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Energy and Buildings 107 (2015) 26–36
Contents lists available at ScienceDirect
Energy and Buildings
journal homepage: www.elsevier.com/locate/enbuild
Preventive conservation of historic buildings in temperate climates.
The importance of a risk-based analysis on the decision-making
Hugo Entradas Silva ∗ , Fernando M.A. Henriques
Departamento de Engenharia Civil, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history:
Received 28 May 2015
Received in revised form 28 July 2015
Accepted 29 July 2015
Available online 31 July 2015
Thermal inertia
Preventive conservation
Risk-based analysis
Historic buildings
Cultural heritage
a b s t r a c t
Historic buildings are usually characterized by a particular microclimate due to their high thermal 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
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 conservation with the progressive assumption of less demanding targets. However these
guidelines were usually defined for specific climates so that when extrapolated to other locations their
application may not be positive.
This research aims to analyze 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 20 ◦ C – 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.
© 2015 Elsevier B.V. All rights reserved.
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 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).
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 20 ◦ C for T and 50% 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
∗ Corresponding author.
E-mail address: [email protected] (H.E. Silva).
0378-7788/© 2015 Elsevier B.V. All rights reserved.
energy efficiency was still not a problem due to the low cost of
energy, and where the risk-based analysis were not extensively
used [2–4]. These targets, even in temperate climates, can only be
achieved using HVAC systems.
It is important to take into account that historic buildings often
show a particular hygrothermal response. These buildings, usually with thick walls and small percentage of transparent surfaces
when compared to the opaque 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
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 1. Main façade of the St. Christopher church (a) and the interior (b).
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].
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 to
the collection needs, searching also achieve the energy economy
without jeopardizing a proper conservation.
surfaces (Fig. 1b). The roof is made with ceramic tiles supported by
timber frames. Inside, there is a rectangular room with 144 m2 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 and 19:30, with
religious celebrations taking place at 18:30. On Sunday, it is open
between 11:00 and 13:00, with a religious celebration at 12:00. On
Monday it remains closed throughout the day.
2. Methodology
2.2. Experimental campaign
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 building and collections was analyzed
and tested according to the natural indoor climate and the targets
defined by the EN 15757, PAS 198 and the historic set-point of 20 ◦ C
and 50%.
To understand the interior microclimatic behaviour an extensive environmental monitoring was conducted. For 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 min and using three different types of
sensors, as it can be seen in Table 1. All of the sensors respect the
uncertainties defined in the European 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
2.1. Site description
The building analyzed was the Church of St. Christopher, a
national monument of Portugal located on the slopes 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 average of 17 ◦ C 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. 1a),
and walls covered with gilded and painted panels in the interior
Table 1
Sensors used in the monitoring system.
Hobo U12-13 (H)
T: ±0.35 ◦ C
RH: ± 2.5%
T: ± 0.1 ◦ C
RH: ± 2%
T: ± 0.5 ◦ C
T&RH Delta T probe
Thermocoulples (T)
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 2. Horizontal plan of the St. Christopher church and location of the T and RH sensors in the main room.
selected (in bold in Table 2): the sensors H on the northern tower
(S.6) and pulpit (S.3) to analyze 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).
2.3. Indoor characterization
For a better understanding and evaluation of the strengths and
weaknesses of the church a microclimatic analysis 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
Table 2
Location of the sensors (the 6 selected sensors are in bold).
Site (Fig. 2)
Height (m)
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)
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
To analyze 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 conclusions. This research was designed in order to evaluate the building and collection response to the natural climate and
according to the targets defined by the EN 15757, PAS 198 and the
pair 20 ◦ C – 50%, with the analysis divided in 5 points: building
response, biological, chemical and mechanical degradation of both
painted panels and sculptures.
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 the building to
respond to the targets. First the historic set-point of 20 ± 2 ◦ C 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 made.
