This paper was published in Studies in Conservation, 52, 199-210 (2007)

Original publication is avaible at: http://www.iiconservation.org

Impact of indoor heating on painted wood: monitoring the

altarpiece in the church of Santa Maria Maddalena in

Rocca Pietore, Italy

Ł. Bratasz, R. Kozłowski, D. Camuffo, E. Pagan

1

Impact of indoor heating on painted wood:

monitoring the altarpiece in the church

of Santa Maria Maddalena in Rocca Pietore, Italy

Łukasz Bratasz a, Roman Kozłowski a, Dario Camuffo b, Emanuela Pagan b

a Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,

ul. Niezapominajek 8, 30-239 Kraków, Poland, e-mail: nckozlow@cyf-kr.edu.pl

National Research Council of Italy, Institute of Atmospheric Sciences and Climate (CNR-

ISAC), Corso Stati Uniti 4, I-35127 Padova, Italy. e-mail: d.camuffo@isac.cnr.it

Abstract

In the church of Santa Maria Maddalena in Rocca Pietore, Italy, the dimensional response of

the wooden altarpiece to variations in indoor temperature and relative humidity (RH) was

monitored between December 2002 and March 2005. Measurements demonstrated that only

the external layer of wood, several millimetres thick, continually absorbs and releases water

vapour following external variations in RH. For massive elements this leads to strong

gradients in the moisture content through wood, a restraint of the dimensional change and a

development of stress, which is the main threat to the integrity of wood and the decorative

layer. Particularly strong RH variations and related high stress levels were produced by the

intermittent heating system based on the inflow of warm air. To incorporate requirements for

preservation, heating systems must provide a localised comfortable temperature in the area

where people are without changing the natural climate of the church as a whole.

Introduction

Originally historic churches had no heating. The demand for heating in churches increased

with the improvement of heating at home. Most churches are heated now and a number of

heating systems have been developed, the choice usually being made between warm air

emitted from floor or wall, radiant heaters - electrical or heated by gas combustion, water-

filled radiators heated from a boiler, underfloor and pew heating [1,2]. Two fundamental

strategies for heating churches can be distinguished: stationary heating, when a church is kept

at a constant temperature, and intermittent heating when a church is heated occasionally over

a short time. A combination of the two strategies is sometimes adopted when a church is

continually kept at a primary low temperature of 8-12 oC and is heated rapidly shortly before

and during services to a comfortable temperature of 15-20 oC [3, 4].

It has been recognised that unheated churches generally preserved their artworks in good

condition over centuries, while rapid signs of degradation were found after heating had been

introduced. Therefore, many conservation authorities conclude that no heating is best.

However, a constant low-level heating might be preferable in countries with mild, rainy

winters as a simple method to avoid exceedingly high humidity and related mould growth.

Another aim of low-level background heating is the reduction of condensation risk on cold

indoor surfaces specially during spring.

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The indoor climates of unheated buildings are essentially governed by the outside climate

modified by the building envelope which impedes heat exchange between outdoors and

indoors, and buffers the external climatic variations by taking up and giving off heat and

humidity. Generally, churches possess large buffering capacities and, if properly maintained

and used, low rates of air exchange. Therefore, their indoor conditions slowly follow the

average outdoor climate. The seasonal cycle is attenuated and the short-term variations are

significantly attenuated. Furthermore, works of art made of moisture-containing materials

sensitive to variations of thermal and humidity conditions, like wood, have adapted over the

centuries to the local climate pattern. This adaptation might have involved a certain degree of

permanent change, as deformation or fracturing, releasing internal tensions in the materials

generated by the variations of relative humidity (RH) and temperature (T). Should a work of

art be brought into a different microclimate – even a well-controlled museum environment –

or should the variations of RH and T increase, it will suffer damage.

The concept of a local natural climate, to which the objects have adapted over their long-

term exposure, was first derived from scientific observations [5, 6] and then explicitly

expressed in the Italian Standard [7] on choice and control of the indoor environmental

conditions favouring conservation of sensitive historic materials. The standard stresses that

for the best preservation of materials sensitive to moisture, the recommended RH ranges

should replicate the long-term local climate and that the RH fluctuations centred on the local

RH level must be kept to a minimum.

