Accepted Manuscript Title: Preventive conservation of historic buildings in temperate climates. The importance of a risk-based analysis on the decision-making process Author: Hugo Entradas Silva Fernando M.A. Henriques PII: DOI: Reference:
S0378-7788(15)30183-3 http://dx.doi.org/doi:10.1016/j.enbuild.2015.07.067 ENB 6061
To appear in:
ENB
Received date: Revised date: Accepted date:
28-5-2015 28-7-2015 29-7-2015
Please cite this article as: H.E. Silva, F.M.A. Henriques, Preventive conservation of historic buildings in temperate climates. The importance of a riskbased analysis on the decision-making process, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.07.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights
Highlights Long-term monitoring of microclimatic parameters in a historic building;
Risk-based analysis to evaluate degradation phenomena;
Hygrothermal capacity of historic buildings to achieve microclimatic targets;
Real risks associated to natural indoor climates in temperate climate;
New classification to evaluate degradation risks and building response.
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*Manuscript
Preventive conservation of historic buildings in temperate climates. The importance of a risk-based analysis on the decision-making process.
Hugo Entradas Silva*, Fernando M. A. Henriques
Departamento de Engenharia Civil, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de
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Lisboa, 2829-516 Caparica, Portugal
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*Corresponding author. Tel.: +351 212948580; fax: +351 212948398.
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E-mail address:
[email protected] (H.E. Silva)
Abstract. Historic buildings are usually characterized by a particular microclimate due to their high thermal
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inertia that may require the use of mechanical systems to control the environment. The guidelines used to define the indoor climate have evolved in recent years, with new methods and deepening the knowledge of the
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behaviour of materials, resulting in the publication of several risk-based methods.
Despite the risk-based methods, the use of guidelines continue to play a leading role on the science of preventive
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conservation with the progressive assumption of less demanding targets. However these guidelines were usually
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defined for specific climates so that when extrapolated to other locations their application may not be positive.
This research aims to analyse the hygrothermal behaviour of an unheated historic building in a temperate climate (Lisbon, Portugal) using a long-term monitoring and applying a risk-based analysis to the natural climate and to the climate limited by the historic set-point of 20ºC-50% and the targets defined by the standards EN 15757 and PAS 198. Finally a classification was defined to assist in the decision-making processes and to verify if it is safe to impose less demanding targets in temperate climates, improving therefore energy economy.
Keywords: Microclimate; thermal inertia; preventive conservation; monitoring; risk-based analysis; historic buildings; cultural heritage
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1.
INTRODUCTION
Historic buildings assume a fundamental role in modern societies, being a symbol of their past, and often used as museums or galleries, bringing together the value of their collections and the history behind the building.
The artefacts that constitute the collections react to temperature and relative humidity, which can induce
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degradation phenomena, namely mechanical, biological and chemical [1]. To avoid the inherent risks and to guarantee a proper conservation it is common to use tight limits of temperature (T) and relative humidity (RH).
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Throughout history, the guidelines were not always based on scientific works, and sometimes the set-points were defined according to experience and observation of the response of objects. The set-point of 20ºC for T and 50%
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for RH, for example, was widely used by conservators for a long time, apparently without scientific explanation, but the truth is that it is still used in several cases [2]. These limits were defined in a particular moment where the
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energy efficiency was still not a problem due to the low cost of energy, and where the risk-based analysis were not extensively used [2][3][4]. These targets, even in temperate climates, can only be achieved using HVAC
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systems.
It is important to take into account that historic buildings often show a particular hygrothermal response. These
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buildings, usually with thick walls and small percentage of transparent surfaces when compared to the opaque
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envelope, present a large capacity to store heat, showing a great effectiveness in damping and delaying thermal cycles. In temperate climates, this behaviour leads to a great thermal equilibrium in short-periods, disturbed only by the presence of disrupting factors such as artificial heating, lighting or human presence [5]. However, their envelope is usually composed by materials with high thermal conductivities, which do not react positively for tight targets of temperature. If the limitations of the envelope are ignored, the consequences may not be positive, with the possibility of surface condensations, for example [6] [7].
