Build Up: This month we are addressing the topic of energy efficient renovation of historic buildings. This building typology needs a specific approach to energy retrofitting. Which would you say are, in general, the main challenges to make this building stock more efficient?

María López Zambrano: There are many factors that hinder the energy improvement of these buildings in different areas. From the point of view of legislation, we find that on the one hand, heritage protection laws are quite conservative in terms of allowing action on this type of buildings, which is positive, these types of regulations have to ensure the safeguarding of these buildings, but at the same time, it complicates the process a lot from the point of view of deadlines and permits. On the other hand, the regulations on energy efficiency and Energy Performance Certifications normally exclude these buildings as a special typology, leaving them in limbo, since if there are no regulations, it is sometimes interpreted as something that cannot be done and when in doubt, it is not done and by extension, it is more difficult for these buildings to be included in subsidies for rehabilitation, special plans, etc.

From a technical point of view, there are solutions that could be used for these buildings, but they are not the same as for a modern building. The hygrothermal behaviour is totally different and so are the strategies to be used. If not done correctly we run the risk of worsening the building instead of improving it.

"The regulations on energy efficiency and Energy Performance Certifications

normally exclude these buildings as a special typology, leaving them in limbo"

In this sense, it is important to train professionals in this type of intervention and also to involve the owners and users so that they understand the best way to use and operate their building to take advantage of things as beneficial as the effect of thermal inertia in providing a more stable indoor temperature throughout the year.

BUP: You have personally been working on an evaluation system of the solutions to be implemented on historical buildings. What could you tell us about this system and how it improves the approach to renovation?

MLZ: The evaluation system idea I have developed, called SESREBIC, functions as a decision-making tool. It was designed for the Spanish heritage and energy regulations. Imagine you have a listed building and you are thinking about a retrofit solution. To see if this solution can be suitable for your building, SESREBIC first checks if the solution is suitable with the protection class of the building element you want to upgrade. The same building may have both protected and unprotected parts, so the same solution in the same building may be accepted in one room and not in another. If the solution is accepted, the evaluation continues and there are eight indicators that will be evaluated: technical feasibility, economic feasibility, implementation, maintenance, energy efficiency improvement, permeability improvement (for windows and doors), embodied carbon and durability. This solution can be compared with two other solutions for the same element, and it is up to the evaluator to decide which indicators are most important in the project to select the best solution.

Obviously, the acceptance of the solution in SESREBIC is indicative, as the heritage commission will have the final say in these cases. The aim of the evaluation system is to give a tool that can combine compliance with the two regulations and be able to include other important aspects for the decision, such as price, embedded carbon, maintenance, etc. The tool is very easy to use and it has been a practical way to show how the evaluation system for energy rehabilitation solutions developed in my thesis works. It is something that could be more complex, adapted to the regulations of other countries and that could be integrated into future Energy Performance Certificates specific for buildings if there is a will to create it in the future.

BUP: How are historic and protected buildings addressed in the current EU energy legislation? Could you provide some numbers of the renovation rate of historic buildings around Europe?

MLZ: In most cases, these typologies are not obliged to comply with energy legislation and there are guidelines and standards (ASHRAE, CEN, local or national) that describe recommended practices, but they are not mandatory. If an owner wants voluntarily to follow the Energy Performance of Buildings Directive (EPBD), it can be very difficult to comply with it. For example, if you have a house built with one metre-deep stonewalls, in most countries you are going to have a default U-value for those walls, probably very high because it is difficult to set a number for something that was built ad hoc manually and sometimes mixing different materials. The standard is quite conservative and usually has a value that sometimes is the same for different types of stones or thicknesses of wall. The same goes for a green roof, earth walls, etc.,  as a result, the Energy Performance Certificate of that house will score very badly. A low score means not having access to subsidies or showing some future buyers or tenants that their building is going to be expensive to maintain, so some owners prefer to avoid this process. On the other hand, energy retrofit solutions, such as certain levels of airtightness in buildings designed to be permeable, make compliance very difficult. So we find as a summary of barriers, the first step to perform an EPC for the building is going to result in a score that has not been adjusted to the actual performance and the second is the solutions that we have to implement to follow the regulations are going to be in most cases detrimental to the building. 

"If an owner wants voluntarily to follow the Energy Performance

of Buildings Directive (EPBD), it can be very difficult to comply with it"

In terms of the number of renovations, about 2.2 million buildings are renovated every year which is 1% of the total European building stock. About 77 million buildings were constructed before 1945. The rate is difficult to say, because for existing buildings, we can look in the EPC databases of the countries, but for heritage, because they do not need to have the EPC it is more complicated to establish an exact number or rate. In the case of Ireland, for example, using SEAI figures, the age profile of dwellings that underwent deep retrofitting was that 22% were built before 1950, and in that we have to include listed and unlisted buildings.

BUP: Renovation and retrofitting are well known to be more energy efficient than demolition and new construction. How could it be further encouraged, at user level, for historic building preservation? Could they reach the same energy performance of a new building?

MLZ: I have worked in heritage rehabilitation in Spain and I continue to participate in research activities and clusters at the European level where I am in contact with owners of these types of buildings and I always come across the same conversation, the building is gorgeous but it is very difficult to live in it because it is not comfortable and it is very expensive to maintain. From my point of view, the best option to ensure that these buildings are preserved is that they have a use. The building that has no use ends up being lost. But it is also important that the building can offer the same things as a new building, from very basic things like having electricity, running water and a bathroom to the fact that the levels of comfort and accessibility can be the same as a new building. And yes, it is possible to get a historic building to achieve the same energy performance as a modern building, obviously not in all cases, but you can always do things to improve it, it is just that sometimes the solutions are not the standards you would use in another building, and you need to look for more options and sometimes just adapt them.