Heating can introduce serious destabilisation to the natural indoor climate in a church. A

stationary heating regime may bring the indoor temperature to a high ‘comfortable’ level

causing a low and variable RH indoors as the cold air outside is drawn in and heated up. The

intermittent heating for liturgical services or cultural events may in turn cause periodic

fluctuations of RH corresponding to the heating episodes when RH drops first from high to

low levels and then returns to high RH after the heating system is switched off. Two general

principles in church heating, therefore, could be formulated to reduce the adverse effects

described:

- heat a church as little as possible during the cold season, or adjust carefully the heat

input just to reduce the excessive dampness and the drop of temperature in winter

- provide localised heating to the areas where people are and maintain a natural or

approximately natural climate in the rest of a church

A broader European research programme [8] was undertaken to develop a novel heating

system consisting of low temperature radiant sources located in pews which would provide

direct confined heat just to people sitting and leave undisturbed the church as a whole. The

work focused on the church of Santa Maria Maddalena in Rocca Pietore, situated at 1143 m

above sea level in the Italian Dolomites. Previously, the church had had an intermittent

heating system based on a forced inflow of warm air. The system operated usually a number

of times a week, for a short period during services. A detailed study of the internal climate of

the church recognised a particularly negative impact of this heating system on the church

fabric and its contents as it generated short-term temperature peaks accompanied by drops in

RH [9].

An important part of the present investigations was a continuous in situ monitoring of the

dimensional response of the main altar, a polychromed and gilded wooden triptych, executed

in 1516-17 by Ruprecht Potsch from Bressanone (Figure 1). The altar was carved in lime

wood (Tilia sp.). Due to its size and location in the church, the altarpiece was particularly

endangered by the microclimatic variations caused by the heating episodes. A few years after

restoration, new deep cracks appeared in the triptych, explicable by the repeated desiccation

of the wood subjected to blown warm air (Figure 2). The inventory of damage made during

the current project revealed an ongoing deterioration of the altarpiece, especially on its upper

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part. The monitoring of the altarpiece recorded not only dimensional changes of carved

elements of different thicknesses, but also the closing and opening of a large crack in the

wood (Figure 3). The results of the monitoring, which ran for more than two years, between

December 2002 and March 2005, are presented here and discussed to provide direct insights

into the response of complex historic wooden objects to short-, medium- and long-term

variations of T and RH which make up the environment of a heated church. A more general

assessment of the idea of the intermittent heating of historic churches will be attempted.

Monitoring the dimensional response of wood

The dimensional changes occurring in two carved elements of differing thickness were of

particular interest. One element was a head – a massive element of cylindrical shape 15.5 cm

in diameter, and the other a finger – a fine element 4 cm long and 0.5 cm thick. The locations

of these elements on the altarpiece are marked in Figure 1. Triangulation laser displacement

sensors were used as they allowed fast, precise, non-contact field measurements. The

principle and features of the triangulation technique, details of the measuring system and

accuracy of the obtained data were described in detail in [10].

The width of a crack in the saint’s head was monitored parallel to the dimensional change

of the head as a whole. The crack was about 2 mm wide and ran 2-3 cm deep into the wooden

structure. A small inductive EX-110V Keyence sensor was attached to one internal surface of

the crack and a metallic reference plate to the other (Figure 3). The sensor recorded changes

of inductivity resulting from the displacement of the reference and thus allowed precise

monitoring of the crack movement within a 2 mm range with uncertainty of about 6 µm. The

measurements were possible for RH values below around 60 % because higher RH caused

significant narrowing of the crack, which disturbed the measurements.

The selection of the carved elements for monitoring was guided by an attempt to follow

the mechanism of the deterioration process for the Rocca Pietore altarpiece driven by the RH

variations. Massive wooden elements - like the head of the sculpture - are most endangered on

the fall in RH. As the moisture diffusion out of wood is not instantaneous, the external layer

of wood dries more quickly than the interior. The gradient of moisture develops and leads to a

differential shrinkage through the wood. The dry external layer is restrained from shrinkage

by the substrate of still wet and swollen wood beneath. This creates mechanical stress and

eventually wood can crack if the tension goes beyond the strength of the material [11, 12, 13].