The knowledge about these subjects is improving and the most recent trends show a higher flexibility, according to developments in the materials science [8]. The experience obtained along the time has shown that in some cases the collections have survived positively, even when exposed to less demanding targets. The ASHRAE specification [9] is a good example, defining five classes and allowing some fluctuations without compromising the collections. The European Standard EN 15757 is other good example, where it was defined a dynamic method to limit the mechanical degradation in organic hygroscopic materials [10].
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The implementation of tight limits has another worrisome consequence: the high-energy consumption needed to keep the building at the desired levels. Nowadays, one of the biggest challenges in historic buildings, such as museums, is to reach an equilibrium between the conservation requirements and the energy economy [4][7], as it is evidenced in the recent British specification PAS 198 [11], where the targets are defined according the collection needs, searching also achieve the energy economy without jeopardizing a proper conservation.
METHODOLOGY
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The methodology used in this work combines a long-term monitoring and a statistical analysis, aiming to characterize the indoor climate of an unheated historic building, and a risk-based analysis. The response of the
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building and collections was analysed and tested according to the natural indoor climate and the targets defined by the EN 15757, PAS 198 and the historic set-point of 20ºC and 50%.
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2.1. Site description
The building analysed was the Church of St. Christopher, a national monument of Portugal located on the slopes
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of the São Jorge Castle (Lisbon – Portugal), under the influence of Mediterranean climate with mild temperatures due the proximity of the Atlantic Ocean. Lisbon has about 260 days of sunshine per year, an annual
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average of 17ºC of temperature, an annual precipitation of 725.8 mm and north prevailing winds [12].
The church was built in the early thirteenth century and maintained its original configuration until the sixteenth century, when it was badly damaged by a fire. It suffered little damages with the 1755 earthquake that shook the entire waterfront of the city [13].
It features thick walls, between 0.7 and 1 m, lined by limestone on the corners and lime mortar renders on the exterior (Fig. 1.a)), and walls covered with gilded and painted panels in the interior surfaces (Fig. 1.b)). The roof is made with ceramic tiles supported by timber frames. Inside, there is a rectangular room with 144 m 2 and 13 m of height, with a flat ceiling. The church presents a sacristy to the north of the central room and a funeral room to the south. The wooden frames single glazing windows have a global area of about 45 m2, which when compared to the 800 m2 of external walls, provide a ratio of 5.6%. No artificial heating systems are available. The roof with no thermal insulation and a small mass appears as a weakness zone.
The church remains usually closed and receives few visitors. It is open from Tuesday to Saturday between 17:00
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and 19:30, with religious celebrations taking place at 18:30. On Sunday, it is open between 11:00 and 13:00,
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with a religious celebration at 12:00. On Monday it remains closed throughout the day.
Figure 1. Main façade of the St. Christopher church (a) and the interior (b).
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2.2. Experimental campaign
To understand the interior microclimatic behaviour an extensive environmental monitoring was conducted . For
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this purpose, a set of sensors for automatic and manual records were used. Measurements were taken from November 2011 to August 2013, with automatic records every 10 minutes and using three different types of
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sensors, as it can be seen in Table 1. All of the sensors respect the uncertainties defined in the European
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Standards EN 15758 [14] for the temperature and the EN 16242 [15] for the relative humidity.
The monitoring system included 25 sensors in the main room (1 sensor type H, 1 T&RH probe and 23 thermocouples) and 1 sensor type H on the northern tower to monitor the external conditions, as it is possible to see in Table 2 and Fig. 2. For the purpose of this paper, besides the focus on the preventive conservation issues, it was considered essential the analysis of the influence of thermal inertia in the building response. For this purpose 6 sensors were selected (in bold in table 2): the sensors H on the northern tower (S.6) and pulpit (S.3) to analyse the relationship between the indoor and outdoor conditions; the sensors T at 0.15 m (S.1) and 10 m (S.5) to evaluate the evolution of the hygrothermal conditions along a vertical profile, verifying the influence of the thermal inertia and the coverage on the air stratification. In addition, it was also compared the hygrothermal evolution of the surface conditions and the risk of surface condensations with the use of a T sensor (S.2) and the one in the middle of the main room (S.4).