The dynamic target defined by the EN 15757 is calculated in
function of the seasonal cycle, calculated as a 30-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 fluctuations to the seasonal cycles it was obtained the target range that
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 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].
aims to limit the mechanical degradation of organic hygroscopic
materials [10].
The PAS 198 [11], a recent guideline published by the BSI, determines some ranges of temperature and relative 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 7 ◦ C to 23 ◦ C,
according to 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.
the internal and surface behaviour taking into account the response
time of the layers and the duration of the fluctuations. The 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 Fig. 3b 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.
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 −29 ◦ C to + 32 ◦ C without plastic deformations. However, it
is known that some materials such as acrylics, alkyds and oil paints
when exposed to low temperatures become brittle. If the temperature of the glass transition (12.8 ◦ C 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 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. 3a. The x-axis presents the RH corresponding to the equilibrium moisture content and the y-axis the RH at the surface. The
method considers a yield strain of 0.004, a conservative value since
0.0055 is the yield strain generally assumed for the majority of the
old woods.
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 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 instantaneous and daily
hygrothermal response of lime wood cylinders. This method relates
2.4.3. Biological degradation
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.
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 species present usually in
buildings. The method was constructed according to three grand
lines: temperature, 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
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.
2.4.4. Chemical degradation
The chemical degradation assumes an important role in conservation science, although in many cases it may not 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.
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 4. Isopleth method of Sedlbauer for the substrate type I: (a) germination time; (b) growth rate [21].
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 difficult to apply, which lead to the
formulation of new approaches. The researchers of the Image Permanence 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. Eq. (1) shows this estimator, denominated as
Preservation Index [25]:
PI =
e(95220−134.9×RH)/(8.314×(T +273.15)+0.0284×RH−28.023
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 [27]. This equation
returns a multiplier factor that compares the real pair of T and RH
with the conditions for the set-point of 20 ◦ C – 50%. This method
does not allows a lifetime prevision, but only a comparison with
the standard values, as it is possible to see:
LMi =
50% 1.3
× e(Ea /R)×(1/Ti +273.15)−(1/293.15)
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 i [◦ C], RHi the relative humidity at point i
[%] and “i”, the data point in data series.
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 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 Weighted
Preservation Index [25,26]:
eLM =
1/N ×
(50%/RHi )
× e(Ea /R)×(1/Ti +273.15)−(1/293.15)
where the eLM is the equivalent lifetime multiplier and N is the
number of data points.
3. 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
(Figs. 5 and 6). As expected, the indoor temperature is much more
stable that the outdoor, both in term of seasonal and instantaneous
fluctuations (Fig. 5a). 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. 5b) and 7.5 days in the
summer of 2012 (Fig. 5c). The thermal inertia compensates the high
thermal conductivity of the existent materials, providing a damping
of 2.6 ◦ C in the winter 2011/2012 (Fig. 5b).
The summer situation is slightly different. As it is possible to
see in Fig. 5c, the delay remains and is consistent with the one of
winter, but the damping is more attenuated (0.6 ◦ C). 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 ceramic tiles that quickly responds to the external fluctuations.
The behaviour for relative humidity is similar (but inverse) to
the one of temperature, with the maximum values occurring in the
winter and the minimum in summer, as shown in Fig. 6.
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. 7a) and summer (Fig. 7b)
from Friday to Monday.
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 convection. In this period various fluctuations were
noted during the opening hours of the church, remaining stable
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 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.
Fig. 6. Indoor and outdoor recorded RH and seasonal cycles.
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 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 than in winter, following more closely the external fluctuations and confirming the influence of the coverage in the internal
The risk of surface condensations is a frequent problem in
buildings with high thermal inertia that can contribute to the
Fig. 8. Psychrometric graphic with air and surface conditions.
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
Fig. 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).
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
real since the higher relative humidities were always lower than
90% both on the air and at the surface.
Table 3
Time and type of response for painted panels and sculptures [28].
