ÐÓ°ÉÔ­´´

WHEN THE National Trust takes on an historic house, the building may
well be neglected, and need urgent attention to stop the roof leaking, the
walls crumbling and the rot spreading. Conservation, though, does not stop
with the structure; it extends to ensuring that any building that houses
precious contents, whether it is a museum or an historic house, provides
a suitable environment for the contents. Most contents are made from materials
that will deteriorate, decay and eventually disintegrate. Excessive light,
pollution, insects, fungi, vermin, the wrong humidity and handling all accelerate
the rate of deterioration. Of these, humidity causes the most long-term
damage. Continuous monitoring of conditions inside and outside National
Trust properties over the past three years has confirmed a relatively simple
picture of how to control the environment within buildings to avoid much
of this damage.

Calke Abbey in Derbyshire is often described as ‘a secret house, where
time stood still’; some of the rooms have changed little since Sir Vauncey
Harpur-Crewe died in 1924, and in many cases since he inherited the property
in 1886. In Sir Vauncey’s time, there were 27 servants at Calke. The house
archives tell us he employed three men just to carry coal to keep the fires
burning, whether the family was in residence or not.

This seemingly unimportant historical fact gives us a clue to why some
houses and their contents have survived, while others have not. In the British
climate, an unheated building presents ideal conditions for mould and rot
to grow, and uncontrolled variations in humidity cause physical distress
and damage. At Calke Abbey, fires were lit in most of the grates. In winter,
these gave out heat at close range, but did not raise the general temperatures
in rooms to the damaging high levels that people now insist on for comfort.
The chimney stacks acted as gentle storage radiators, keeping the whole
of the fabric a few degrees warmer than the outside. The kitchen was a constant
heated core to the building, and warm air was kept moving throughout as
people went about their daily business. By good fortune, the environmental
conditions at Calke were just those that sophisticated monitoring and analysis
now tell us are right for long-term conservation.

After Sir Vauncey’s death, his heir was faced with large death duties.
The Harpur-Crewe family was forced to reduce the number of servants to six,
and could afford to occupy only some of the rooms. The building and its
contents gradually deteriorated. When Sir Vauncey’s grandson Charles died
in 1981, the family offered the house and surrounding parkland to the nation
in lieu of substantial taxes. The government accepted. There were also contributions
from the National Heritage Memorial Fund, an anonymous benefactor and a
public appeal, which secured the house, contents, garden, park and surrounding
land for the Trust, and provided for the repairs that were urgently needed
and for future maintenance.

The National Trust for Places of Historic Interest and Natural Beauty,
to give it its full name, is a charity founded in 1895. As well as owning
about a quarter of a million hectares of land in England, Wales and Northern
Ireland, it is responsible for some 200 historic houses and castles.

When the Trust took on Calke Abbey in 1985, it established that parts
of the house date from before 1700 – the Harpur-Crewe family bought the
estate in 1622 – although they are effectively disguised by rebuilding that
took place between 1701 and 1704. The house was remodelled at the end of
the 18th century. In 1841-42 there were further improvements. After then,
there were no significant alterations or even repairs until we began work,
which we did very soon after we acquired the abbey, in 1985.

The restoration of the building itself involved replacing the lead roof,
repairing the cornice and other stonework, and attending to structural repairs
inside the house. There were four epicentres of dry rot. We replaced all
the affected timber, but did not cut out any sound timber, as is often done,
beyond the outbreak itself: we wanted to preserve as much of the building
as possible. The fungi that cause dry rot will grow only when the moisture
content of the wood creeps up to more than 20 per cent. Our policy is to
prevent this from happening by keeping the building watertight and controlling
the environment within the house. This approach is similar to that adopted
in the Royal Pavilion at Brighton (‘Slash and burn in the dry rot jungle’,
New ÐÓ°ÉÔ­´´, 19 April 1984).

