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BUI LDING CONTRACTORS
are comparatively slow-acting and represent
long-term rather than immediate threats,
which makes them more difficult to anticipate
and identify. However, despite their chronic
rather than critical nature, their effects can
be severe and irreversible and the cost of
rectifying widespread damage of this type can
be extremely high.
Most of the risks are not uncommon and
could be anticipated at a very early stage of
the project. However, problems tend to occur
because the right questions are not asked
at the right time. At an early stage of any
project, the question that should be asked is
‘What are the possible negative impacts of
the proposed intervention and how can we
mitigate them?’ In most cases, as long as the
risks are identified sufficiently early, the design
can be adapted with only limited impact on
the overall outcomes of the project.
BUILDING PERFORMANCE
ASSESSMENT
The process by which the relevant issues are
examined can generally be referred to as a
building performance assessment. In simple
cases, this can be performed by HLF grant
applicants with their existing professional
advisers being armed with the correct set of
questions. In the proposed guidance notes
which the HLF is currently preparing, the
key risk areas are identified and relevant
questions are outlined. For the more complex
cases or those where the risk is of serious and
irreversible damage, it may be necessary to
have a more detailed assessment undertaken
by a specialist.
In most cases the building performance
assessment involves answering four basic
questions:
• How should the building
perform in its natural state?
• How is the building currently performing?
• What are the likely effects of
the proposed project on the
environment and performance?
• If the effects are potentially negative,
what mitigation measures can be put
in place which will allow the project
to proceed while minimising risk to
the historic fabric and artefacts?
The nature and extent of a formal building
performance assessment can vary significantly
depending on the complexity of the project
and the relevant risks. A simple assessment,
which is likely to be appropriate in all but
the most complex projects, usually involves
a site examination and building survey and a
short report, and takes no longer than one or
two days. Thermal imaging and spot readings
for humidity, liquid moisture, light and UV
radiation are commonly used in investigations
of this type. In the more difficult cases
environmental monitoring, materials analysis
and other techniques may be necessary.
Investigations of this type take longer (most
environmental monitoring programmes
last a minimum of 12 months) and are more
costly and are therefore only appropriate
in a minority of cases. In all events, it is
very important to ensure that the type of
assessment carried out is appropriate for the
project and that the information which comes
out of the work is of practical use.
As well as being a risk assessment exercise,
a well-designed building performance
assessment may also be able to offer significant
benefits to the project. For example, advice
may be provided on how best to improve the
thermal performance of the building and to
therefore achieve greater comfort for the users
with reduced energy input (something which
often benefits the historic fabric as well). Advice
may also be provided on how using different
decorative coatings (for example limewash or
distemper instead of emulsion) might maintain
a more porous building structure, improving
moisture buffering and reducing the risk of
condensation. In fact, the results of a good
building performance assessment should be
to improve the environmental aspects of the
project rather than simply to prevent damage.
One area where this is particularly
relevant is heating. Project proposals often
state that there is an intention to increase
or improve the heating of the building. In
fact, it is rarely the case that people wish
to use expensive energy to heat a very large
airspace with only a small number of people
at floor level. Indeed this can often result in
significant risk to the building fabric. Rather,
it is usually the intention of the project
to provide thermal comfort for that small
number of people in the lowest two metres of
the building at the minimum cost (in financial
and carbon footprint terms), with minimal
harmful impact on the historic building or
any artefacts. To achieve this, it is important
to understand issues of energy loss and energy
input, as well as how the building is to be used,
by whom and for what periods of time.
Another area where problems
commonly occur is with the proposed use
of compartmentalisation and screens in
a historic building which was originally a
single large space or series of large spaces.
When used effectively, screens and internal
walls can be a useful tool in containing
the local microclimate and ensuring that
comfortable conditions are produced in
the most heavily used areas of the building
and where they have least impact on the
rest of the building. However, the same
containment can also focus harmful
conditions on sensitive fabric and artefacts,
increasing the risk of damage to them.
Combined with increases in visitor numbers
or the installation of catering facilities,
interventions which have the potential to
have a benign impact on the building can
swiftly become the focus of the problem.
A well-designed building performance
assessment can provide advice not only on
how to avoid harmful effects but also on how
to achieve the most effective environmental
controls for the benefit of both the users and
the historic fabric.
It is important not to confuse this type
of assessment of the building environment
with that undertaken to improve energy
performance. In many cases an energy
performance assessment has a primary aim
of increasing user comfort and minimising
carbon footprint with the building considered
as a neutral vessel within which the controls
take place. Often the measures which best
improve energy performance also increase
the risk to the building and artefacts.
A common example is the use of fan convector
heaters, which can swiftly change the air
temperature. This is often a benefit in terms
of comfort and energy use, but the same
sudden fluctuation in temperature and
humidity can have a particularly damaging
impact on historic structures and their
contents. Therefore, it is important that the
building environment and conservation
issues are addressed alongside any energy
performance environmental surveys.
This new emphasis on building
performance has been designed to draw the
attention of HLF grant applicants to the issues
associated with building environment and
the relevant risks. It is intended that it should
create little additional work in most cases and
should help to avoid time-consuming and
costly reworking of the design at a later stage,
which has sometimes occurred when these
issues have been overlooked. Ultimately, the
aim is to improve the long-term outcomes of
conservation and development projects for the
building user, while minimising the risk to the
building and artefacts.
TOBIT CURTEIS runs Tobit Curteis Associates
LLP , 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 Historic England and is the National
Trust’s advisor on wall paintings.
SARA CROFTS is an architect and head of
historic environment at the Heritage Lottery
Fund . The HLF uses money
raised by National Lottery players to help
people across the UK explore, enjoy and
protect the heritage they care about.
Thermal imaging and spot readings for humidity,
liquid moisture, light and UV radiation are
essential features of a typical building performance
assessment. (Image: Tobit Curteis)