Environmental Performance

Assessing the Impact of Heritage Projects on Building Environment

Tobit Curteis and Sara Crofts

  Builders at work in a timber-framed guildhall  
  Finchingfield Guildhall, Essex: previously on the Buildings at Risk Register, the Grade I listed medieval building was restored with the help of HLF funding and is now home to a range of facilities including a museum, function room, shop and the village library. (Photo: Finchingfield Guildhall Trust)  

As well as improving the condition of the historic fabric of a building and its collections, publicly funded conservation projects often aim to change the way the building is used for the benefit of all. Indeed, improving and widening access and use is often a key requirement of projects supported by public funding.

However, changing the way in which a historic building is used, even modestly, can have a disproportionate effect on the building environment, posing challenges for the conservation of fabric and artefacts.

It is therefore important that the potential impact of the proposed changes is fully understood and that, where the changes increase risks to the building, mitigation measures are built into the design at an early stage.

The impact that changes in use have on the building environment and fabric is complex and not well understood. As a result, there have been many cases where the risks were not recognised until late in the design process. This can be disruptive, causing significant delays and additional costs.

While such problems can arise in all types of projects, they are most prevalent in smaller projects where a large professional team may not be in place. In order to avoid this disruption and to promote more effective projects which will recognise and be designed around potential risks, the Heritage Lottery Fund (HLF) is developing measures that would encourage potential grant applicants to consider these issues more fully.

In the simplest cases, it is intended that applicants should be encouraged to carry out impact assessments themselves or in conjunction with their architects or other suitable professionals. Detailed guidance will be also be given about the issues involved and how they might affect the project. For more complex cases it is likely that specialist support would be required and guidance will be given on how to commission appropriate survey work. This short article gives an overview of the issues which generally cause difficulties and the approaches that are likely to be suggested in the forthcoming HLF guidance.


Historic buildings are vulnerable to irreversible damage from factors including water, light, heat and pollution which can be referred to collectively as building environment. Sometimes this damage is from obvious sources such as water leaks, and the results – crumbling plaster, mould growth or timber decay – are plain to see. In other cases, environmental damage is less easy to anticipate and is associated with apparently innocuous practices such as the way in which the building is heated or doors being left open. Although it is often slower, this kind of damage can still be serious and may even be irreversible.

Many of the buildings we are dealing with are poorly suited to modern use or are awkward to use for specific functions, and are often thermally inefficient. Some degree of alteration is usually unavoidable, either to secure a new use for a redundant building, or to ensure that a building remains viable and that its future is secure.

However, the very measures which are used to improve conditions can have unintended consequences, and it is important to understand that, in many cases, we are trying to use the building in a way in which it has never been used and for which it was not designed. The history of conservation is littered with examples where measures intended to make a building more comfortable for its users have resulted in costly and expensive failures over the longer term.

That said, historic buildings, by their very nature, have survived for many years and, in many cases, are remarkably resilient and adaptable and can even help achieve the required environmental aims. Thick masonry walls can provide excellent thermal buffering, and their porous limewash and plaster finishes can control moisture levels in the air without the need for mechanical air conditioning. Furthermore, much of this control is passive and can be achieved at little or no cost. If they are used well, these characteristics can reduce the building’s carbon footprint and are sustainable over the long term.


There are many areas of risk associated with modifying the building environment, but the most common are linked to changes in heating design and control, changes in visitor numbers and use patterns, alterations to internal structures (partitioning and enclosures) and the introduction of catering facilities. In all cases, the underlying risk is the destabilisation of environmental conditions.

Fluctuating relative humidity can cause an increase in salt activity resulting in the deterioration of stone, plaster and other inorganic structures, as well as dimensional response in timber causing deformation and cracking. Significant increases in relative humidity, sometimes in hard to access areas such as under and in roof spaces, can increase risk of microbiological growth and encourage chemical deterioration such as underside corrosion on lead roofs.

Many of these potential causes of decay are comparatively slow-acting and represent long-term rather than immediate threats, which makes them more difficult to anticipate and identify. However, despite their chronic rather than critical nature, their effects can be severe and irreversible and the cost of rectifying widespread damage of this type can be extremely high.

Most of the risks are not uncommon and could be anticipated at a very early stage of the project. However, problems tend to occur because the right questions are not asked at the right time. At an early stage of any project, the question that should be asked is ‘What are the possible negative impacts of the proposed intervention and how can we mitigate them?’ In most cases, as long as the risks are identified sufficiently early, the design can be adapted with only limited impact on the overall outcomes of the project.


The process by which the relevant issues are examined can generally be referred to as a building performance assessment. In simple cases, this can be performed by HLF grant applicants with their existing professional advisers being armed with the correct set of questions. In the proposed guidance notes which the HLF is currently preparing, the key risk areas are identified and relevant questions are outlined. For the more complex cases or those where the risk is of serious and irreversible damage, it may be necessary to have a more detailed assessment undertaken by a specialist.