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Table 1. Sensors used in the monitoring system. Uncertainty T: ± 0.35ºC RH: ± 2.5% T: ± 0.1ºC RH: ± 2% T: ± 0.5ºC Table 2. Location of the sensors.
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Height [m] 5.3 3.1 3.9 7.5 (S.4) 0.15 (S.1); 1.5; 3.9; 7.5; 10 (S.5) 3.9 (S.3) 1.5 (S.2); 3.9 3.9 7.5 5.3 (S.6)
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Site (Fig. 2) a b c d e e e e f g h -
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Sensors 1 horizontal profile (T) 1 sensor – door of sacristy (T) 1 horizontal profiles (T) 1 horizontal profiles (T) 1 vertical profile 1 sensor – northern pulpit (H) 2 wall surface sensors (T) 2 floor surface sensors (T) 1 horizontal profiles (T) 1 horizontal profiles (T) 1 sensor in the chorus (T) 1 sensor – northern tower (H)
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Sensor Hobo U12-13 (H) T&RH Delta T probe Thermocoulples (T)
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2.3. Indoor characterization
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Figure 2. Horizontal plan of the St. Christopher church and location of the T and RH sensors in the main room.
For a better understanding and evaluation of the strengths and weaknesses of the church a microclimatic analysis
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was performed in order to verify the interactions between the interior and exterior climates, the influence of the thermal inertia and to check the air stratification, the disrupting factors and to assess the risk of surface condensations.
To analyse the indoor RH fluctuations a constant water vapour concentration along the main room was assumed, a premise that seems reasonable given the low occupancy and ventilation rates of the building. The data collected by the sensor located in the northern pulpit were used as reference to calculate the water vapour concentrations for each moment.
2.4. Response of the building and collections The use of standards and guidelines is a good tool, especially when it is not possible to make detailed studies for each location. However, it is necessary to note that the guidelines do not always fit all objects and climates [16][17]. Sometimes it becomes necessary to carry out a detailed risk-based analysis in order to draw strong
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conclusions. This research was designed in order to evaluate the building and collection response to the natural climate and according the targets defined by the EN 15757, PAS 198 and the pair 20ºC - 50%, with the analysis divided in 5 points: building response, biological, chemical and mechanical degradation of both painted panels and sculptures.
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2.4.1. Building response This research aims to compare the influence of four different set-points in the conservation and in the capacity of
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the building to respond to the targets. First the historic set-point of 20±2 ºC and 50±5% was used, followed by the targets defined by EN 15757 and PAS 198. Finally an analysis according to the natural indoor climate was
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made.
The dynamic target defined by the EN 15757 is calculated in function of the seasonal cycle, calculated as a 30-
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day run average, and the short-term fluctuations, calculated by the exclusion of the 14% major differences between the recorded data and the seasonal cycle. Adding the 7th and 93th percentiles of the short-term
of organic hygroscopic materials [10].
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fluctuations to the seasonal cycles it was obtained the target range that aims to limit the mechanical degradation
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The PAS 198 [11], a recent guideline published by the BSI, determines some ranges of temperature and relative
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humidity taking into account various factors as the chemical, mechanical and biological degradation, energy efficiency and human comfort. For this purpose it was defined a range of temperature from 7ºC to 23ºC, according the energy considerations. For the RH it was defined a range from 30% to 65% to take into account the mechanical stability and the energy considerations. Finally it was used the data records of the church without any limits.
2.4.2. Mechanical degradation
The fluctuations of T and RH originate changes in the moisture equilibrium of the organic hygroscopic materials that can lead to important degradation phenomena. It is possible to find some targets in the bibliography aiming to limit this phenomenon, but a risk-based analysis applied for each case is indispensable.
The temperature does not usually appear as a key factor for conservation. For example, hided glues can survive fluctuations from -29ºC to + 32ºC without plastic deformations. However, it is known that some materials such
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as acrylics, alkyds and oil paints when exposed to low temperatures become brittle. If the temperature of the glass transition (12.8ºC for the acrylics) will be respect, the risks of mechanical degradation due the T fluctuations remain very low [18].