The described mechanism is not pertinent to finer, freely moving wooden elements which

experience a uniform moisture distribution on drying or wetting due to their small size, and

hence little internal restraint and stress. It was important to precisely know the response of the

altarpiece to the natural climatic conditions prevailing in the church to which the object had

adapted throughout its history. The wooden sculpture was in perfect condition some ten years

ago, when it was restored. The subsequent recent damage can be attributed to the use of

intermittent warm air heating.

Results and discussion

The seasonal cycle of the local climate

The climate in the church in Rocca Pietore is essentially governed by the outside climate,

the protective and buffering properties of the building envelope and the use of the church. All

indoor air parameters were measured at many sampling points within the church, and the

nearest point to the altar was a probe placed in its central part at 3.5 m above the floor. The

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natural ventilation rate in the church, when the heating was off, was 1.3-1.4 air change per

hour (ACH) which can be regarded as comparatively high. For example, ACH of around 0.1

is typical for churches with airtight plastered stone vaults and 0.5 – 0.75 for churches with a

wooden vault [1]. A high air exchange between the interior and the outside is illustrated in

Figure 4 by plots of indoor and outdoor moisture content in 2003 expressed as humidity

mixing ratio (MR), i.e. grams of water vapour per kilogramme of dry air. The MR inside

varies between about 2 and 13 g/kg which is in accordance with the external seasonal

variations. Figure 5 shows plots of T and RH inside and outside during 2003. The variations

both in T and RH indoors are evened out when compared with outdoors due to impeded air

exchange and the buffering effect of the building.

The seasonal cycles of both T and RH indoors result from the external seasonal variation

and heating the church in the cold period. The interior temperature varied seasonally from

about 1 oC in January to 24

oC in August with a marked increase in the cold period caused by

heating. The heating episodes are visible as upward blips in the indoor temperature record.

RH shows the same yearly cycle as temperature which ranges from about 35 % in March to

72 % in June. Average RH is about 55 %. It should be stressed that the low-high RH cycle

indoors is caused by heating in winter as the RH outside does not show any distinct seasonal

variation and fluctuates irregularly around a calendar year average of 70 %. The winter

heating brings down the indoor RH by approximately 15-20 % into the range between 30-50

% instead of 50-70 % during the unheated period. However, as shown below, short episodes

of quick warming-up, giving rise to drastic drops in RH, are the main cause of damage to the

wood rather than the average drop in RH during winter.

The seasonal cycles in the indoor RH are reflected in dimensional changes of wooden

elements as illustrated in Figure 6. The figure compares plots of the dimensional response of

wooden elements of the altarpiece with the T and RH variations. The diameter of the thick

wooden head varied within a range of about 0.4 % - a contraction of 0.2 % on the maximum

decrease in RH to 34 % in March was followed by an expansion of 0.2 % on the rise of RH up

to 73 % in summer, if the mean head diameter was assumed as a zero level.

Wood is anisotropic and its moisture-related dimensional changes vary in its three

principal structural axes – longitudinal, or parallel to grain, radial and tangential. The

monitored sculpture can be viewed as carved from a cylindrical tree stem; therefore the

diameter of the head most likely coincides with the radial direction in the wood. The

laboratory measurements of the radial moisture-related expansion was for lime wood

1,38±0,05*10-3

per percent by weight uptake of water. If an equilibrium moisture content in

wood had been attained, the RH variation of 39 % during the seasonal cycle would have

caused water content variation of 7.5 % by weight. This should have led to a wood movement

within a range of about 1 % i.e. 2.5 times larger than actually observed. The smoothing of the

response of the element with respect to the RH change was due to the slow moisture diffusion

into and out of the wood and the resulting dimensional restraint which did not allow for the

equilibration of a 15 cm thick element even to the very slow, year-long variation of RH.

The movement of the fine wooden finger was confined to a much broader range of about

2.5 % - the contraction of 1.75 in winter was followed by the expansion of 0.75 in summer.

