Interview with Christina Johanna Hopfe

Christina Hopfe is a full Professor in Building Physics at TU Graz and has been the head of the institute of Building Physics, Services and Construction (IBPSC) since May 2020. She has previously worked in Germany, the Netherlands, USA, and the UK. She is a Chartered Engineer, and a Fellow of CIBSE and IBPSA. Christina has been a director at large of IBPSA-World since 2014. Since October 2017 she has been the chair of the IBPSA communication committee.

Her research specializes in building performance simulation, low and plus energy buildings and communities, uncertainty quantification in building simulation models, optimization techniques, and advanced teaching methods for building performance. She is author and co-editor of the ‘Passivhaus Designer’s Manual’; shortlisted as Routledge bestselling books in architecture in 2016-2018. In 2013, she was one of 28 nominees shortlisted in ‘Stars of Building Science’ in the BRE’s Virtual Academy of Excellence. She is also the winner of the CIBSE Dufton Silver Medal for 2021, awarded for papers relating to fundamental research.

LHC: I will start with a question which was also asked in the paper “Ten questions concerning residential overheating in central and Northern Europe” [1] published in Building and Environment in 2023, where you were one of the eight authors: What is overheating in buildings?

CJH: In simple terms, overheating occurs when the occupants of a building are too hot, due to the thermal conditions indoors. In terms of causality, I would add that it is important to distinguish between the concepts of seasonal and chronic overheating in buildings. Seasonal overheating is well known and arises in summertime as a combination of warmer air temperatures and more intense solar radiation. Summertime overheating is exacerbated by climate change, since more heat is retained in the atmosphere and oceans due to rising concentrations of green-house gases. But there is also the concept of chronic overheating, and that means that you can have year round overheating in some buildings, which is caused by high internal gains, largely stemming from the building fabric and the mechanical systems. Despite differences, both concepts lead to elevated internal temperatures and therefore can be classified as overheating.

LHC: Let’s concentrate on the seasonal overheating related to climate change. You lived a few years in UK and now in Austria. Does each country have a different definition of overheating of buildings or is there any standard definition?

CJH: It really depends on the country and the context and that’s because people have different expectations and also because most people adapt, at least to some extent, to the environment they are accustomed to. Some national guidelines state a maximum temperature (e.g. CIBSE Guide A gives 28°C as the maximum allowable temperature, which should not be exceeded for more than 1% of the occupied hours), other standards refer to the dynamic thresholds, such as the operative temperature. In the Passivhaus Standard (which originates from Germany), the indoor temperatures cannot exceed 25°C for more than 10% of the hours of a year (Figure 1).

Figure 1. Adaptive overheating thresholds defined in standards and the range of static criteria adopted in EU countries (Reprint from [1]).

It is important to note that there is often a difference between criteria for domestic and non-domestic buildings and in the former between living rooms and bedrooms because of the sensitivity of the occupants to heat at different times of the day and the effects on sleep deprivation and health.

LHC: OK, so each country has different definitions. Let’s go to another question. There is a quite often used narrative (at least in the Netherlands) that low energy dwellings built according to the latest standards overheat more than conventional buildings built much earlier. Do you agree with it?

CJH: This is a question that, as a scientist, I could not answer with a simple yes or no because it is much more nuanced.

First of all, you need to think about how you define a low energy building. Typically, they have very low U-values and they’re also often airtight, meaning they don’t have a lot of air leakage. Studies using simulation demonstrate that such buildings may be more sensitive to increased temperatures and high internal gains, but if properly designed they can also keep heat out for longer and retain coolth. The problem is therefore more related to poor design decisions.

LHC: Can you give an example?

In Austria, we have a lot of old school buildings with recent extension of one or more extra classrooms. These newly built classrooms are often built with much lower ceiling heights than in the traditional school buildings and with large glazed areas which are poorly shaded. Quite often these large openable windows, which are meant to be fully open but in reality, there are safety and practical issues which limit this possibility.