In most cases the building performance assessment involves answering four basic questions:

  • How should the building perform in its natural state?
  • How is the building currently performing?
  • What are the likely effects of the proposed project on the environment and performance?
  • If the effects are potentially negative, what mitigation measures can be put in place which will allow the project to proceed while minimising risk to the historic fabric and artefacts?

The nature and extent of a formal building performance assessment can vary significantly depending on the complexity of the project and the relevant risks. A simple assessment, which is likely to be appropriate in all but the most complex projects, usually involves a site examination and building survey and a short report, and takes no longer than one or two days. Thermal imaging and spot readings for humidity, liquid moisture, light and UV radiation are commonly used in investigations of this type.

  Thermal image showing heat loss from the facade of a timber framed building  
  Thermal imaging and spot readings for humidity, liquid moisture, light and UV radiation are essential features of a typical building performance assessment. (Image: Tobit Curteis)  

In the more difficult cases environmental monitoring, materials analysis and other techniques may be necessary. Investigations of this type take longer (most environmental monitoring programmes last a minimum of 12 months) and are more costly and are therefore only appropriate in a minority of cases. In all events, it is very important to ensure that the type of assessment carried out is appropriate for the project and that the information which comes out of the work is of practical use.

As well as being a risk assessment exercise, a well-designed building performance assessment may also be able to offer significant benefits to the project. For example, advice may be provided on how best to improve the thermal performance of the building and to therefore achieve greater comfort for the users with reduced energy input (something which often benefits the historic fabric as well).

Advice may also be provided on how using different decorative coatings (for example limewash or distemper instead of emulsion) might maintain a more porous building structure, improving moisture buffering and reducing the risk of condensation. In fact, the results of a good building performance assessment should be to improve the environmental aspects of the project rather than simply to prevent damage.

One area where this is particularly relevant is heating. Project proposals often state that there is an intention to increase or improve the heating of the building. In fact, it is rarely the case that people wish to use expensive energy to heat a very large airspace with only a small number of people at floor level. Indeed this can often result in significant risk to the building fabric.

Rather, it is usually the intention of the project to provide thermal comfort for that small number of people in the lowest two metres of the building at the minimum cost (in financial and carbon footprint terms), with minimal harmful impact on the historic building or any artefacts. To achieve this, it is important to understand issues of energy loss and energy input, as well as how the building is to be used, by whom and for what periods of time.

Another area where problems commonly occur is with the proposed use of compartmentalisation and screens in a historic building which was originally a single large space or series of large spaces. When used effectively, screens and internal walls can be a useful tool in containing the local microclimate and ensuring that comfortable conditions are produced in the most heavily used areas of the building and where they have least impact on the rest of the building.

However, the same containment can also focus harmful conditions on sensitive fabric and artefacts, increasing the risk of damage to them. Combined with increases in visitor numbers or the installation of catering facilities, interventions which have the potential to have a benign impact on the building can swiftly become the focus of the problem.

A well-designed building performance assessment can provide advice not only on how to avoid harmful effects but also on how to achieve the most effective environmental controls for the benefit of both the users and the historic fabric.

It is important not to confuse this type of assessment of the building environment with that undertaken to improve energy performance. In many cases an energy performance assessment has a primary aim of increasing user comfort and minimising carbon footprint with the building considered as a neutral vessel within which the controls take place. Often the measures which best improve energy performance also increase the risk to the building and artefacts.

A common example is the use of fan convector heaters, which can swiftly change the air temperature. This is often a benefit in terms of comfort and energy use, but the same sudden fluctuation in temperature and humidity can have a particularly damaging impact on historic structures and their contents. Therefore, it is important that the building environment and conservation issues are addressed alongside any energy performance environmental surveys.

This new emphasis on building performance has been designed to draw the attention of HLF grant applicants to the issues associated with building environment and the relevant risks. It is intended that it should create little additional work in most cases and should help to avoid time-consuming and costly reworking of the design at a later stage, which has sometimes occurred when these issues have been overlooked. Ultimately, the aim is to improve the long-term outcomes of conservation and development projects for the building user, while minimising the risk to the building and artefacts.



The Building Conservation Directory, 2016


TOBIT CURTEIS runs Tobit Curteis Associates LLP, a practice specialising in the conservation of wall paintings and the diagnosis and control of environmental deterioration in historic buildings. He is an external consultant for the Building Conservation and Research Team at Historic England and is the National Trust’s advisor on wall paintings.

SARA CROFTS is an architect and head of historic environment at the Heritage Lottery Fund. The HLF uses money raised by National Lottery players to help people across the UK explore, enjoy and protect the heritage they care about.

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