The research published by Mecklenburg et al. [19], where the authors assess the climate-induced mechanical damage of some materials of painted panels, is a good example of how it is possible to evaluate the mechanical
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degradation in function of RH fluctuations. The allowable RH fluctuations that do not lead to plastic deformations of the basis materials (for the cottonwood) is shown in Fig. 3.a. The x-axis presents the RH
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corresponding to the equilibrium moisture content and the y-axis the RH at the surface. The method considers a
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yield strain of 0.004, a conservative value since 0.0055 is the yield strain generally assumed for the majority of the old woods.
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Despite their effectiveness on the painted wood analysis, the method shows some simplifications that compromise their application on more elaborated objects and where the moisture gradient from the core to the
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surface is important, as in the case of sculptures. Moreover, this method does not consider the influence of the cycles, considering only the full response of the materials. Accordingly Jakieła et al. [20] have modulated the
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instantaneous and daily hygrothermal response of lime wood cylinders. This method relates the internal and surface behaviour taking into account the response time of the layers and the duration of the fluctuations. The
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fluctuations of moisture content assume a fundamental role in the mechanical response of the objects, i.e., for example, when the surface is drying, the core remains with higher moisture content, resulting in high tensions. The stresses decrease when the moisture content of the core is closer to the one of the surface.
The allowable fluctuations of RH in function of the starting RH are shown in the Fig. 3.b) for a step fluctuation of RH. It was decided to use this approach instead the daily fluctuation, since it is the more adverse and conservative scenario.
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Figure 3. Mechanical degradation: a) Mechanical response of the base layer of painted panels due the RH fluctuations [19]; b) Mechanical response of lime Wood cylinders simulating the sculptures response due a step RH fluctuations [20]. 2.4.3. Biological degradation
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The biological degradation is one of the most important causes of building pathology and usually is directly linked to the mould growth that occurs for high relative humidities.
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Several authors have studied this theme, and the isopleth method defined by Sedlbauer [21] has generated a great consensus. The author defined this method to predict the mould growth, considering a great number of fungal
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species present usually in buildings. The method was constructed according to three grand lines: temperature,
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relative humidity and the substrate quality, which must coexist along a certain time period, such as demonstrated in the so-called Isopleth diagrams that give the boundaries of T, RH and time needed for spore germination and mould growth. In the so-called Isopleth diagrams, the Lowest Isopleth for Mould – LIM – is the minimum limit for the mould activity. It is important to note that if the conditions for growth are good but the time of exposure is insufficient to lead a germination the growth will not occur, unless a previous contamination had existed.
The method considers four different classes of substrate: 0 - optimal culture medium; I - biologically recyclable building materials; II - biologically adverse building materials and III - building materials that are neither degradable nor contain any nutrients [22].
According to the conservative principles of the method, and to the specificities of the historic buildings it was decided to make an analysis based on the substrate I, as is represented in Fig 4.
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Figure 4. Isopleth method of Sedlbauer for the substrate type I: a) germination time; b) growth rate [21].
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2.4.4. Chemical degradation
The chemical degradation assumes an important role in conservation science, although in many cases it may not
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be considered, probably due to the difficulties to maintain the adequate levels of T and RH, and to the fact that these levels may be in contradiction with those used to limit the mechanical degradation.
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The hydrolysis, one of the major manifestations of the chemical degradation, is commonly characterized by the Arrhenius equation [23][24], that allows the calculation of the degradation rate. Sometimes this equation is
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difficult to apply, which lead to the formulation of new approaches. The researchers of the Image Permanence
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Institute (IPI) developed some experiments and defined an empirical equation that allows to estimate how long it is necessary for cellulose acetate to evidence significant signals of deterioration, as discoloration, embrittlement and other changes that involve loss in appearance or functionality [25][26]. The IPI has established a value of 45 years as the minimum to guarantee a proper conservation. The equation 3 shows this estimator, denominated as Preservation Index [25]:
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e (95220134.9RH ) /(8.314(T 273.15)0.0284RH 28.023 365
(3)
with RH in % and T in degree Celsius
However, it must be kept in mind that the method defined by the IPI is an empirical result based on specific data for cellulose acetate and it is therefore not possible to extrapolate the results to other materials. Therefore it will be used only as a qualitative classification method. In this work, it was decided to use a different method – the concept of Lifetime Multiplier, define by Michalski
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[27]. This equation returns a multiplier factor that compares the real pair of T and RH with the conditions for the set-point of 20ºC-50%. This method does not allows a lifetime prevision, but only a comparison with the standard values, as it is possible to see:
1.3
Ea
1
1
50% e R Ti 273.15 293.15 LM i RH i
(4)
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where LMi is the Lifetime multiplier at point i, Ea the activation energy [J/mol] – 70 for the yellowing varnish and 100 for degradation of cellulose [27][28], R the gas constant (8.314 J/mol K), Ti the temperature at the point
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i [ºC], RHi the relative humidity at point i [%] and ―i‖, the data point in data series.