Sometimes the opening is even interfering with the shading concept. These design shortcomings are often the reasons why newly built constructions may be more likely to overheat in comparison to an older building (which is more draughty, with higher ceilings, less glazing). The point is not to avoid building low energy buildings, because we urgently need to reduce our energy consumption, but to make sure new buildings are properly designed to contend with warmer summers as well.

LHC: Speaking about low energy buildings, you are from Germany where (in cooperation with Sweden) the concept of Passivhaus was conceived. You co-edited a book on Passivhaus.[2] What are design principles for Passivhaus to make sure that the users will not have problems with overheating?

CJH: Passivhaus’s main aim is to provide thermal comfort whilst minimising operational energy consumption. This is achieved through compliance with one of two main criteria. In simple terms a Passivhaus is certified by having either a peak load of ≤ 10 W/m² or an annual specific heating demand of ≤ 15 kWh/m²a in addition to a limit on the total primary energy demand. In climates where active cooling is needed, the space cooling energy demand requirement roughly matches the heat demand requirements, with an additional allowance for dehumidification.

LHC: Can Passivhaus buildings overheat?

CJH: All things being equal a Passivhaus is no more likely to overheat than any other dwelling, however in a well-insulated airtight building you need to be mindful not to add unwanted heat gains due to their high thermal inertia (Figure 2).

Fig.2. South-facing window-to-wall ratio and cost-optimised solutions to prevent current and future overheating in Passivhaus buildings. Comparing the frequency of overheating (% hours) above 25 °C under present 2020 (P) and future 2070 (F) climate conditions shows that ‘cost ideal’ solutions which avoid overheating require low south facing window areas (circa 15% glazing). (Reprint from [6]).

Preventing overheating in a Passivhaus (or any other low energy building) has a lot to do with early-stage design decisions and subsequent user behaviour. For example, in one of the first certified Passivhaus social houses in Wales, temperatures in an upstairs bedroom were found to exceed 26°C, which came as a surprise, considering the relatively mild and overcast climate in Wales.

In this one particular case, it turned out, it was a little girl’s bedroom, and she admitted that she never opened the window because she was afraid of spiders crawling into the room. Once they put a fly screen in the window, the problem was solved. It shows the importance of understanding individual user behaviour, which is something that cannot always be predicted by a computer model.

LHC: Can we use building performance simulations (BPS) to predict overheating risk in buildings?

CJH: In general, yes, but as my previous example shows, we can’t always account for user behaviour. There are many different types of models with different amounts of uncertainty. There are for example, black – and white box models. Black box modelling is driven by data from a specific building and has been shown to reliably forecast indoor temperatures during heat waves if sufficient time series data is available.

In terms of white box (or physics based) models, there are both steady state and dynamic models. Steady state models typically operate at a monthly or daily level, which is often sufficient for energy balance calculations, but can struggle to accurately predict the precise time and extent of overheating. Dynamic or transient models deal better with complex time-varying loads and in principle are able to predict the extent and timing. As with all models, whatever one enters into the model will have an impact on the outcome. In general, I think BPS is very useful in this respect if you combine it with an established methodology such as CIBSE TM59 but still one must always be aware of the limitations and assumptions embedded in any model.[3].

LHC: You mean CIBSE Design methodology for the assessment of overheating risk in homes TM59 from 2017?

CJH: Yes. This is for domestic buildings and sets out compliance criteria, internal gains profiles, guidance on ventilation, weather files, windows and shading to create a model of the building that can then be simulated with BPS.

LHC: When you speak about input data, a part of it are climate data. Is climate change taken into account here or not?

CJH: Yes and no. The focus is mostly on the dry-bulb or operative temperature when it comes to these assessments. I think we often neglect the importance of (increasing) solar radiation and humidity. At Loughborough University we were evaluating the impact of climate files in terms of daylighting and overheating [4]. We found out at this time that solar radiation data in climate files was not sufficiently represented, when comparing standard data sets with monitored data. In developing new climate files, rather than deriving the irradiance components from estimates of cloud cover by making use of the Skartveit-Olseth radiation model we could show that overheating predictions were much closer to reality[1].

