The Survey and Identification of
|Figure 1: The Chinese Palace, Oranienbaum, Russia; localised deterioration patterns characteristic of low level liquid water sources
Adverse environmental conditions
are among the most significant influences
on the deterioration of historic buildings.
Broadly speaking, the causes of such damage
can be separated into those factors associated
with the failure of the building envelope and
those caused by the internal microclimate,
although in practice the two areas are generally
interdependent. While there are many
environmental factors which can cause damage,
the most significant, and the one that will be
discussed in most detail in this article, is damp.
If water-related deterioration is to be controlled,
it is essential that the precise cause and
mechanism of the damage is understood so that
control measures can be designed accurately.
Damaged buildings and artefacts
display a range of symptoms which may be
characteristic of the type of deterioration.
However, the symptoms are not necessarily
specific to a particular deterioration source.
It is therefore often easier, and in the short
term cheaper, to treat the symptoms rather
than to identify and control their source. The
widespread use of cementicious render and
bituminous paint to control rising damp in
churches in the 19th and early 20th centuries
is an all too common example of this approach.
As the source of the deterioration was often left
untreated, it should have come as no surprise
that the damage simply returned.
A HISTORY OF MISUNDERSTANDINGS
The history of building conservation is littered
with examples of inappropriate treatments
being applied to misunderstood problems.
In many cases these treatments have caused
further damage to the buildings they were
intended to help. In some cases this is caused
by a simple misreading of the symptoms,
while in others it is the result of inappropriate
A classic example of the former is the
misidentification of rainwater penetration on
an external wall, when in fact damage is being
caused by condensation resulting from the
localised cooling of the wall by the evaporation
of rainwater from the external surface. This
can lead to expensive pointing or structural
repairs being undertaken when a modification
of the internal microclimatic conditions or the
application of thermal insulation to the wall
may be more appropriate.
In other cases, the cause of deterioration
may be understood but the ways in which
the remedial measures will work are not
fully appreciated. This is sometimes the case
with the gravel-filled trenches often known
as French drains. Designed for draining
waterlogged soil, these were invented in
the mid-19th century by Henry French, an
American agricultural engineer. In the context
of building remediation, French drains can
also be useful as a means of reducing the
horizontal transport of moisture from the
surrounding ground. However, in many cases
this type of drainage is installed in an attempt
to aid evaporation from a wall suffering from
rising damp. In this case, the system has only a
limited effect as the moisture content of the air
between the stones quickly reaches saturation,
preventing any evaporation into the gravel.
In these examples, as in many others, a
careful assessment of both the damage and the
proposed remedial measures can be a useful
and cost-effective step in accurately identifying
the underlying causes of deterioration and
designing effective controls.
|Figure 2: Mapping electrical capacitance readings and patterns of deterioration on the 13th century wall paintings at Holy Rood Church, Ampney Crucis, Gloucestershire
The first step in assessing any suspected
moisture damage in historic buildings is to
examine the nature and distribution of the
damage. Different moisture sources generally
result in different patterns and types of
damage. Broadly speaking, damage associated
with liquid water is localised and severe, while
that caused by microclimatic factors is more
widespread and evenly distributed.
Most of the basic investigation can be
achieved with a careful study of the building
envelope and the moisture transport routes.
The design of any successful building envelope
should enable rainwater deposited on the outer
shell to be transported away to a safe location.
If there is water damage on the internal
surfaces, it suggests that either the design
itself is faulty or that the system is damaged
A similar approach can be taken to the
basic assessment of the internal microclimate.
If the building envelope is functioning
successfully, then, in the absence of artificial
influences such as heating and ventilation, it
should achieve a predictable level of buffering
between the internal and external conditions.
If there is instability in the microclimate,
then it is probable either that the envelope
is not functioning correctly or that artificial
influences are destabilising the situation.
MONITORING AND MEASUREMENT
In many cases the simple identification of
the probable sources of deterioration may be
enough to allow suitable control measures to be
designed. In some cases, however, the precise nature of the deterioration needs to be more
accurately defined and in these instances fuller
technical investigations may be necessary.