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In order to facilitate the analysis and to evaluate the annual response, an equivalent Lifetime Multiplier was used that returns a unique value, representing the influence of all year. Instead to the use of the arithmetic average, it
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was decided to calculate the equivalent value by the average of the reciprocal values of lifetime multiplier, increasing the influence of the points with worse conditions, as it was made by the IPI to calculate the Time
(5)
1 1.3
Ea
1
1
1 N 50% e R Ti 273.15 293.15 N i 1 RH i
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eLM
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Weighted Preservation Index [25][26]:
3.
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where the eLM is the equivalent lifetime multiplier and N is the number of data points.
RESULTS
3.1. Indoor characterization
The monitoring process is one of the most important steps of the hygrothermal rehabilitation, allowing a profound knowledge about the behaviour and limitations of the buildings.
The analysis of the recorded data showed the relationship between the outdoor (S.6) and indoor (S.3) conditions (Fig. 5 and 6). As expected, the indoor temperature is much more stable that the outdoor, both in term of seasonal and instantaneous fluctuations (Fig. 5.a)). The influence of the thermal inertia in delaying and damping of the seasonal cycles was confirmed, with a delay of 7.1 days in the winter of 2011/2012 (Fig. 5.b)) and 7.5 days in the summer of 2012 (Fig. 5.c)). The thermal inertia compensates the high thermal conductivity of the existent materials, providing a damping of 2.6ºC in the winter 2011/2012 (Fig. 5.b)).
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Figure 5. Indoor and outdoor temperatures and seasonal cycles: a) annual behaviour; b) effect of the thermal inertia in winter; c) effect of the thermal inertia in summer. The summer situation is slightly different. As it is possible to see in Fig. 5.c), the delay remains and is consistent
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with the one of winter, but the damping is more attenuated (0.6ºC). This fact can be justified by the high solar radiation in the summer and the low mass of the coverage, composed by a wood ceiling and a ventilated roof of
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ceramic tiles that quickly responds to the external fluctuations.
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The behaviour for relative humidity is similar (but inverse) to the one of temperature, with the maximum values
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occurring in the winter and the minimum in summer, as shown in Fig. 6.
Figure 6. Indoor and outdoor recorded RH and seasonal cycles.
In order to understand the internal fluctuations, measurements were taken in a vertical profile at 0.15 (S.1) and 10 m (S.5), and presented for four typical days in winter (Fig. 7.a) and summer (Fig. 7.b) from Friday to Monday.
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Figure 7. Vertical profile of the indoor air temperature and relative humidity at 0.15 m (S.1) and 10 m (S.5) in: a) winter (January) and b) summer (August).
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It was possible to verify different behaviours in winter and summer. During the winter, the indoor conditions remain more stable, with the highest temperatures near the floor, being possible to note the effects of air
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convection. In this period various fluctuations were noted during the opening hours of the church, remaining
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stable otherwise, enhancing the influence of the external factors, as the human presence.
In summer, the temperatures near the ceiling are higher, what can be justified by the increase of the exterior
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temperature and by the higher number of hours of sunshine. During this period air stratification by temperatures was noted, with a gradual increase from the floor to the roof. The conditions during this period are less stable
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the internal behaviour.
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than in winter, following more closely the external fluctuations and confirming the influence of the coverage in
The risk of surface condensations is a frequent problem in buildings with high thermal inertia that can contribute to the degradation of several materials and the deterioration of the interior environment.