LHC: Let’s go back to the paper we mentioned at the beginning. It is about the overheating problem in Central and Northern Europe. When I first saw it, I thought why not South? And then I realized, that they have buildings which are designed for hot weather and people are used to closing the outside shading (which all houses have!), facades are mostly white, not dark brown like in the Netherlands…

CJH: Well, we had to limit the scope of the paper somehow. But you are spot on in that the building design in a southern European context often differs significantly from central and northern European building design. I have an example of a building in the South of France (Figure 3) that I took once on holiday and often use with my students. Their tasks: List where does this building get its shading and cooling from and then discuss in small groups which one may be the most significant.

fig.3

It gets them thinking about climate responsive design and why these things are not just decorative after-thoughts, and they learn a lot from it.

LHC: We are back at the design. From your point of view, from your experience, what do we have to do to avoid overheating in new buildings?

CJH: In my lectures I always say that preventing overheating must happen on three levels: (1) city scale, (2) building scale and (3) personal scale. At the city scale we look for example at green and blue infrastructure, including cooling corridors (which allow cool air from the hinterland to break up the urban heat island), at the building scale it is about maximising natural ventilation, self-shading, use of ceiling fans, hybrid-ventilation systems and low-energy cooling devices. And last-but-not-least, at the personal level. That’s to do with what we do as occupants to control our thermal environment and to modify our behaviour to accommodate warmer temperatures. This might mean relaxing in the afternoons (such as for example in Spain), and using portable water-sprayers (for evaporative cooling) or fans (for localised air movement). But also, things like you posted on LinkedIn a year ago about the T-shirt day [5] and wearing more casual clothing in the office.

We need to fully exploit the simple ideas, which have been successfully used for centuries in the buildings in the south of Europe. If we adopt that approach it will mean that our buildings will have much lower cooling loads and any active systems can be sized down accordingly, it also means they will also be far more resilient to the risks of future power outages.

LHC: The last question: Is it true that you are going to organize the Building Simulation conference in 2027 in Vienna?

CJH: Correct. IBPSA-DACH (Germany, Austria and Switzerland) is proposing to host Building Simulation 2027 for the very first time in Vienna, Austria, a city, which has been ranked the world's most liveable city for the 11th consecutive time. Graz University of Technology (TU Graz) and Vienna University of Technology (TU Wien) will co-host the conference.

Cited papers and book

[1]     J. Taylor et al., “Ten questions concerning residential overheating in central and Northern Europe,” Build. Environ., vol. 234, 2023, [Online]. Available: https://doi.org/10.1016/j.buildenv.2023.110154.

[2]     C. Hopfe and R. McLeod, Eds., The Passivhaus Designer’s Manual A technical guide to low and zero energy buildings. Routledge, Taylor & Francis Group, 2015.

[3]     K. Mourkos, R. S. McLeod, C. J. Hopfe, C. Goodier, M. Swainson, Assessing the application and limitations of a standardised overheating risk-assessment methodology in a real-world context, Building and Environment, 2020, https://doi.org/10.1016/j.buildenv.2020.107070.

[4]     E. Brembilla, C. Hopfe, J. Mardaljevic, A. Mylona, and E. Mantesi, “Balancing daylight and overheating in low-energy design using CIBSE improved weather files,” Build. Serv. Eng. Res. Technol., vol. 41, p. 014362441988905, Nov. 2019, https://doi.org/10.1177/0143624419889057.

[5]     L. Hensen Centnerová, “Cool the person, not the building!”, LinkedIn, 2023.

[6]     J. Forde, C. J. Hopfe, R. S. McLeod, R. Evins, Temporal optimization for affordable and resilient Passivhaus dwellings in the social housing sector, Applied Energy, 2020, https://doi.org/10.1016/j.apenergy.2019.114383.