Dr Martina Pacifici on retrofitting older houses for modern sustainability in the 2024 Building Conservation Directory

May 24, 2024

Read the article in The Building Conservation Directory 2024 here 

Passive and active strategies to upgrading energy performance while holding embodied carbon.

Dr Martina Pacifici, Sustainability Lead, ADAM Architecture 

We know that, compared with other countries in Europe, the UK’s building stock is in a sorry state; old, thermally inefficient and run down. Data from a study by German company tado GmbH, based on a survey of 80,000 homes across Europe, shows that UK homes lose up to three times more heat and lose it faster than their European neighbours. For example, the average UK home loses three degrees after a period of five hours with a temperature of 20 degrees inside and zero degrees outside. Conversely, a Norwegian home will lose 0.9 degrees over the same period. Basically, in winter British heating systems need to work much harder and use more fuel to keep temperature constant.

That’s because the UK has some of the oldest housing stock compared to EU member states. Approximately 38 per cent of UK homes were built pre-1946 compared to 24 per cent for Germany and Sweden, and these homes won’t perform any better without some degree of intervention by way of retrofitting. This is the preferred approach to avoid the release into the atmosphere of the embodied carbon locked within ageing building stock. A major step up is required for many dwellings to move from existing high energy demand levels to a performance level that matches our European neighbours.

UK buildings are also responsible for approximately 18 per cent of the UK’s greenhouse gas emissions through their use of oil and gas for heating and hot water. These emissions need to be cut significantly by 2030 to help meet legally binding climate goals. According to scientific studies, we need a reduction of seven per cent on our current emission rate to hit climate change targets. However, during Covid and a period when travel and other emissions sources were significantly reduced, we achieved a reduction of only six per cent. We therefore need major change if we want to shift the dial to where it needs to be.

In the UK, 80 per cent of the homes that we will be using in 2050 have already been built but our new-build standards are not yet aligned to net zero, so homes we are building now will need to be retrofitted before 2050.

Retrofit approaches may vary a lot depending on the building’s age. Older buildings (broadly pre-1919) require different understanding, skills and material solutions to allow for improvement. This is due to the challenges posed by their peculiar building physics, which are different from those in houses of more modern construction. Most modern buildings depend on impermeable barriers to control the movement of moisture and air through the building fabric, but older traditional buildings tend to absorb moisture from their surroundings and release it depending on environmental conditions. Buildings of traditional construction have greater ‘thermal inertia’ than their more modern counterparts, heating up and cooling down more slowly, and this ability to ‘buffer’ moisture and heat can help to even out fluctuations in humidity and temperature.

Listed and historic properties are special, not least because they have additional significance and are of exceptional interest in a social and heritage context. Despite these sensitivities, many are family homes with their occupants requiring comfort, heat, light and a clean, healthy environment while also ensuring that the historic character of their property is maintained.

This security of tradition is important but it means that we have to find ways to make these buildings not just energy efficient and fit for comfortable living, but also able to secure their embodied carbon footprint, while of course respecting the important heritage and significance the listing recognises. This is a balance that is sometimes difficult to achieve.

However, replacement or repair of old systems, or the introduction of new systems to improve safety, comfort and energy performance still have implications in terms of their embodied and operational carbon footprint. They should be calculated and steps taken to minimise them as much as possible.

Extensions and most alterations to the exterior of a building require planning permission. Houses are allowed a little more freedom and can carry out small alterations under ‘permitted development’ rights, but these can be removed by the local authority where the building is in a conservation area. A building might, for example, require a planning application to change the windows or to add an insulated render. Where a building is protected as a listed building, local authority consent is needed for all alterations, including any alterations to the building, inside and out, which might affect its significance.

Looking at energy efficiency measures for listed properties, we must consider what level of intervention is necessary and indeed possible. The first step is to understand the significance of these properties and thereby the constraints in which work must be sensitively interwoven. There is then a chance to really begin to understand what might be achieved, whether it is changes around the fabric of the building, appropriate repair or modern retrofit. We also need to understand how these buildings have been built and how their fabric deals with moisture in particular, as most traditional structures need to breathe. This in itself presents a challenge when looking at improving the performance of these sensitive buildings.