Analysing Fig. 8 it is possible to conclude that the surface conditions are more stables than the air, demonstrating once again the influence of the thermal inertia. Taking into account only the superficial RH fluctuations related to the T differences between the air and the surface, the risk of surface condensations has never been real since the higher relative humidities were always lower than 90% both on the air and at the surface.
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Figure 8. Psychrometric graphic with air and surface conditions.
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3.2. Response of the building and collections
3.2.1. Building response
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To qualify the hygrothermal capacity of the building a performance index denominated BR – building response was used (based on [29]) that calculates the percentage of time in which each target is supported by the natural
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defined range.
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response of the building. Lower values of this ratio indicate the need for a greater amount of energy to ensure the
The 3 set-points and the BR indexes for T, RH and combined T plus RH are presented in Fig. 9, showing that the 20ºC-50% target is satisfied only during 0.5% of the year, thus requiring an extreme consumption of energy.
The target defined by the PAS 198, from the mechanical and energy points of view, presents a better behaviour, but is only satisfied in a short period – 37.9%. It was noted that the ranges defined as economics for a certain country may not be for other locations. The dynamic target of the EN 15757 allows the better behaviour among the three guidelines, with a BR index of 75.3%. The BR values for the natural climate are obviously 100% (according to the definition of the index).
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Figure 9. Building Response (BR) according with the 3 set-points in analysis: 20ºC-50%, EN 15757 and PAS 198. a) Temperature; b) Relative humidity. 3.2.2. Mechanical degradation
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Sometimes the analysis of mechanical risks is based on the air fluctuations, but it is important to note that the dimensional changes of hygroscopic materials occur in function of their moisture content. Usually the objects do
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not respond immediately to the air fluctuations; often the equilibrium is reached after several hours, days or even weeks [20] and depends on the adsorption/desorption characteristics of the materials. It is important to analyse
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the response time for each material, taking into account the fact that the interior and the surface layers do not respond at the same time.
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To enable the analysis in function of the response of the materials instead the air fluctuations, Martens [28]
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developed an equation to predict the RH of objects for each moment:
RH response,i
RH response,i 1 1
RH i n/3
(6)
1 n/3
Where RHi is the RH of the air, n is the number of data points logged during the response time of the material in analysis, RHresponse correspond to RH of the object, assuming that in the end of the response time the object reaches the equilibrium with the air conditions.
For the current research, the response of two types of wooden objects was evaluated: painted panels and sculptures. The respective time and type of response are shown in the Table 3.
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Table 3. Time and type of response for painted panels and sculptures [28]. Artefact Painted Panel Sculptures
Type of response Surface response Full response Surface response Sub-surface response causing maximum stresses
Response time 4.3 days 26 days 10 hours 15 days
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For the painted panels it was used the methodology defined by Mecklenburg et al. [19] and illustrated in Fig. 3.a. To evaluate the risk along the time, it was considered that the water content in the inner of the objects varies in
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function of their response time, thereby changing the internal restrictions, instead of considering a global
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restriction in function of the yearly average, for example. So, the x-axis was changed and now represents the response of the core and the y-axis represents the response of the surface layer, as can be seen in Fig. 10.a).
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In parallel, a risk analysis was performed for the sculptures using the method defined by Jakieła et al. [20] for a step change in RH, taken into account the higher response time of the core in relation to the superficial layers.
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The RHresponse of the core was used as a starting RH and the one at the surface as the ending value, as seen in Fig.
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10.b).
Figure 10. Mechanical risks: a) painted wood; b) sculptures.
Applying these two methodologies to the four conditions in analysis (natural climate, 20ºC-50%, PAS 198 and EN 15757) it was possible verify the influence of each one on the mechanical degradation.
The results showed that the target 20ºC-50% allows a perfect mechanical behaviour. PAS 198 allowed a perfect response for the painted panels, while for the sculptures there is a short period in plastic behaviour corresponding to 0.6% of time.
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The dynamic target defined by the EN 15757 enabled a perfect response for the sculptures and a general good behaviour for painted panels with a plastic behaviour in compression being attained during only 5.3% of the time.