The observed movement even exceeded the range of 1.4 % calculated from the radial swelling

coefficient measured in the laboratory - apparently an extra component due to a movement of

the entire hand disturbed the measurement of the long-term dimensional change of the finger,

in spite of several precautions in the configuration of the experiment to record just the isolated

relative dimensional change of each particular element [10]. However, the dimensional

changes of the finger recorded over shorter periods during which the disturbance was less

relevant were in close accord with the calculated changes. Figure 7 shows an example of

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measured and calculated movements of the finger in August 2003 in which longer and shorter

RH variations occurred in the church.

At this point it should be added that the thermal contraction or expansion affects the

overall dimensional change of wood as the pattern of the RH and T variability is the same

during the year. As shown below, the variability in temperature rapidly affects the structure of

the wood when compared to the rate of heating, so there is no smoothing effect on the

response to any temperature variation. The thermal expansion coefficient of the lime wood

measured in the laboratory was about 7±1*10-5

per oC for radial as well as for tangential

directions. Therefore, the temperature variation from the minimum of –1 oC in winter to the

maximum of 25 oC in summer caused the dimensional change of 0.2 % as compared to 1-1.4

% caused by the RH variation as calculated above. Therefore thermal expansion had a

comparatively minor effect on the overall dimensional change of the wood.

Short-term variations

The monitoring allowed a comparison of the response of wooden elements to irregular

variations in RH for periods shorter than a seasonal cycle. The plots of the dimensional

response of two wooden elements - the head and the finger – to three such microclimate

variations, recorded typically in the church in Rocca-Pietore, are shown in Figure 8.

The first was a natural weekly variation due to a spell of dry weather lasting several days.

An episode in July 2004 with a relatively strong fall in RH of 10 % followed by the change in

the opposite direction was selected. During the period between December 2003 and March

2005 only about 15 such strong cycles were recorded. The wooden head responded weakly to

this variation and the contraction recorded was 0.1 % as compared to the calculated full

response of 0.6 %.

The second short-term variation selected was an indoor diurnal cycle caused by a strong

external cycle in the summer, when solar radiation was intense, with a drop in RH of 12 %

between night and day. Almost no dimensional response of the head was recorded for this

natural diurnal cycle.

Finally, a short, man-made cycle due to a single heating episode for a service in January

2003 was selected. The heating was operated for a total duration of some 90 minutes and it

generated a quick increase of temperature from 4 to 21oC accompanied by a 27 % fall in RH

from 54 to 27 %, followed by a slow return of both parameters to their initial values when the

heating was switched off.

During this heating episode the head expanded due to the temperature increase but no

shrinkage due to the fall in RH was detected. In contrast, the finger exhibited a rapid,

unrestricted movement which agreed well with the calculated dimensional change. The finger

first expanded, due to the increase in temperature, and then shrank due to the decrease in RH.

Monitoring the crack

A large crack in the saint’s head was continuously monitored for changes in width, i.e.

narrowing and widening. Results for the winter 2003 are plotted in Figure 9 as an example.

The crack width increased on a decrease in RH due to a shrinkage of the external layer of the

sculpture and a resulting tensile stress acting tangentially. Conversely, the crack size

narrowed on an increase in RH as the external layer swelled and a tangential compressive

stress appeared. The expansion and contraction of the crack was rapid and followed the short

microclimate fluctuations as shown in Figure 10. This observation is direct evidence of stress

continuously engendered by climatic variations in the outer zone of the wooden statue. The

movement of the crack was measured as -0,047 mm or 1*10-4

of the circumference of the

sculpture per 1 % of RH change. The entirely unrestricted swelling/shrinkage at the

6

circumference of the sculpture was on average 4*10-4

per 1 % of RH change. Therefore, the

crack is able to reduce by only 25 % the tangential stress appearing in the external wooden

layer during the RH variations. In consequence, the threshold of allowable microclimate

fluctuations which wood can ultimately endure without damage, increases in the same

proportion.

Conclusions

The elements of the altar responded to the variability of indoor T and RH. The moisture

content in the wood varied according to whether RH increased or decreased in the proximity

of the altar. The variation in the moisture content caused dimensional changes in the wood;

the response, however, was characterised by ranges and rates which varied considerably with

the thickness of the wooden elements. Fine wooden element 0.5 cm thick expanded and

contracted quickly and completely even during short-term RH changes. Therefore, the

external layer of all wooden elements of the altar, at least several millimetres thick, will be

strongly affected by any, even rapid, change in RH.

