for the calculation of existing buildings prior to renovation [EuroPHit].

Even in the case of listed buildings, at least some individual measures can be implemented in the vast majority of cases: the roof can be insulated and a ventilation system can be installed. Sometimes new windows or interior insulation are possible. As part of the 3EnCult project, which ran from 2010 to 2014, besides Italian palaces we also looked at listed residential buildings and how they could be upgraded using Passive House components; a half-timbered building in Appenzell was one example of this [3EnCult].

The big advantage for listed buildings is that Passive House components can be used to remedy defects and prevent structural damage. The buildings are improved and can be utilised again.

Various cost-effective solutions using Passive House components have already been implemented throughout Europe and worldwide, including some examples from the EU Sinfonia project (see Laszlo Lepp's article in this Protocol Volume).

The approaches to achieving rapid, cost-effective EnerPHit renovation were demonstrated in the outPHit EU project using various practical examples. Here, too, measurements were carried out to monitor performance. [outPHit]

The Passive House Planning Package (PHPP) can be used for all buildings, including renovations. Over the years, it has been gradually expanded to include planning aids, particularly for modernisation. The PHPP is suitable for energy balancing of the initial state BEFORE renovation, as well as for the renovated EnerPHit state, even up to the Passive House standard. Many intermediate steps can be mapped by filling out the variants sheet and activating it in the other PHPP input sheets. In addition, there is a comparison worksheet in which two variants can be compared with each other, also in terms of their costs. The PHPP calculates the energy balance straightforwardly and accurately. This tool is comprehensible and transparent due to its Excel format. The PER methodology is also established in the PHPP:

The PER demand provides information about the extent to which a building will strain the future, fully renewable energy system ([Passipedia_PER], [Feist 2014], [Feist 2021], EnerPHit 2024, pages 43–47). It indicates how much renewable primary energy, e.g. PV or wind power must be generated in order to be able to supply the amount of final energy delivered at the property boundary, including conversion and storage losses (from summer to winter and from day to night), because electricity must be converted for storage. It therefore depends on when the electricity is needed ; electricity for heating is particularly valuable, while PV electricity is relatively abundant in summer and can be used for e.g. building cooling without problem. The PER demand is therefore a meaningful and future-oriented assessment criterion. However, the PE demand, which will become obsolete in the future, will continue to be calculated in the PHPP and reported in the verification sheet; for the time being, it may be used for certification as an alternative to the PER demand.

The PHPP is a transparent tool for future-oriented, climate-friendly building renovation with an energy balance, variants, comparison, EnerPHit Retrofit Plan and DIN V 18599 transfer sheet. With the PHPP, it is possible to makes informed planning decisions using robust energy modelling. This prevents incorrect optimisation. Energy efficiency and renewable energy complement each other! In the PHPP, this is realised so that one can choose from three Passive House and EnerPHit classes: Classic, Plus and Premium.

Passive House and EnerPHit (“Classic”) do not require the generation of renewable energy on site. The main characteristic of the two classes Plus and Premium is the fact that they also generate renewable energy.

Classification is based on the PER assessment. The classes for renovation with Passive House components (EnerPHit) were defined similarly to the Passive House classes, see Fig. 4 Passive House/EnerPHit classes.

Figure 4: Passive House/EnerPHit classes

EnerPHit Classic buildings have a PER demand of </= 60 kWh/(m²a) + an additional amount for a higher heating (/cooling) demand compared to Passive House buildings, EnerPHit Plus buildings have a PER demand </= 45 + an additional amount for a higher heating (/cooling) demand compared to Passive House buildings, and EnerPHit Premium buildings have a PER demand of </= 30 kWh/(m²a) + an additional amount for a higher heating (/cooling) demand compared to Passive House buildings. These classes do not affect the key criteria for Passive House/EnerPHit efficiency of the building (heating, cooling, airtightness). These remain unchanged and are the same for all classes.

In the PHPP verification sheet, the user simply selects the energy standard, the energy method and the class. In the PHPP sheet “PER”, a graph shows where the building ranks in terms of energy efficiency and renewable energy production and how far it is from the classification limits for the classes, see Figure 5. This helps with further optimisation.

Figure 5: PER chart from PHPP10 based on the example of a residential building ¹⁾

Depending on whether renewable energy and/or Passive House components are used in the renovation, the cross moves further to the left (= reduction in energy demand) and/or further upwards (= generation of renewable energy) with each renovation step.

Passive House and EnerPHit – theory and practice agree with each other. Since 1996, numerous comparisons between consumption measurements and energy balance calculations using the PHPP have demonstrated this (see, for example, [Johnston 2020] and articles in this Protocol Volume). A very good correlation exists when the same boundary conditions are applied (the PHPP calculation uses an indoor temperature of 20°C for the heating case; higher average indoor temperatures are often measured).

