Page 164 - The Building Conservation Directory 2013

1 6 4 t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
4.2
Services & Treatment : Heating & Lighting ServicesSensible Heating
Balancing energy consumption, comfort and conservation
Tim Bowden
B alancing the conflicting
requirements of occupant comfort,
protection of historic fabric and energy
consumption can be challenging. This
article considers the key issues and gives
examples of how they can be addressed
in a number of historic building types.
Protection of historic
fabric and contents
Historic buildings are sensitive to changes in
environmental conditions. This is particularly
true in those that contain organic materials
in their construction or contents such as
furniture, pictures, timber panelling and
leather components such as those found in
church organs.
The most important environmental
parameter in a historic building is relative
humidity (RH), which should ideally be in
the 40–65 per cent range. When RH is too
low, cracks can form in organic materials and
furniture joints tend to become loose. When
RH is too high there is an increased risk of
mould growth, dry rot and insect infestation.
Understanding the prevailing conditions
in a building is key to providing effective
control. Therefore, monitoring temperature
and humidity in the main spaces, ideally over
at least a full year, is important to provide an
understanding of the current conditions and
allow the impact of any proposed changes
to be monitored against a base case. Factors
such as moisture entering the building due
to poor maintenance of rainwater disposal
systems will affect the internal environment.
Only when the existing conditions in the
building have been established can the impact
of operational changes such as increased use
or the addition of a café, with its associated
moisture generation, start to be understood.
Temperature, humidity and comfort
Temperature and humidity are linked. If a
volume of air is cooled, its relative humidity
will rise up to the point of 100 per cent
RH, which is known as its dew point, and
further cooling will cause the water vapour
to condense out. This is demonstrated when
warm air touches a cold single glazed window
in the winter months and condensation forms
on the surface of the glass.
The external environment changes during
the seasons, being generally cold in winter
with lower RH and warm in summer with
higher RH.
When buildings are heated in winter
to make them more comfortable for the
occupants, the general RH in the space is
reduced below the ideal humidity range for
conservation and this presents an increased
risk to organic contents and fabric.
Controlling risk to contents
The temperature and humidity in a space
can be controlled by mechanical systems
(air conditioning) but this is expensive in
terms of capital cost, space requirements,
maintenance and energy consumption. Air
conditioning is therefore only appropriate for
very specific conservation applications, such
as in archive stores that meet British Standard
BS5454 and some museums and galleries
where temperature and humidity must be kept
within narrow parameters.
One approach that is often used in
historic buildings is ‘conservation heating’.
It has been found that heating the internal
space of a building to a few degrees above the
external ambient air temperature generally
maintains the internal relative humidity
within the ideal range.
Using this method, the heating devices
are required to raise the temperature by
only a few degrees (rather than the 20oC for
a conventional heating system, assuming
0o C external temperature and 20oC internal
comfort temperature). The heat output from
the heating devices is low and therefore energy
consumption is minimised.
To fully influence the internal humidity
this type of heating must be employed
constantly. Also, in summer when the external
RH is high (>65%RH), to reduce the internal
RH levels the temperature in the space
needs to rise further. Applying heating when
external temperatures are high should only
take place when it is essential. For normal
applications a fixed maximum temperature
should be agreed above which no further
heating will be applied. RH levels outside
the ideal range are likely to occur and are
acceptable for limited periods.
Thermal inertia
Buildings themselves can also moderate
changes to the internal environment.
According to the principle of thermal mass, a
building acts as a sponge absorbing thermal
energy when the surroundings are higher
in temperature and releasing it when the
surroundings are cooler. This is also applicable
to moderating changes to the building’s
moisture content. Smaller items such as
furniture and pictures that are not part of the
building structure are likely to show signs
of unfavourable conditions more quickly
due to their low thermal/hygroscopic mass.
Sustained heating of a building to occupant
comfort levels will gradually reduce the
RH in the building. Intermittently heating
a space and having wide swings in internal
temperature (and therefore humidity) with
items of low thermal/hygroscopic mass is
likely to lead to cracking.
Generally the building temperatures
that give the ideal conservation
conditions are lower than the comfort
requirements of occupants.
Nostell Priory, Wakefield uses a conservation heating approach. By heating the internal space so that it is a few
degrees above the external ambient air temperature, the internal relative humidity can be maintained within a
suitable range for the preservation of the historic interior and the significant collection of Chippendale furniture.