On the other hand, the overall dimensional reaction of the massive wooden element 15 cm

thick was much slower as the uptake or release of the water vapour was too slow to produce a

uniform moisture content through wood. For short-term cycles, natural or due to heating

episodes, practically no change in the overall dimension of the element was observed in

association with changes in RH. The slow overall moisture movement out of or into the wood,

combined with an immediate response of the external layer, led to strong radial gradients in

moisture content. This in turn resulted in a restraint of the dimensional change and stress

development at the external layer of the wood. At a sufficient level of stress the external layer

might suffer from mechanical damage, such as cracking, a principal deterioration feature of

the wooden sculptures.

The level of the stress depends on the amplitude and rate with which the RH varies. A

systematic numerical modelling of the phenomenon for a cylindrical object, imitating a

wooden sculpture, was reported elsewhere [14]. The obtained stress levels for the natural

fluctuations were found to be much smaller than the critical magnitude corresponding to the

elastic limit of the wood, i.e. the yield point. In contrast, the fluctuations produced by the

intermittent heating with the inflow of warm air gave rise to stress levels beyond the

allowable limit. The modelling further predicted that the most endangered massive wooden

elements of the altarpiece could endure significant RH fluctuations of the amplitude of up to

25 % when the initial RH level was 70 %.

The cracking of wood, which has occurred in the past, can increase the allowable

threshold in the microclimate variations [6] as the crack movement will release some of the

stress at the external layer of the wood and facilitate diffusion of the water vapour into and out

of the wood. However, only substantial cracking seems to bring about a significant

‘adaptation’ of a sculpture to the environmental variations.

The results of the present study add new arguments to the discussion of advantages and

disadvantages of intermittent heating in historic churches. Rapid heating of a church for

services is sometimes claimed to be a good strategy because ‘it reduces the relative humidity,

but for such a short time that the painted woodwork does not have time to respond’ [4]. The

lack of an overall dimensional response of massive wooden elements to fast RH changes

gives a false impression of ‘stability’ of the objects. In reality, their external zone, at least to

the depth of several millimetres, continually absorbs and releases water vapour, changing

their size. The resulting gradient of moisture content and stress is the main threat to the

integrity of wood and to the decorative layer.

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There is a similarity and not a difference between the RH buffering exerted by wood and

by a painted porous wall, described in [4]. The danger in both cases arises from the moisture

movement in the surface layer: in the case of the wall, it will cause salt crystallisation

threatening the adhesion of a wall painting; in the case of wood, it will cause stress leading to

mechanical damage.

Intermittent heating can be economical. It can be also the least damaging strategy for

heating historic churches under the condition that it provides a localised comfortable

temperatures in the area where people are without changing the natural climate of the church

as a whole. The novel heating system developed within the European Project Friendly

Heating (2002-2005) provides direct confined heat to people seating in the pew area. It brings

an enormous improvement in the condition of the altarpiece in Rocca Pietore [15].

Other heat sources like properly positioned electric overhead radiant heaters, warming up

the floor and seats, can be advocated as providing localised heat without adversely affecting

painted walls and the contents of churches. However, care should be taken that sensitive

works of art, responding rapidly to changes in T and RH, like paintings on canvas or wood

panel, are not exposed to intense infrared radiation as they can be damaged by the direct

increase of the temperature and by the associated loss of moisture. Finally, the best form of

localised intermittent heating should be developed individually for each church building as

each constitutes a special case.

Acknowledgements

This research was carried out within the FRIENDLY HEATING project (contract EVK4-CT-

2001-00067), supported financially by the European Commission 5th Framework Programme,

Thematic Priority: Environment and Sustainable Development, Key Action 4: City of

Tomorrow and Cultural Heritage. Thanks are due to Laszlo Bencs (University of Antwerp)

and Henk Schellen (Technical University, Eindhoven) for their information and advice

concerning ventilation. We thank also Mons. Giancarlo Santi and Don Stefano Russo from the

Italian Episcopal Conference in Rome, Don Giacomo Mezzorana of the Diocese in Belluno

and Don Attilio De Zaiacomo, the parish priest in Rocca Pietore, for their assistance in this

study.

