Historic churches are a class of building which embody significant cultural heritage; as a result, they are historically listed in recognition of the contribution they make to the built environment [1]. Frequently composed of different building materials and techniques, these have been adapted over hundreds of years to fit stylistic changes alongside congregational and maintenance requirements. Churches are often home to important artefacts and design features that require consideration in terms of conservation, yet historic churches seldom possess heating and ventilation systems (HVAC) which effectively monitor and control temperature and humidity to preserve artefacts’ life and condition.

Where space heating is used in historic churches, it often focuses on human thermal comfort rather than the building and artefact requirements, inducing large fluctuations in temperature and relative humidity [2]. Space heating systems attempt to condition large volumes of air in the church in an often futile effort to meet modern comfort expectations [3]. Despite construction utilising high thermal mass materials, the sporadic nature of church services results in an environment which is often below human comfort levels [4]. The repeated heating of such high thermal mass buildings often introduces degrading processes in high value artefacts such as paintings, fabric, artwork and musical instruments like the organ [5].

In order to protect and preserve historic buildings for the future, major alterations to their fabric and character are usually prohibited. Historic buildings such as churches have been exempted from energy efficiency targets because they are constructed using methods which do not meet energy performance benchmarks [6]. In addition, undertaking any retrofitting of energy saving measures may result in damage to the historic fabric and artefacts contained within ^[7][8][7,8]. Historic building fabric is porous, requiring a careful management of ventilation to prevent excessive moisture accumulating in the fabric and internal space ^[9][10][9,10]. British Standard EN 15759-1: Heating churches, chapels and other places of worship suggests that the ideal solution is to maintain suitable environmental conditions over a constant period, although it does not specify temperature and relative humidity ranges, rather focusing on the type of systems available and their general effect upon the conservation of the building and its contents [11].

One heating option discussed in EN 15759-1 is conservation heating. This is a strategy some historic properties implement to focus on the preservation of fabric and artefacts through controlling relative humidity. The control of moisture condensation and frost is deemed more important in conservation than human comfort levels [11]. Conservation heating aims to meet the standard for environmental control set out by the National Trust but results in a wide range of room temperatures [12]. The operation of heating solely to preserve church artefacts risks degrading human comfort and limiting the use of the space at a time when churches are looking to utilise buildings for the wider community [13]. If churches are to be heated regularly in response to increased usage patterns, research is required to understand if the internal environment of historic churches is suitable to meet the needs of conservation, while attempting to meet human comfort demands.

Guidelines and standards exist for museums and art galleries to maintain good conservation standards through the operation of HVAC systems. The British Standards Institution BS EN 15759-1:2011 [11] establishes that relative humidity (RH) is generally the most critical parameter from a conservation point of view, and shall therefore be kept at a defined level and as stable as possible. While not every museum and art gallery have sophisticated HVAC and/or building management control systems installed, they are, at an institutional level, focused on best practice in managing conservation and maintaining collections in good order. Therefore, the option to consider if the specific temperature and relative humidity ranges outlined in the standards applicable to the museum and conservation sector are relevant when historic churches look towards the increased operation of less sophisticated retrofitted HVAC systems to meet increased usage patterns.

