Holistic and Sustainable Solutions to Conservation
|Monitoring St George: A probe installed by English Heritage
monitors variations in the relative humidity and ambient
temperature of a magnificent but highly vulnerable wall painting
in the castle chapel at Farleigh Hungerford near Bath.
The deterioration of historical building
materials is attributed to changes
in their environment. The majority of
environmental problems are associated
with those defects in the fabric that
lead to water penetration, condensation
and dampness in the building fabric.
Severe salt efflorescence, damp staining,
blistering of finishes and timber decay
in buildings are mainly the result of
However the causes of deterioration
are also influenced by the building’s
internal environment. Humidity,
temperature and ventilation all
contribute to this microclimate, which
will vary depending upon the building
structure and the envelope of the
internal building fabric.
There is little point in dealing with
decay if the causes of decay are
not dealt with first. Indeed, it is often
necessary to treat the cause alone.
When dealing with historic building
fabric the historic value of the original
material often justifies retaining
partially decayed material, provided
that neither its integrity, nor that of
the building of which it is part, is
jeopardised in any way.
Where the causes of decay are
not obvious it is necessary to carry
out a thorough study of the environmental
conditions to identify the cause of decay. This
is done by employing a range of hand-held
instrumentation, physical sampling and sensor
technology to monitor various parameters
within the fabric of the building.
Environmental monitoring may also be
justified where the recurrence of a defect
is unlikely to be detected before extensive
damage has been caused, for example in the
roof space above an auditorium. In this case
long-term environmental monitoring will be
ENVIRONMENTAL MONITORING, METHODOLOGY AND OVERVIEW OF APPROACH
The first step in the investigation of a problem
building is to carry out a thorough inspection
of the building for defects. Then:
- Establish moisture contents in affected
materials, such as timber, plaster, masonry,
insulation materials and textiles.
- Establish the humidity, temperature and
dew point in the environment, both
internally and externally. (The dew point
is the point at which air-borne moisture
condenses due to a fall in temperature, for
example in a porous masonry wall which is
cold on one side and warm on the other.)
- Investigate in greater detail as necessary
the moisture profiles in large dimension
timbers and across masonry masses.
This information can be determined by:
- Measuring moisture contents of timber
with resistance based moisture meters.
Probes can also be used to measure
moisture contents at depth in large section
timbers and those built into masonry.
- Surface moisture readings in plaster and
masonry using moisture meters. These will
indicate if a wall is dry but can give false
readings of dampness (see below).
- Where possible, mortar samples should be
taken of the areas affected to determine
accurately the moisture and salt content of
the masonry. This does, however, have the
disadvantage of not being non-destructive.
- Data loggers used to measure the
environmental parameters (temperature,
humidity and dew point in
particular) both internally and
- Specialist probes used to measure
moisture across masonry walls.
The results of all or some of the above
tests will establish the cause and enable
a solution to the problem to be put
Mortar sample analysis
Mortar sample analysis is one of the
most important tools in establishing
accurately the moisture levels in masonry
and plasters. Where moisture levels are
high it is also possible to determine how
long there has been a damp problem
from the salt content, a high salt content
indicating a long-established problem.
Mortar sample analysis can also be
useful to determine the type of salt when
trying to establish whether there is a
genuine problem with rising dampness.
However taking samples of mortar or
plaster for analysis has the disadvantage
of causing some damage, and might
not be appropriate where, for example,
ornate plasterwork is concerned.
Timber moisture contents
Timber moisture contents above 20 per
cent indicate unacceptably high moisture
levels in the building. If this is a general
moisture level rather than a localised one then this is
likely to be associated with high humidity in
the building. Localised high readings are more
likely to be associated with a building defect.
For instance, high readings in the built-in end
of a timber would indicate that the wall was
damp, posing the threat of future timber
decay. The options are to isolate the timber
from the wall, provide an air gap around the
timber to allow the timber to breathe, or to
eradicate the source of damp and monitor the
timber as the wall dries out. The option selected will be determined according to each
Masonry moisture monitoring profiles
|An example of a resistograph measurement of moisture content in timber
Measurement of the moisture across the
thickness of a wall is a specialised task as there
are no instruments available off the shelf for
carrying this out. Tailor-made probes are used
containing hygroscopic materials (materials
which absorb moisture). These are placed in
the wall at varying depths and sealed off from
the outside environment. After some time the
probes are removed and their moisture content
analysed. This method will give an indication
of moisture levels across the thickness of the wall and combined with temperature and humidity readings both
internally and externally will give an indication of the moisture source. However, it must be pointed out that the use of hygroscopic
material to measure moisture is inaccurate at higher moisture levels.
Environmental data loggers
Data loggers measuring temperature and humidity are useful to
determine whether there is, for instance, an abnormally high humidity
or whether there is a risk of condensation in a building.
If readings are taken on both the interiors and exteriors of the
building, dew points can be calculated within materials such as masonry
STABILISING THE HISTORIC ENVIRONMENT
For the holistic and sustainable conservation and preservation of historic
buildings, stable environmental conditions are important.
Once investigations have been completed, a strategy can be devised
to stabilise the building’s environment. Various building works may
be required to prevent further water penetration and to maximise
ventilation to damp-affected materials. Correction of these building
defects, combined with measures to dry out the wet areas and to protect
any decorative interior finishes by allowing ventilation of the wet areas,
will prevent further deterioration. If thoughtfully and competently
carried out, such work may extend the life of the building indefinitely and
Until the drying out of the building fabric and its associated timber
elements is completed, any other actions to remedy the deterioration
problems will be ineffective and a waste of time and resources.
In some situations it may well be necessary to introduce both
continuous long-term monitoring and preventative maintenance. Long
term monitoring may be necessary for the following reasons:
- To provide information on the state of moisture equilibrium and
balance (moisture sources, reservoirs and sinks) in the building’s
environment, its fabric and its structural elements as it dries out.
- To allow co-ordination and scheduling of work stages to prioritise
remedial work to achieve acceptable levels of moisture in the
masonry and timber and to prevent future deterioration problems.
- To allow a cost-effective, long-term holistic approach to
environmental stabilisation of the historic environment.
- J Singh, Building Mycology: Management of Health and Decay in Buildings,
Spon, London, 1994
- J Singh, 'Dry rot and other wood-destroying fungi: their occurrence, biology,
pathology and control', Indoor and Built Environment, Vol 8, Number 1, p3, 1999
article is reproduced from The Building Conservation Directory, 2001
DR JAGJIT SINGH, Director of Environmental Building Solutions Ltd, is an independent consultant specialising in building health problems, heritage conservation and
environmental issues. His current research focuses on inter-relationships of building
structures and materials with their environments and occupants.
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