A simplified version of the PHPP has been developed specifically for small, less complex buildings (single-family homes): the “easyPH” tool. It can be used for both new buildings and renovations and, also allows EnerPHit verification in particular.

easyPH consists of just 8 PHPP worksheets. The central worksheet is “easyPH”, which uses a series of standardised assumptions to reduce the number of inputs required for the overall energy calculation of the building. To facilitate geometric input, easyPH can be used with designPH, the SketchUp plugin for PHPP.

easyPH is included in the PHPP 10.6 Master Bundle. This beta version of easyPH will initially only be available in English (as of June 2025). Nevertheless, easyPH can be used with immediate effect for worldwide certification of new buildings and renovations with just one or two residential units and a ventilation system.

But building owners should get a rough idea of the situation before the specialist gets down to business.

ENBIL The ENBIL energy consulting tool provides a quick and easy initial overview.

Figure 6: Left: ENBIL input mask Right: Results including recommendations and costs (ENBIL, PHI)

The online version of ENBIL (Figure 6) is a free, intuitive energy consulting tool for the renovation of residential buildings; rough building data is entered and energy savings and the cost-effectiveness of measures are displayed. Export of the PHPP calculation allows continue work with the PHPP energy balancing tool without needing to re-enter data: ENBIL(German).

While an energy balance using the PHPP is based on physical principles and is fully validated, since 2024 in Germany, proof of thermal protection must be provided with a calculation in accordance with DIN V 18599 together with the application for a subsidy from BAFA (Federal Office for Economic Affairs and Export Control) or the KfW bank. This means that two calculations must be made. To minimise duplication of work, it makes sense to continue planning with the PHPP and then use the 18599 transfer sheet, which is integrated into the current PHPP version. With the PHPP one gets a reliable energy balance calculation, and verification (according to the German minimum standard GEG or subsidy standard BEG) in a second step. The DIN V 18599 verification procedure is NOT suitable for optimising the building energy balance in a meaningful and cost-effective way, nor was it conceived for this purpose. It is a verification tool. Essential parameters simply aren't depicted, because only a building twin is created with specified component parameters that must be complied with. Thus, for example, neither the significant influence of the compactness of the building nor the effect of changing window sizes or window orientations can be determined.

Furthermore, drawing a comparison between buildings is not possible with the DIN V 18599 methodology. In contrast, this is very easy with the PHPP; the specific heating demand per square metre of living space can be seen at a glance in the verification sheet. If this value is lower than that of another renovation, the project is more energy-efficient.

In terms of climate protection, the EnerPHit standard makes optimum use of the potential for improving existing buildings. The high quality of the components avoids the problem of the lock-in effect. The climate protection targets set by Europe and Germany are reached! However, the financial incentive for implementing the EnerPHit standard rather than a mediocre quality is lacking in terms of investment costs. The only exceptions to this are some stipulations for building components if one takes advantage of the BAFA's subsidy for individual measures, see Fig. 7.

Figure 7: Comparison of component qualities: BAFA grants for individual measures and EnerPHit component method

With project planning according to the PHPP (simplified calculation method!), the EnerPHit criteria can be reliably met and the reality (heating, cooling, renewables, etc.) can be accurately depicted. Energy consultants can use this tool to transparently demonstrate the energy and cost savings that can be expected. At the same time, the EnerPHit component values stored in the PHPP can be used as verification for BEG funding, because EnerPHit criteria are the same or more stringent.

EnerPHit buildings deliver what they promise! This has been confirmed by measurements on several occasions. In 2005, a project in Tevesstrasse in Frankfurt was renovated cost-effectively using Passive House components, scientifically monitored and re-measured [Tevesstrasse_PHI]. As little technology as possible, but as much as necessary! For example, heat recovery ventilation systems are an important component in EnerPHit renovations.

The success obtained with the EnerPHit standard, which is impressive both in ecological and economic terms, is also due to the Passive House components and their interplay. The ventilation system with at least 75% heat recovery works well if the building is very airtight and operates with balanced air flows. The predicted heating energy savings through heat recovery (HR) will then be achieved. This is particularly worthwhile in cases where there is a high occupancy density, which simultaneously requires a high air exchange rate [IEA Annex_71_Renovation_MFH_Gießen_2023]. Even more importantly, HRV reduces the heating load, as explained in Rainer Pfluger's contribution to this Protocol Volume.