2. Building Fabric

Historic churches are monuments to traditional building practices and skills. Often composed of different materials, building techniques and styles, many church buildings have been in regular use for hundreds of years. Alterations and repairs have taken place during the lifetime of the building in response to changing needs and the degradation of fabric components. Hoping to remedy the problems inherent to the traditional construction, modern building technologies and materials have been applied to traditional buildings, resulting in new and more complex problems. In many cases, a lack of knowledge related to the function of traditional buildings and their materials has played a part in creating problems for building operators [14]. In some cases, these individual interventions do not appear to singularly cause a problem, but in combination, they are damaging to the building ^[15][16][15,16]. It should be recognised that historic buildings possess more complex interactive bioclimatic properties when compared to modern equivalents [7]. Christian church design and layout generally conform to the established parameters inherent to the faith. The majority of Christian churches were built with the nave oriented west to east, typically having the main entrance at the west end of the church. Site restrictions and peculiarities result in some churches which do not conform to these design principles [17]. Cathedrals also adhere to the same principles of orientation, but with somewhat grander designs: walls are thick masonry with deep window and door reveals. The availability of quality building materials on a local level has resulted in varying construction techniques across Europe. Flint construction and infill feature heavily in Dorset, Wiltshire, Hampshire, Sussex, Kent, Surrey, Berkshire, Suffolk and Norfolk (UK) and have been used in many of those region’s churches [18]. Examples of typical construction materials can be seen in Figure 1. Historic churches are constructed without insulation in the walls or roof structures. Retrofitting insulation is possible to reduce thermal transmittance. However, not all church buildings are suitable for such insulation due to solid floors or ornate/decorated ceilings. Depending upon the ceiling design and construction, insulation may be possible but is likely to be impractical elsewhere, as well as potentially damaging to fabric. Haupl et al. [19] suggest that calcium silicate possesses the necessary capillary action required to allow moisture, which may condense on the cold side of the insulation, to diffuse on the warm side. Failing to account for the possibility of interstitial condensation may allow moisture to become trapped and lead to damage risk. Uniformly applying insulation across the whole wall may not account for the local microclimates associated with different fabric components and construction [20]. Despite the ability to respond to the local climatic conditions, over time, human thermal comfort requirements have resulted in the installation of space heating systems. Although the system is specified to heat the building fabric and the volume of air, the design of historic churches often leads to human discomfort, as large vertical temperature gradients exist, and inadequate mean radiant and operative temperatures are experienced [21]. Wall thickness and material choice dictate the thermal performance of a major part of the church building. A list of construction materials and wall thicknesses in historic European churches and religious buildings identified during the literature review can be found in  Table 1. Correctly establishing the thermal properties of locally sourced materials, given the shortage of thermal conductivity data in the UK, thickness of the walls and accounting for voids in the construction, contribute to reduced confidence when attempting to understanding the overall thermal performance. It is suggested that standard calculations for U-values underestimate the thermal performance of traditionally built buildings ^[22][23][22,23].

3. Artefacts

Historic churches represent a rich legacy of liturgical development covering hundreds of years. With changing fashions, the interior decoration of the buildings and the artefacts were adjusted or obscured. In some cases, church ownership passed to different denominations with their own stylistic view point. In addition to the usual artefacts present within UK churches, there are some remarkable survivors, despite the changing views on religious art and decoration, representing rare examples of Christian artwork ^[30][31][30,31]. To maintain conservation standards, the selection of the guidelines reviewed here set out the tolerance to fluctuating relative humidity. By setting limits on changes to the RH in a 24 h period, damage to artefacts can be avoided, especially those which undergo mechanical changes, such as wood. Laboratory tests on wood samples have shown that wood takes one day or more to adapt to new environmental conditions [32]. Schito et al. [33] attributed deterioration of a painting on the Scrovegni Chapel walls and wooden objects in the Roslyn Chapel to high values of relative humidity. Therefore, the duration of the fluctuation is important in the preservation of wooden artefacts. The high moisture content in wood also has the potential to foster development of rot fungi and wood borers [34]. The desire to control relative humidity and the resulting damage to artefacts and building fabrics prompted the concept and application of conservation heating in National Trust properties. Historic properties normally experience relative humidity levels between 60–80% [35], which is above the maximum that many artefacts can tolerate for long periods without experiencing degradation. Over time, the tolerance and range has been adjusted due to the financial implications of providing heating to larger properties and a greater understanding of the behaviour of artefacts with the duration and range of fluctuations in relative humidity. Human comfort is considered secondary to control of relative humidity in National Trust properties; therefore, comfort boost heating has been established during visiting times to maintain an environment conducive to visitors and staff. The drawback of such humidistat controlled heating systems is the increased temperatures required in summer months to keep relative humidity within acceptable ranges. At a time when humidity is highest in the outdoor environment, and often coinciding with higher air temperatures, heating a property up to 30 °C to maintain 58% RH is difficult to justify. Therefore, the proposal to limit room temperature to 22 °C in summer and allow the RH to increase has been the National Trust policy since 1994. However, with climate change, the percentage of time that RH control is lost in a calendar year has increased [35]. Most historic churches were built without heating systems; therefore, artefacts may have resided within the natural indoor environment for many generations. With the advent and installation of space heating systems, the historic environment has been changed to one that favours human comfort. Legner and Geijer [36] highlighted the increasing frequency of the conservation required for wooden artefacts when space heating was installed in old Swedish churches. The most common damages reported were the cracking of paint and desiccation cracks. The desire to achieve human comfort may cause degradation of heritage items [2], although materials respond differently to changes in temperature and relative humidity. Items made of wood, for example, take longer to respond to changes than other artefact types. A comparison of the recommended temperatures and relative humidity ranges for artwork, displays and internal spaces are detailed in Figure 2. The data are sourced from four separate studies encompassing microclimate, thermal comfort, hygrometric and climate control research in historic buildings. Artefact sensitivity to changing humidity levels ranges from extreme (inlaid furniture, wooden musical instruments, wooden sculptures and paintings on panels) to low (metals, glass and stone) ^[38][37]. Figure 2 demonstrates the wide-ranging minimum temperatures that separate items can tolerate, some as low as −20 °C, according to Larsen and Brostrom ^[37][38]. While temperature fluctuations themselves may not induce damage, the resultant change in relative humidity dictates the safe range for room and building temperatures. Other authors and sources providing figures for safe temperature ranges tend towards the general accepted ranges for museum class items on display, namely the 15–25 °C range, which is the temperature acceptable for human occupation. Figure 2 allows the individual bands to be seen in comparison to others of the same or different category. It is clear that an area for temperature and relative humidity exists where most artefact types will be safe, although short term fluctuations outside these bands are allowable in many standards and guidelines.