It was verified that these latter three targets do not lead to a dangerous response, but doubts about the response with the natural climate remain. Applying the methodologies to the natural climate without any restrictions, it
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was possible verify a general good response. For the painted wood a plastic behaviour in compression was attained in 6.1% of the time, where the limits of RH in the worse conditions were exceeded by 4.7%. For the
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sculptures the duration of the plastic behaviour was even shorter. The objects have plastic behaviour in tension
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during 0.8% of the time.
Despite the better performance of the tightest ranges, one can conclude that in any case the risk of mechanical
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damage is not too high, showing that the natural climate present in the unheated building do not lead to high
3.2.3. Biological degradation
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risks of physical damage.
The biological degradation is a frequent risk in old buildings, not only directly in the objects, but also on the
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interior surfaces. Since a correct analysis should be conducted for the surface conditions, it was considered that
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the object responds instantaneously to air fluctuations. The isopleth system for the substrate type I was used. Applying a more conservative case would mean that if it is secure for this condition all the others should also be secure [22].
The representation of the four conditions in analysis is plotted in the so-called isopleth diagram, as it can be seen in Fig. 11.a. As expected the environments defined by the 20ºC-50% and PAS 198 targets do not lead to any risk. The results for the EN 15757 and the natural climate are less conclusive, being possible to see that in certain periods the conditions for the spore germination is reached, but without conclusions about the time of exposure.
Trying to obtain a more conclusive result it was decided to use the concept of mould risk factor (MRF). To determine the MRF, it was assumed that for each reading above a certain isopleth the counter would start. The MRF is obtained by summing the reciprocal of the time needed to the germination for each point above the isopleth. In this case, where the data were recorded for every 10 minutes, each reading above the isopleth of 16
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days, for example, is pondered as 1/(16 x 24 x 6) [22][26]. The result of a running sum allows the computation of the global MFR. All values above the isopleths, even if not consecutive, are added, since some spores can survive in unfavourable conditions and resume growth after those periods. The germination occurs when the MRF reaches the 1-value. After the germination, if 24 h or more of dry conditions occur the running sum must be restarted, since it is expectable that moulds do not survive more than 24 h in dry conditions [26].
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Observing the MRF evolution on Fig. 11.b a value of 0.29 for the environment defined by EN 15757 and 0.35
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for the natural climate are reached, both below the 0.5-value, considered as the boundary of the safe zone [26].
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Figure 11. Evaluation of biological risk: a) representation of the data records on the isopleth diagram; b) representation of the germination factor. 3.2.4. Chemical degradation
As it was referred for the mechanical degradation, the objects do not reach the equilibrium from the environment instantaneously. According with the IPI a running average of 24 h for T and of 30 days for RH [26] was used to obtain the response of the collections and to calculate the lifetime multiplier. While the mentioned work deals only with the degradation of the cellulose, it was decided to use it in the present study to obtain a general view about the influence of the four environments on the evaluation of chemical degradation.
The evolution of the lifetime multiplier for the four environments is presented in Fig. 12. It is possible to note that the bad conditions from this point of view occur for the high temperatures of summer but not for the high relative humidities of winter. Considering the concept of equivalent lifetime multiplier, that enhances the influence for the worst cases, it was verified that the environment from the set-point 20ºC-50% presents the best
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conditions, as expected, since the multiplier relates the current records to the stationary condition of 20ºC and 50%.
The climate limited by the PAS 198 assumes the second better response to the chemical degradation. Curiously, the environment defined by EN 15757 presents a worse scenario than the one for the natural climate. This fact
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can be justified by the imposition of the lower limit of temperatures that contributes to a lower value of eLM.
Figure 12. Chemical degradation: lifetime multiplier for varnish and cellulose.
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According the Image Permanence Institute (IPI) the lowest Preservation Index to guarantee satisfactory conservation conditions for 45 years, that corresponds approximately to the target 20ºC-50%, i.e. a eLM equal to
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1.
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This study shows that any of the used targets conduct to perfect conservation conditions, as far as chemical degradation is concerned. Despite the existing risks, it is necessary to take into account that not all collections are equally sensitive to chemical degradation; a detailed analysis for each case is required, to understand if there is a real need to improve the internal environment.