One of the simplest forms of moisture
investigation involves the use of hand-held
electrical resistance or capacitance meters.
These instruments, which are generally
designed for measuring the moisture content
of timber, are sometimes dismissed as being
of limited use in building investigation
as they can be influenced by the presence
of electrolytes such as hygroscopic salts.(1)
However, they are non-invasive, quick to use
and provided they are used by a practitioner
skilled enough to interpret the data, they are an
invaluable basic measurement tool.
Simple resistance and capacitance data
plotted onto plans and elevations can provide
a very useful overview of the correlation
between current moisture distribution and
elements of the building structure or rainwater
disposal system. Such data can also give a clear
indication of whether damage is being caused
by an active moisture source or is simply
the residual symptom of a historic problem
If it is necessary to examine the moisture
conditions within the structure of the
wall (for instance, to differentiate between horizontal transfer of rainwater and localised
condensation, as discussed above) then various
core sampling techniques can be employed.
These range from micro cores, often used
to examine the very precise stratigraphic
distribution of salts near the wall surface, to
deep cores used to establish the pattern of
moisture throughout the wall.
Salt analysis can also be a useful tool.
Different salts are obtained from a variety of
sources and, by identifying the salt mixture
present, it is often possible to establish the
moisture source. For example, nitrates are
often associated with decomposing organic
matter and therefore the localised distribution
of nitrates near the base of a wall is often
indicative of capillary rise of ground water, or ‘rising damp’. An even distribution of sulphates
on the surface of the wall may well be an
indication of industrial pollution or a coal-fired
heating system. In both cases the source
may be historic, but because the salts remain
sensitive to variations in moisture content, the
damage may remain active.
In addition, different types of salts
change between their solid and liquid states
at different humidities and the identification
of the salt type can therefore be a big help in
identifying when damage is likely to occur.
Historic buildings generally contain high
levels of hygroscopic salts both in the original
and added building materials. While the salts
remain in their liquid or solid phase, they
cause little or no damage. It is the transition
between the two phases resulting from changes
in moisture content which causes damage to
porous building materials.
In practical terms, this means that a
building which remains either moist or dry
(irrespective of temperature) may suffer from
little salt damage. However, when the moisture
conditions change, for instance as a result of
heating or ventilation, the salts can change
state and damage can occur.
These issues can be examined by
a carefully designed programme of
environmental monitoring. Monitoring, as
opposed to measuring, involves collecting a
large set of data over a prolonged period so
that the pattern of microclimatic change can
be observed. If the heating is turned on or a
window is opened, for example, how does this
affect the microclimate and how in turn does this affect the deteriogens (Figure 3)?
It is not simply the activity of salts that
can be examined in this way but any aspects
of the building or artefact that are sensitive
to changes in the microclimate. Timber
structures (particularly painted structures)
are extremely vulnerable to microclimatic
instability, as are organic materials which are
susceptible to microbiological attack. Of course,
the microclimatic parameters of concern in
historic buildings are not merely moisture
and temperature; light is also a significant
factor both as a deteriogen for photosensitive
materials and as a source of radiant heat.
Clearly, it is essential to define specific
questions about the deterioration as precisely
as possible before undertaking a monitoring
programme if useful and practical information
is to be produced. All too often, monitoring is
undertaken without precisely defined aims,
resulting in large quantities of data being
produced with no clear conclusions.
|Figure 3: Monitoring the environmental conditions on medieval stained glass at Canterbury Cathedral (All images supplied by Tobit Curteis Associates
The control of environmental deterioration
is not the subject of this article, but a brief
overview may be useful.
In many cases, particularly with liquid
moisture issues, implementing effective control
measures is simply a matter of accurately
identifying the source of the problem and then
targeting basic building repairs accordingly.
Problems can arise from the incorrect use of
certain techniques, such as the French drains
discussed above, but these are the exceptions.
Controlling the microclimate is rather
more complicated due to the often competing
requirements of the building and those of
the users. One of the most difficult areas is
reconciling the desire for comfort heating with
the need to maintain conditions conducive to
the conservation of sensitive building fabric.