4. Occupants

With the widespread installation of central heating in homes across the developed world, individuals routinely expect to find optimal thermal comfort widespread in public buildings. It is estimated that 80–90% of people spend their time indoors ^[40][48]. Thermal comfort is a subjective measure based on several criteria: temperature, thermal radiation, humidity, activity, clothing and air speed ^[40][41][42][48,49,50]. Ethnicity, health, body type, fitness and acclimatisation all further contribute to the complex nature of thermal comfort. Personal adaptations such as clothing, duration of stay and activity can overcome temperatures outside individual comfort ranges ^[43][41]. Different denominations require varied levels of activity from occupants. Orthodox services usually require the congregation to stand throughout ^[44][40], while Catholic and high Anglican services may involve frequent changes between seated, standing, kneeling and walking to the alter. Presbyterian services call on the participant to be seated the majority of the service. When higher levels of activity are undertaken, sensitivity is decreased, and the risk of thermal discomfort is lowered ^[45][51]. Elderly or sick persons may need a higher room temperature to feel the same comfort as other younger and healthier occupants ^[46][52]. It is evident that temperature is of chief concern to occupants, while relative humidity matters only when very low (<30%) or very high (>80%). Occupants typically expect temperatures in the range of 18–22 °C ^[42][50]. When human comfort expectations are examined, it can be foun the context of this study, we find that only a minor conflict arises between the demands of the occupants and those of the artefacts and artworks. The difficulty is that artefacts are susceptible to relative humidity changes in a way humans are not ^[42][50]. Andersen et al. ^[47][53] observed, in a study of 48 young males, that participants did not perceive changes in relative humidity when it was altered from 70% down to 10% in controlled clean air at 23 °C. However, the altered RH% did cause a change in the perceived temperature, despite being 23 °C throughout. The location of the church may also lead to exposure to wind and moisture. In the case of Kilmelford Parish Church, a church located 700 metres from the sea, the prevailing south west wind brings in damp air due to the entrance on the west gable. The positioning of the door leads to significant draughts and heat loss as occupants enter or leave the building [16]. With the alter typically being placed at the east end of the church, the entrance to the building is often exposed to the UK’s prevailing south westerly wind. A room with many cold surfaces is inherently uncomfortable for human occupation. The amount of heat transferred to the surface is dependent on the temperature difference and the duration of occupation ^[46][52]. ISO 7730 [28] advises that vertical surfaces’ radiant asymmetry should be kept to less than 10 °C from air temperature. In winter those sitting near cold windows and surfaces may feel uncomfortable. It is common to feel a downdraught from a cold window, therefore radiators are often fitted on the wall below the window to counteract these draughts, providing they cover the entire width of the window ^[46][52]. Historic churches unfortunately meet all the criteria for thermal discomfort: insufficient local thermal control, poor insulation, large vertical temperature gradients and inadequate mean radiant and operative temperatures. Where heating systems are installed, they are unable to guarantee thermal comfort due to intermittent usage, large heat loss through the building fabric, significant room height, single glazed windows and infiltration loses [21]. Heat is often concentrated in the upper areas of the church, while cold air is retained at pew height ^[48][54]. Although thermal comfort is a subjective quality expressing satisfaction with the thermal environment ^[45][51], standards do exist to measure human comfort and meet those demands. ASHRAE Standard 55, EN15251 and ISO 7730 all use the same method of Predictive Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) to determine human comfort levels. The PMV predicts the mean value in votes from a large group of people on a seven-point thermal sensation scale. The seven points are: +3 Hot, +2 Warm, +1 Slightly warm, 0 Neutral, −1 Slightly cool, −2 Cool, −3 Cold. PPD is derived from PMV and predicts the percentage of people who are likely to feel too warm or too cold in the given environment ^[45][51]. ASHRAE Standard 55 specifies the thermal conditions in which 80% or more of the occupants within a space will find the environment thermally acceptable based on the heat balance model of the human body. This model is influenced by measurable factors: humidity, air speed, air temperature, radiant temperature, metabolic rate and clothing insulation. It does not account for subjective measures like those mentioned previously ^{[40][41][42][49]}[48,49,50,55]. In cold seasons, churches are often heated very quickly before services in order to achieve a comfortable environment for all. However, this produces a highly variable temperature throughout the church ^[43][50][41,56]. Short rapid heating events are unable to heat the fabric of the building; thus, the cold radiant temperatures from the surfaces of the church remain a risk to those nearby. High temperatures in summer are often associated with high relative humidity inside historic buildings. Despite the interior being at a comfortable temperature for occupation, in the summer, users may still find the church uncomfortable. This aspect of thermal comfort was reported by Martinez-Molina et al. ^[51][57] when assessing visitors’ thermal comfort at a museum based in a historic building. The hotter the temperature outdoors the colder visitors felt inside.