3.3. Global evaluation
Following the individual analysis performed previously it was decided to summarise the results and to create a classification method that may allow a better understanding of the global phenomena and an easier process for the comparison of results. This new classification is divided in five categories, where 5 represents the ideal conditions and 1 the worst case, and evaluates five different parameters: the hygrothermal building capacity – BR (Building response); the mechanical degradation of painted wood and sculptures; the biological degradation MRF (mould risk factor) and the chemical degradation - eLM (equivalent lifetime multiplier). No weights were attributed to the parameters, neither a final classification, considering that each building and collection present
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particular needs and the final evaluation should be made case by case.
This classification was based on the method published by Martens [28] that defines three classes and evaluates the risk of chemical, biological and mechanical degradation. However, some changes were introduced highlighting the inclusion of a new parameter (BR - building response) and the addition of 2 more classes.
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For several years the conservation science theories were based on tight limits of temperature and relative humidity, admitting that any possibility of loss in the collections was not acceptable. Nowadays a new consensus
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seems to be reached leading to a higher resilience of the collections to microclimate fluctuations. New standards, guidelines and risk-assessment methods based on laboratory tests are emerging, as those used in the current
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paper. Despite these advances some experts argue that the actual approaches, although less demanding, remain too conservative [30]. The rational being that if some materials of permanent expositions have survived for years
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to environmental changes, even before the proliferation of the acclimatization systems where the indoor climate was only controlled by the building envelope, what is the reason not to survive in the present days?
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The addition of 2 new classes can be used to overcome these questions, since in a 5-class scale it is possible to
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hardly be done on a 3-class scale.
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maintain the extremes, while allowing to take into account more intermediate behaviours, something that can
For the mechanical risk, class 5 represents a perfect behaviour, always in the elastic region, while class 1 represents the failure obtained when subjected to tension strengths. The middle zone of the scale considers 3 classes, where class 4 (good response) shows a plastic response, but only in compression and in less than 10% of the time. Class 3 (some risks) extends the limitation of the class 4 allowing a plastic response both in compression and in tension. Class 2 (potential risk) represents a behaviour in plastic region in more than 10% of time but without reaching failure.
The concept of MRF, based on the research developed by Sedlbauer [21, 22] was used to quantify the risk of biological activity and was divided in 5-classes, influenced by the limits defined by the Image Permanence Institute [26]. In the origin of the so-called isopleth method, it was defined that the germination only occurs if the ideal conditions of temperature and relative humidity were reached for a certain time, representing a 1-value of MRF. So, two classes were created for this region, class 2 for a 1-value, representing a potential risk and class 1 for a MRF higher than 1 representing a high risk. In the other extreme class 5 represents an ideal behaviour for a
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MRF equal to 0. Class 4 is less demanding, allowing germination levels to be exceeded for short periods as long as MRF is lower than 0.5. Under normal circumstances the limits defined for class 3 may be sufficient to prevent biological growth; however it is prudent to keep a careful monitoring to avoid unexpected occurrences.
For the evaluation of chemical risks the concept eLM was used, relating the risks of a given ambience (characterized by T and RH) with the reference established for 20 ºC – 50%. For this purpose a classification was
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defined around the mid value 1 (class 3 – some risks), with two classes for increased life expectancies (classes 4 and 5) and two for shorter life expectancies (classes 1 and 2). The definition of the intervals for each class was
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based on the classification of the Image Permanence Institute [26].
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Finally 5 classes for the building response factor (BR) were proposed, representing the percentage of time in which the use of HVAC systems is not necessary. For class 5 (ideal) a value of 100% was defined,
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corresponding to the absence of any energy requirements to control the indoor climate. The limits for the other classes are presented in table 4. A careful analysis should be made for classes 1 and 2 checking if the existing
unnecessary heat losses.
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The classification is presented in Table 4.
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temperature/relative targets are not too tight or if there are problems with the envelope that may lead to
Table 4. Microclimatic classification according the building response and a risk-based analysis. Category
BR [%]
Ideal
5
100
Good
4
[90;100[
Some Risk
3
[75;90[
Potential Risk
2
[50;75[
High Risk
1
10% Failure
MRF: Mould risk factor
MRF
eLM
0
>2.2
1