Components that were developed for new Passive House construction [Passive House components] are used in EnerPHit renovations. In addition, components specifically for customised solutions were made available for building renovations. These include sealing of beams (airtightness) for example, which is important for creating an airtight layer on and around the wooden beam head in an attic/false ceiling. Furthermore, there are component certification criteria for renovation systems for retrofitting or step-by-step additions to existing components.

Element façades, i.e. modular façade elements that do not require scaffolding, and building services components add to the product range for those interested in building renovation.

The PHI component team not only certifies products, it also carries out thermal modelling, provides reliable information for the PHPP and advises on improvement of components. Cooperation with manufacturers is very productive as this allows products to be effectively assessed, improved if necessary and ultimately made certifiable.

The PHI puts together training courses (theory and practice) with training materials that are offered in an extensible format [list of course providers] and that can be concluded with the Certified Passive House Tradesperson examination.

Furthermore, increasingly simple component solutions help to speed up implementation.

A functioning quality assurance process gives customers the certainty that EnerPHit actually means EnerPHit. Independent checking of planning is guaranteed by PHI-accredited building certifiers, whose numbers are increasing each year.

The dual control principle is applied because everyone makes mistakes. In the best case, errors are detected early on with the second review of the calculations. Liability for planning and correct execution remains with those responsible. Cost-effective certification is carried out on the basis of mutual trust (photo documentation instead of on-site inspections).

Many examples have been implemented that are worth emulating. In addition to the pilot projects and studies already mentioned, three of the most recent renovations are worth highlighting.

1) Passive House Plus apartment building in Cologne (Germany), a serial complete renovation, ID 7540
Affordable energy costs were the set and achieved goal of this serial complete renovation: the flat-rate rent (i.e. rent + low energy costs) is no higher than before, when tenants had to bear the rent plus high energy costs. The project was presented at the 27th International Passive House Conference in Innsbruck [27.IPHT] by architect Michael Kölmel (Zeller Kölmel Architekten GmbH) and is also featured in the EnerPHit brochure. It is also included in Protocol Volume No. 61 of the Research Group for Cost-effective Passive Houses “Serial energy retrofits according to Passive House principles” [AkkP 61].

2) Passive House Plus semi-detached house Wagner-Lubs in Aachen (Germany), ID 6544
This semi-detached house is also a cost-effective complete renovation, which was exemplarily optimised with regard to ventilation based on the cascade principle, air distribution via old chimney shafts and heat transfer via radiators. Thanks to very large but low-cost insulation thicknesses, this retrofit even achieved the Passive House standard.

3) Cost-effective social housing in accordance with the EnerPHit Plus standard (not certified) in Darmstadt, ID 7452
This PassiveSocialPlus building renovation was completed in 2019. This is a renovation project in which low warm rents (rent + heating costs) were achieved through the consistent reduction of energy consumption. The implementation of the warm rent model eliminates the need to read energy consumption meters, which saves operating costs. The project was scientifically supported by the Institute for Housing and the Environment in Darmstadt. The detailed final report examined the project from many angles. The combined EnerPHit Plus measures result in significant energy savings, and that in turn means enormous cost savings: monthly energy bills are approximately 40% lower than those of comparable social housing in Darmstadt. As a result, tenants have much less to worry about when it comes to utility bills! The building thus also contributes to combating energy poverty in Germany.

Further examples of cost-effective renovation of residential buildings are presented in the article by Laszlo Lepp.

Internationally, Passive House/EnerPHit is an exponentially growing market. The iPHA (International Passive House Association), a network initiated by the PHI, already has over 7,000 members from 58 countries and 23 partner organisations, which in turn actively promote Passive House/EnerPHit in their national languages at the local and national levels.

It is important to facilitate energy efficiency measures, especially for existing buildings. Good ideas and the elimination of obstacles are often more important than financial support. Take this example from Hamburg, where no permit is required for installing exterior wall insulation with a thickness of up to 25 cm. Eventually this may be extended to 40 cm of insulation, which would be helpful for prefabricated wall elements.

We must all work to counteract the explosion in construction costs: meaningful energy-efficient building renovations contribute to reducing the overall costs. Tried and tested simplifications should be chosen, obviously in compliance with the necessary requirements for sound insulation, fire safety and thermal protection, since they simplify our lives (living, sleeping, working, playing) in many ways. Weakening or abolishing them, or even promoting minimum standards, would send the wrong signal. The way we treat those most affected by energy poverty demonstrates our respect and responsibility towards our fellow human beings. A healthy society should aim to make life worth living for both current and future generations. Passive House and EnerPHit buildings offer a viable way to achieve this goal.

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Research Group 62 - Cost-Effective Refurbishments

Refurbishment with Passive House components

Certification as Quality Assurance