BUP: Although building typology is the driver to define retrofitting strategies, under your expertise, could you propose some key cost-effective retrofitting strategies that could be implemented at user level?

MLZ: The strategy will be very similar to any other typology. First, the cheapest thing is to find out how you use your energy and think about whether you are able to reduce your demand. For that, new technologies are quite useful, like monitoring, apps, smart devices, smart meters, etc. Change schedules, switch off devices, wear more clothes at home and lower the temperature on thermostats. Traditional passive strategies are always best - if your climate is cold, try to make the rooms with the most natural light the main rooms. If your climate is hot, shade is your best ally, use blinds to keep the temperature down during the hottest hours, and use ventilation at night to cool down during the day. Secondly, check if you can afford to spend on more efficient appliances and systems, the best example is lighting, which is very easy to do yourself and the most cost-effective.

Normally, the ones that have the biggest impact on the monthly bill are the use of renewable energies or changing the heating system for a more efficient one, but if we think about traditional buildings, they were designed to be as efficient as possible; I like to call them primitive Passivhaus. These buildings rely on thermal inertia to maintain a constant temperature throughout the year; it is what we would call the cave effect or what we feel on a hot day when we enter a medieval cathedral. Therefore, if our measures are in favour of enhancing this effect, we will be achieving something similar to what modern passive houses do, which require little demand for heating/cooling, which is what normally increases the monthly energy consumption bill the most. How to do this? Insulating the exterior with breathable materials usually works quite well. If possible, it is always better to make sure that our building has the lowest possible demand before implementing active measures such as new HVAC systems. It is the classic example of the bucket that we want to fill with water and it is full of holes, we will need less water to fill it if we cover the holes first, a building would work the same way. All this is always taking into account that each case, also for the protection of the building, will have to be studied to see which measures can be more effective.

BUP: The Green Deal highlights how existing buildings play a fundamental role to maximise the objective of decarbonisation and resource conservation for 2050. What would you propose to be the right steps to achieve this goal in historic buildings?

MLZ: There is no doubt that the energy refurbishment of existing buildings must be a priority to achieve decarbonisation targets for several reasons. One is because the demolition of buildings to build new ones implies more carbon emissions than refurbishing existing ones. Another reason is because in many cases the focus is on building on the periphery of cities, leaving many empty buildings in historic centres. This leads to additional mobility problems in cities, hindering public transport needs and thus increasing the use of private vehicles. This would run counter to the compact city model or the eco-neighbourhood concept. In the case of heritage buildings, we have countless historical cases with abandoned buildings, waiting to be demolished to build flat blocks. Europe is the old continent, a large part of our tourist attraction and of course part of our identity are our historic districts. I speak of districts because we often think of the palace or the cathedral, but there are many smaller buildings that make up the cityscape and we are losing them. An energy renovation in a historic building usually has a double impact; it benefits the owner, but also the environment, making it more attractive.

The road ahead is complex because there are many gaps to be filled before the renovation of historic buildings becomes mainstream, but some steps have already been taken. Unfortunately, money is always the problem and, in most cases, the solution. If it is more profitable for an owner to renovate than to abandon the building and then sell the land to build flats in a central location, the owner will choose renovation. We cannot have empty buildings if we now have problems with housing in Europe, including public authorities. It makes no sense to build a new public building if the administration owns some other vacant one. It is an exercise in long-term development and investment planning; to facilitate the process to approve retrofit and to make specific regulations for energy in heritage to be enforced, with different targets and strategies.

"We cannot have empty buildings if we now have problems with

housing in Europe, including public authorities"

In addition, more research on the subject is needed. This is our contribution from the IERC and Tyndall trying to promote projects that study ways to make better use of energy in heritage buildings and identifying future lines of research such as those included in the Heritage and Regeneration Position Paper published in 2022 by the European Technology Platform for Construction, Built Environment and Energy Efficient Buildings (ECTP).

BUP: Finally, could you highlight some good examples of historic renovation around Europe?

MLZ: I always like to give as an example one that was part of my thesis, the Monastery of Santa María de Retuerta in Sardón de Duero (Spain). It was a Romanesque monastery built and inhabited by Premonstratensian monks until 1835 when it was expropriated. For years, it was used as a livestock farm until 1998. Thanks to the fact that it was in use, it remained standing, although not in the best condition. In 2012 it was completely refurbished, taking into account that it is a building considered BIC (Bien de Interés Cultural – Asset of Cultural Interest) in Spain, which is the highest level of protection. One would think that little could be done in this case, but this was not the case at all. The monastery is now a 5-star hotel called Abadía Retuerta LeDomaine with a Michelin-starred restaurant. Converting the abbey into such an establishment meant that comfort levels and energy use were important concerns when designing the energy refurbishment. In 2013 it received the European Union's Europa Nostra Award in the category of Conservation and Restoration. Some of the solutions adopted were the use of the ancient heating system of the Roman hypocaust and the incorporation of sustainable energy, its panels integrated into the environment, produce energy equivalent to that needed to supply 60 homes, making the complex self-sufficient.