Carbon must be at the core of all our thinking. All our old heritage and listed buildings, whilst they will have the benefit of a significant lifespan, are essentially acting as stores for embodied carbon. We must therefore think very carefully about the improvements that we make or are required to make. Assessments of embodied carbon through carbon accounting can also allow designers and energy consultants to compare the different design strategies and approaches to redevelopment and upgrading, for example, light versus deep retrofit.

In general terms, the case for refurbishment in short life buildings or those with a lifespan of 30 to 60 years is sound, as it will take many years for an efficient new building to draw level with an efficient refurbishment given the high energy and carbon required in new build. The same goes for older buildings, too, and by extending the life of our historic assets we can materially reduce the need for high carbon materials, technologies and activities. Historic England says that ‘the longer a building and its component parts last, the less embodied carbon is expended over the life of the building’, so, where required, repair, maintenance and upgrading will be the right approach in terms of keeping embodied carbon ‘locked up’.

Given all the constraints, we know that the best results are delivered by retrofit practices taking a whole building approach using an understanding of a building in its environmental, cultural and economic context. This delivers balanced solutions that save energy and carbon, sustain heritage significance and maintain a comfortable and healthy indoor environment.

This means a site-specific approach where opportunities and constraints can vary widely depends on context. A whole building approach makes use of passive and active strategies to achieve a truly sustainable renovation and upgrading of a building’s energy performance, while limiting embodied carbon emissions during retrofit, repairs and replacement.

A passive strategy is one that capitalises on the natural elements of a site including sun and wind patterns to provide natural heating and cooling of spaces through the different seasons. An active strategy goes much further and relies on mechanical processes. These may involve adding new systems to generate energy, as well as incorporating improvements to the existing systems of the building.

Passive strategies
Passive strategies support a fabric-first approach to reduce the space heating demand by focusing on fabric improvement. There are a number of possible passive interventions applicable to a project. Not all may be applicable and some may be unnecessary or hard to justify financially or in embodied carbon terms, given their low impact. Table 1 illustrates a list of passive strategies that may be applied to historic buildings depending on their listed grade and specific constraints. Figure 1 illustrates these interventions in the context of a typical architectural section of a listed property.

Active strategies
Given that most historic buildings have a greater reliance on their fabric for moderating their environment rather than on mechanical systems, then installation of active technologies can be challenging. However, their implementation can massively improve a building’s performance. Active technologies demand high capital costs and a noticeable increase in embodied carbon due to their manufacture and complex end of life disposal. For these reasons, systems should be selected and sized carefully to give value for money and achieve optimal performance.

Figure 2 illustrates a range of active interventions in the context of a typical architectural section of a listed property. Some of these technologies may be impossible to install in a listed property while, for others, their application will require discussion and creative solutions so as not to compromise either the building’s historic fabric or damage its sense of place.

For hot water and heating, one solution is the installation of heat pumps (ground or air sourced). Their technology is proven and reliable, but generally they deliver water at a lower temperature than a gas boiler and are better suited to underfloor heating than conventional radiators. The size/capacity of the heat emitters (radiators or underfloor heating) must be carefully matched to the heat demand of the building to ensure that the heat pump can deliver sufficient energy. Threading new pipes through historic fabric can be a challenge but there are often opportunities to think creatively and work with the existing fabric that allows work to be carried out without any damage to the building.

Heat pumps typically produce two to three times as much heat energy than the electrical energy they require to work. The integration of the heat pump with a hot water tank to heat water is highly recommended as a heat pump cannot provide the instantaneous supply of a combi boiler. Also, the requirement for additional space can be particularly difficult in smaller dwellings where the introduction of a dedicated energy centre or the use of the basement to locate the equipment may be needed. Heat pumps can be coupled with photovoltaic systems to increase the efficiency of a property, but consent for the panels will be required where the building is listed.