Type of response
Response time
3.2. Response of the building and collections
Painted panel
Surface response
Full response
4.3 days
26 days
3.2.1. Building response
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 response of the building. Lower values
of this ratio indicate the need for a greater amount of energy to
ensure the defined range.
The 3 set-points and the BR indexes for T, RH and combined T
plus RH are presented in Fig. 9, showing that the 20 ◦ C – 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).
Surface response
Sub-surface response causing
maximum stresses
10 h
15 days
3.2.2. Mechanical degradation
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 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
analyze 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.
To enable the analysis in function of the response of the materials instead the air fluctuations, Martens [28] developed an equation
to predict the RH of objects for each moment:
RHresponse,i =
RHresponse,i−1 + RHi /(n/3)
1 + (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 Table 3.
For the painted panels it was used the methodology defined by
Mecklenburg et al. [19] and illustrated in Fig. 3a. To evaluate the
risk along the time, it was considered that the water content in
the inner of the objects varies in function of their response time,
thereby changing the internal restrictions, instead of considering
a global 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. 10a.
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. 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. 10b.
Applying these two methodologies to the four conditions in
analysis (natural climate, 20 ◦ C – 50%, PAS 198 and EN 15757) it
was possible verify the influence of each one on the mechanical
The results showed that the target 20 ◦ C – 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.
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 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 sculptures
the duration of the plastic behaviour was even shorter. The objects
have plastic behaviour in tension 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 damage is not
too high, showing that the natural climate present in the unheated
building do not lead to high risks of physical damage.
Fig. 9. Building Response (BR) according with the 3 set-points in analysis: 20 ◦ C – 50%, EN 15757 and PAS 198. (a) Temperature; (b) relative humidity.
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 10. Mechanical risks: (a) painted wood; (b) sculptures.
3.2.3. Biological degradation
The biological degradation is a frequent risk in old buildings, not
only directly in the objects, but also on the interior surfaces. Since
a correct analysis should be conducted for the surface conditions,
it was considered that 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. 11a. As
expected the environments defined by the 20 ◦ C – 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 min, each reading above the isopleth of 16 days, for example,
is pondered as 1/(16 × 24 × 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].
Observing the MRF evolution in Fig. 11b a value of 0.29 for the
environment defined by EN 15757 and 0.35 for the natural climate
are reached, both below the 0.5-value, considered as the boundary
of the safe zone [26].
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
Fig. 11. Evaluation of biological risk: (a) representation of the data records on the isopleth diagram; (b) representation of the germination factor.
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Fig. 12. Chemical degradation: lifetime multiplier for varnish and cellulose.
environment from the set-point 20 ◦ C – 50% presents the best conditions, as expected, since the multiplier relates the current records
to the stationary condition of 20 ◦ C 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 can be justified by the imposition of
the lower limit of temperatures that contributes to a lower value
of eLM.
According to the Image Permanence Institute (IPI) the lowest
Preservation Index to guarantee satisfactory conservation conditions for 45 years, that corresponds approximately to the target
20 ◦ C – 50%, i.e. a eLM equal to 1.
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 summarize 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
particular needs and the final evaluation should be made case by
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
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 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 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 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?
The addition of 2 new classes can be used to overcome these
questions, since in a 5-class scale it is possible to maintain the
extremes, while allowing to take into account more intermediate
behaviours, something that can hardly be done on a 3-class scale.
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 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 ◦ C – 50%. For this purpose a
classification was 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 based on the classification
of the Image Permanence Institute [26].
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, 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 temperature/relative targets are not too tight or if there are problems with the envelope
that may lead to unnecessary heat losses.
The classification is presented in Table 4.
Applying this classification to the four targets, it is possible to
conclude that the set-point 20 ◦ C – 50% presents the best response
to the mechanical, biological and chemical degradation, but their
tight limits are too exigent for the building that can conform the
target ranges only in 0.5% of the year, revealing high requirements
of energy to achieve the limits.