Suppliers

Laser displacement sensors: Micro-Epsilon Messtechnik GmbH & Co., Koenigbacher Strasse

15, D-94496 Ortenburg, Germany.

Inductive sensor for monitoring the crack: Keyence Corporation Japan, website:

http://world.keyence.com

References

1 Schellen, H., Heating Monumental Churches, Indoor Climate and Preservation of

Cultural Heritage, Eindhoven Technical University, Eindhoven (2002).

2 Bordass, W., and Bemrose, C., Heating your church, Church House Publishing,

London (1996).

3 Pfeil, A., Kirchenheizung und Denkmalschutz, Bauverlag GMBG, Wiesbaden and

Berlin (1975).

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4 Padfield, T., Bøllingtoft, P., Eshøj, B., and Christensen, M., Chr., ‘The wall paintings

of Gundsømagle church’ in Preventive Conservation: Practice, Theory and Research,

IIC, London (1994) 94-98.

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Sciences 23, Elsevier, Amsterdam (1998).

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lighting and people in re-using historical buildings: a case study’, Journal of Cultural

Heritage, 5 (2004) 409-416.

7 Italian Standard UNI 10969, Cultural Heritage - Environmental conditions for

conservation. General principles for the choice and the control of the indoor

environmental parameters. Part 1 Microclimate (2002).

8 CNR - Istituto di Scienze dell'Atmosfera e del Clima, Padua, Italy Friendly heating:

both comfortable for people and compatible with conservation of art works preserved

in churches. www.isac.cnr.it/friendly-heating/.

9 Camuffo, D., Sturaro, G., Valentino, A., and Camuffo, M., ‘The conservation of

artworks and hot air heating systems in churches: are they compatible?’, Studies in

Conservation, 44 (1999) 209-216.

10 Bratasz, L., and Kozlowski, R., ‘Laser sensors for continuous in-situ monitoring of the

dimensional response of wooden objects’, Studies in Conservation, accepted for

publication, (2005).

11 Siau, J.F., Wood: Influence of moisture on physical properties, Department of Wood

Science and Forest Products, Virginia Polytechnic Institute and State University,

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13 Mecklenburg, M.F., Tumosa, C.S., and Erhardt, D., ‘Structural response of painted

wood surfaces to changes in ambient relative humidity’ in Painted Wood: History and

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14 Bratasz, L., Jakiela, S., Kozlowski, R., ‘Allowable thresholds in dynamic changes of

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9

Fig.1. The main altar of the church of Santa Maria Maddalena in Rocca Pietore, Italy. The

numbers indicate the position of wooden elements monitored for dimensional change:

(1) the saint’s head, (2) the Child’s finger.

1

2

10

Fig.2. A crack in the saint’s head, result of the microclimatic stress.

11

Fig.3. An inductive sensor and a metallic reference plate glued to internal surfaces of a crack

to monitor changes of its width.

12

Fig.4. Indoor and outdoor moisture content in 2003 expressed as humidity mixing ratio

(MR). The plots were smoothed by calculating every five minutes an average of the

data points in the two adjacent 24 hour periods.

13

Fig.5. Indoor and outdoor climate in 2003 as plots of T and RH smoothed as in Figure 4.

14

Fig.6. Relative dimensional change (∆l/lo) of the saint’s head and the Child’s finger during

2003. Records of indoor T and RH are shown for comparison. Averaging as Figure 4.

15

Fig.7. Relative dimensional changes of the Child’s finger recorded every five minutes in

August 2003 are compared with the values calculated on the assumption of full

unrestricted response.

16

Fig.8. Relative dimensional changes (∆l/lo) of the saint’s head (A) and the Child’s finger (B)

in response to three short variations of climate: weekly variation due to a change of

weather outside, indoor diurnal cycle in summer and fluctuation caused by a heating

episode in winter involving an inflow of warm air. Records of indoor T and RH are

shown for comparison.

17

Fig.9. Change in the width of the crack from January to March 2003. Records of indoor T

and RH are shown for comparison.

18

Fig.10. Change in the width of the crack in response to three short variations of climate shown

in Figure 8.