Various research projects are presently being
undertaken in the UK and across Europe
examining sensitive heating for historic
churches. The results show broadly that
acceptable conditions can be achieved either
through establishing stable ambient heating
throughout the building (generally based on
convective heating systems which heat air),
or by very localised heating which only heats
the users of the building (generally based on
radiant systems which heat surfaces). It is the
fluctuation in overall ambient temperature
(and therefore relative humidity) which tends
to be the most damaging microclimatic factor
and should therefore be avoided.
Ventilation, or internal/external air
exchange, is an equally powerful tool for
microclimatic control, but it is generally less well understood than heating. While
controlled air exchange can be used to improve
internal microclimatic conditions, the type
of ventilation usually recommended in
historic buildings is largely uncontrolled. The
introduction of unstable external air into an
otherwise well buffered building will inevitably
destabilise the internal microclimate. Similarly,
the introduction of humid external air into all
but the wettest interiors will increase moisture
levels rather than decrease them.
It should be remembered that during the
drying process following the successful control
of a moisture source, salt activity and other
deterioration may increase over the short term
before conditions stabilise. The conditions
will always need to be monitored during
this period and, in severe cases, emergency
conservation may be required.
The successful control of environmental
deterioration relies on a precise understanding
of the causes of deterioration and accurately
designed control measures. To be most
effective, investigations should be carried
out at the earliest stage of the project so that
recommendations for control can form part
of the basic project design, rather than being
added at a later stage.
As with so many aspects of building
deterioration many of the issues involved
with environmental damage are relatively
straightforward. Most cases can be dealt with
by a basic assessment of the building and the
rainwater disposal system. Where further
investigations are required, it is essential that
the questions are very carefully defined in
advance and that the appropriate investigative
techniques are employed.
- A Arnold and K Zender, ‘Monitoring Wall
Paintings Affected by Soluble Salts’, in The
Conservation of Wall Paintings, Proceedings
of a Symposium Organized by the Courtauld
Institute of Art and the Getty Conservation
Institute, London, 1987
- T Curteis, ‘Environmental Conditions in
Historic Churches: Examining their Effect
on Wall Paintings and Polychrome Surfaces’, Transactions of the Ecclesiastical Architects
and Surveyors’ Association, Volume 5, 2004
- T Curteis, ‘Painted Wood in Historic Buildings
with Uncontrolled Environments: Active
Deterioration and Passive Conservation’
in Post prints of Polychromed Wood, a
Conference Organised by the Institute of
Conservation, Stone and Wall Paintings
Group, London (forthcoming)
- G Massari and I Massari, Damp Buildings Old
and New, Rome, 1993
- C Price and P Brimblecombe, ‘Preventing Salt
damage in Porous Materials’, Preventive
Conservation: Practice, Theory and Research.
Preprints to the IIC Ottawa Congress,
- S Staniforth, B Hayes and L Bullock, ‘Appropriate Technologies for Relative
Humidity Control for Museum Collections
Housed in Historic Buildings’, Preventive
Conservation: Practice, Theory and Research.
Preprints to the IIC Ottawa Congress,
- G Thomson, The Museum Environment,
Butterworth-Heinemann, London, 1994
- G Torraca, Porous Building Materials, ICCROM,
(1) The resistance meter measures the electrical
current between two pins. This can be
affected either by moisture content or
varying concentrations of electrolytes
including commonly found hygroscopic
salts. The capacitance meter measures the
dielectric constant in the substrate in contact
with the sensor plate, typically to a depth of
10-20mm. The dielectric constant is largely
affected by water content and to a lesser
degree by the presence of salts and other
article is reproduced from The Building Conservation Directory, 2008
TOBIT CURTEIS trained in the conservation
of wall paintings at the Courtauld Institute
of Art. He runs Tobit Curteis Associates, a practice specialising in
the conservation of wall paintings and the diagnosis and control of environmental deterioration in historic buildings. He is an external
consultant for the Building Conservation and Research Team at English Heritage and is the National Trust’s advisor on wall paintings.
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