Given the phasing out of conventional gas boilers and where future heat demand of a building during colder periods cannot be met by a heat pump alone, a hybrid boiler/heat pump could be considered, but only if the fabric is to be improved to a point where the boiler is no longer required before gas supplies end.

Biomass is another option and wood burning stoves and boilers may seem to offer attractive low-carbon alternatives. However, sustainable timber takes time to grow and needs to be transported and there is, at best, a short-term carbon penalty for using biomass. Proper evaluation is rarely given to the embodied carbon in harvesting and processing the crop and in transporting it. Biomass use also impacts on air quality and its production competes with our farmland resource for food production.

Hydrogen could also help us to move away from gas without the requirement to improve the fabric performance but currently it has many unknowns. At present, most hydrogen is produced from fossil fuels, and with carbon capture it is neither a cheap nor an easy option for domestic heating and would be discounted for any retrofit taking place in the short to medium term.

Direct electric heating systems use electrical energy without any supporting mechanisms such as heat pumps. Electric heating and hot water systems can be attractive due to their simplicity and typically, installation comes at a lower capital cost when compared to a ‘wet system’. However, the building will be significantly more expensive to run when compared to either a boiler or heat pump.
A direct electric system also results in higher peak loads and so is not desirable at scale for the grid system. The use of storage heaters can help to mitigate this, but generally direct electric should only be considered where heat pumps are not feasible and where heat demand is very low.

District heating systems for heating and hot water are available in certain locations, but many networks are powered by gas. Combined heat and power (CHP) systems, too, require a transition plan away from fossil fuels. If heat pumps are used as an alternative generation plant, the lower temperature of the hot water generated can present issues such as heat loss so such systems will need to be carefully modelled.
Heating networks can be expensive and/or unreliable but could be considered where large-scale retrofit is taking place, for example, as part of a neighbourhood regeneration scheme.

For larger historic properties, an ‘energy centre’ can often be created on site using redundant buildings while retaining their historic fabric. The centre can provide heat, electricity and cooling to a group of buildings through a decentralised network with no need for an individual plant room in the main building itself. Heat losses for such local distribution networks can be mitigated by reducing the distance between the energy centre and the main buildings. Table 2 summarises the active strategies that can be deployed.

In reality, the solution may lie in not one or the other but a combination of both passive and active strategies and, as said, every project comes with its own challenges and opportunities. While broadly some of the solutions and optimal ways forward may be similar, the case for every property should be addressed on its own particular circumstance and an appropriate strategy for raising its energy performance level should be developed accordingly. When it comes to historic and traditional property there is no one size fits all. A too rigorous fabric-first approach, following Passivhaus standard for example, could be unrealistic, especially when applied to listed properties. Systems could be incompatible with the significance of the building or the components affected, and in some cases, the embodied carbon of insulation systems can be too high to be offset by the operational savings, particularly where a component has a relatively short service-life.

Systems could be incompatible with the significance of the building or the components affected and embodied carbon of insulations too high to be offset by the operational savings achieved by their application.

The right selection of passive and active actions is project dependent, working with a team that has researched, understood, and aligned the proposed strategies with the principles of construction of historic buildings. As part of this design process, the use of building performance evaluation (BPE) can help to inform decision-making and evaluate the individual and collective impacts of the solutions chosen. BPE can be undertaken in the form of dynamic energy modelling studies, steady state calculations, life-cycle carbon assessments (LCA) and post-occupancy evaluation (POE) with the scope to assess energy performance, carbon footprint and occupant comfort. They allow a building’s future performance to be predicted and also make comparisons with the established design targets.

There is much to consider but what is certain is that accounting of embodied carbon and assessing the different strategies, whether active or passive, in approaching retrofit of our historic and listed buildings are absolutely key in adopting the right approach to the preservation and safeguarding of our heritage and built environment.