5. Hygrothermal and Microclimates

Many historic churches are now heated on a regular cycle during the winter season. However, it is worth noting that space heating was not a feature when they were designed and built. Therefore, it is safe to assume that a lack of heating will not have a detrimental effect upon their condition, given the hundreds of years many have survived without being heated ^[52][58]. Due to the design features inherent in most Christian church buildings, Curteis [3] reports broadly similar internal environmental microclimates existing in historic cathedrals of different size and location. Conditions at the west end of the nave are more unstable, and artefacts located in that area of the building are at greater stress due to this being the main door of the church. Smaller churches experience greater instability when large groups of people occupy the space due to a smaller internal volume and complete exchange of air from the entrance door [3]. Historic England also highlights the existence of various microclimates in historic buildings. This is due, in part, to older buildings often having walls composed of more than one material, resulting in different performance characteristics ^[53][59]. The UK climate is challenging for historic buildings. With an annual average relative humidity of 80%, the UK is dominated by moisture-laden maritime depressions: rapidly changing conditions interspersed with high-pressure stable situations ^[54][60]. Problems related to moisture within building fabrics are a common theme for historic building stock. The masonry walls of historical buildings lack damp insulation and were designed to allow the absorption and evaporation of moisture. As a result they exhibit high humidity associated with outside conditions, rising damp or rain penetration ^[9][10][9,10]. In San Jan Bautista Church, Talamanca de Jarama, Spain, the indoor relative humidity (RH) rises from around 45% to about 70–75% on rainy days, where it remains for around 48 h. During rainy periods the area experiences over 90% relative humidity, with a yearly average of 57.1% ^[55][61]. When studying the cathedrals of England, Curteis [3] stated that typical churches have 85% RH at 10 °C. Furthermore, winter typically sees lower RH during the period the heating system is operative. With high relative humidity comes the potential for mechanical stress, mould growth and degradation in susceptible items. Conservation heating is one strategy to control relative humidity in historic buildings. It is used by the National Trust to reduce the risk to collections and building fabrics. The main function of conservation heating is to keep the building envelope at a suitable temperature to avoid moisture condensation or frost, which is a frequent problem in buildings with high thermal inertia [11].