The dynamic targets defined by EN 15757 allow a better
response of the building, that can achieve the limits without any
other measure, active or passive, during 75% of the time, conducting to a high energy economy. Despite the more permissive targets,
the EN 15757 presents a good response of mechanical and biological
H.E. Silva, F.M.A. Henriques / Energy and Buildings 107 (2015) 26–36
Table 4
Microclimatic classification according to the building response and a risk-based analysis.
Some risk
Potential risk
High risk
BR (%)
Painted wood and sculptures
Plastic: Only compression. % of time in plastic region <10%
Plastic: Compression and/or tension. % of time in plastic region <10%
Plastic. % of time in plastic region >10%
BR, building response; MRF, mould risk factor; eLM, equivalent lifetime multiplier.
Table 5
Classification and comparison between the four approaches in analysis.
20 ◦ C;50%
EN 15757
PAS 198
Painted wood
P: C. = 5.3%
P: C. = 6.1%
P: C.+T. = 0.6%
P: C.+T. = 0.8%
E, elastic response; P:C., plastic response in compressions; P. C.+T, plastic response in tension or in tension and compression.
biological and chemical response of the collections. It was possible
to achieve some relevant conclusions, namely:
Fig. 13. Graphical representation of the microclimatic classification of the four
approaches in analysis.
Using the PAS 198 it is possible to observe an ideal mechanical
behaviour for the painted woods and in terms of mould growth. For
the sculptures there are some risks but not extremely dangerous.
Despite the use of T ranges considering energy conservation and RH
ranges according with mechanical and energy requirements, the
building responds positively to this target only in 37.9% of the year,
demonstrating that guidelines should not be used widely without
previous validation.
For the natural climate it was possible to note the absence of
mould risk and a good mechanical behaviour for painted panels,
while for sculptures there are some risks. It was also possible to note
that the elastic limits are exceeded only in 0.8% of time, something
that can hardly be seen as a dangerous behaviour. This classification is presented in Table 5. The results are also represented in a
decision-make diagram (Fig. 13).
4. Conclusions
This research analyzed the influence of 3 targets (20 ◦ C – 50%;
EN 15757 and PAS 198) in an unheated historic building in temperate climate and made a comparison between those targets and
the natural climate by using a risk-based analysis. It was evaluated the hygrothermal capacity of the building and the mechanical,
• There was a perfect mechanical response of the collections when
the set-point 20 ◦ C – 50% was applied. The dynamic target of
EN 15757 lead to a perfect response for sculptures and a plastic response in 5.3% of the year for the painted wood. The target
defined by PAS 198 allows perfect conditions for the painted
wood, with plastic response in 0.6% of the year for sculptures. The
natural climate without any constraints lead to a plastic response
in 6.1% of the year for the painted wood and in 0.8% for sculptures.
• There were no biological risks for the four conditions.
• All targets allow chemical risks especially from May to October
when the temperatures are higher. Since not all the collections
are equally sensitive to chemical degradation, a detailed analysis
may be required to understand if there is a real need to improve
the internal environment.
• The target of 20 ◦ C – 50% is very demanding in terms of the
hygrothermal response of the building, being reached only during 0.5% of the time, while the EN 15757 is reached in 75.3% and
PAS 198 in 37.9%.
It was possible to conclude that the more demanding set-points
require the use of strong HVAC systems and high energy consumptions that often are not required by the collection. Some guidelines
conceived for particular climates may not result if applied in other
locations, hence the need for a previous validation for each climate
before they are used.
Finally, it was concluded that a detailed knowledge about the
hygrothermal response of each building and a risk-based analysis
could lead to energy savings without compromising the conservation of the collections.
The study was co-financed by COMPETE funds in its FEDER component and by the budget of the FCT – Foundation for Science
and Technology under the research project PTDC/ECM-COM/3080/
2012. The study received support from the FCT – Foundation
for Science and Technology under the PhD scholarship PD/BD/
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