We monitor water penetration into the building by comparing the relative
humidity of the air inside with that outside. If the moisture content of
the air inside the building, calculated from its relative humidity and temperature,
exceeds that of the air outside, this indicates that water is leaking into
the building. At Calke, we use battery-powered recorders with solid-state
memories, which log readings of both temperature and relative humidity.
These recorders are more convenient than thermohygrographs, which need calibrating
frequently and a member of staff to change the paper chart every week. The
dataloggers take hourly readings, and the memory will hold 16 000 readings,
equivalent to six months’ worth when all four channels (two for temperature
and two for relative humidity) are recording. In practice, we extract the
data into portable computers every two or three months. Back in the office,
we can analyse the readings and print them out in graphical form to give
us advance warning of conditions that could promote a new outbreak of dry
rot.

Relative humidity and temperature are particularly important to preserving
the contents of a building, too. The higher the temperature, the more water
the air can hold. The maximum amount of water that the air can hold at a
given temperature is known as the saturated moisture content. Relative humidity
is a measure of the actual moisture content of air compared with the saturated
moisture content at that temperature. When it exceeds 65 per cent, biological
agents of destruction will become active. Moulds and fungi start to grow,
and insects such as woodworm and silverfish breed more prolifically. Metals
corrode more quickly, and silver, copper and bronze tarnish sooner than
they should. As the relative humidity changes, organic materials, such as
wood and textile threads, absorb or give out water and alter their size
and shape. Cycles of change will eventually result in mechanical damage,
particularly if an object is made of different materials. For example, the
veneer on furniture splits and lifts, and paintings flake.

Data from the Meteorological Office show that, outdoors in Britain,
the relative humidity averages between 80 and 90 per cent all year round.
There are two ways of bringing it down to below 65 per cent inside a building:
raising the air temperature or removing some of the water vapour it contains.
In areas that do not have to be heated for comfort, dehumidification is
the preferred method of reducing relative humidity because it uses a third
of the energy to achieve the same level of control . The drawback, however,
is that draughts must be eliminated from the space to be dehumidified, otherwise
fresh damp air replaces the dry air at such a rate that the dehumidifier
becomes inefficient. At Calke, we have installed heating in the showrooms
to control the relative humidity; in the ‘museum’ rooms, where particularly
precious and sensitive objects are exhibited, dehumidification works better.
We also used dehumidification to protect the contents of the house during
the three years of building work.

The contents of Calke Abbey were in various stages of decay and disintegration
when the National Trust assumed responsibility for the collection. We can
be fairly certain that a great deal of this damage occurred in the past
60 years, when the house was unheated and the relative humidity high. Most
of the 9000 objects on the inventory at Calke Abbey required urgent attention,
and a programme of conservation for the contents ran parallel to that for
the building.

The work on the restoration of the building was so extensive that it
was essential to empty the house. There was room in the attics of a nearby
National Trust house, Sudbury, for some of the contents, and others went
immediately to conservators’ studios. Most, though, had to be stored somewhere
at the abbey itself.

As it happened, there is a large indoor riding school attached to the
house that was just large enough to take the 100 wooden cases into which
the contents of the house were packed. Readings from the solid-state memory
recorders showed that the packing cases kept the relative humidity constant,
providing a stable environment for their contents, but that the humidity
was at levels that would encourage mould to grow and insects to proliferate.
We had to reduce humidity to ensure that there were some contents left to
put back in the house after we finished building work.

We wanted to install dehumidifiers because they use less energy than
heaters. First, though, we had to eliminate some of the draughts. We experimented
with a ‘tent’, built by fixing polythene sheets over a frame of scaffolding
and timber battens within the riding school. It contained two modest dehumidifiers,
each with a capacity to condense three litres of water a day at 10 Degree
C and 80 per cent relative humidity. Over six weeks, this arrangement reduced
the relative humidity within the tent to 65 per cent. Outside the polythene,
the values varied up to 95 per cent with a mean level of 80 per cent. After
the success of the first tent, we draught-proofed the remaining space within
the riding school and installed further dehumidifiers. During the three
years of operation, the dehumidifiers consumed a total of 7500 kilowatt-hours
of electricity, at a very modest 35p to 40p per day.

After three years, the building was restored and we could move the contents
back. Most of the showrooms at Calke cannot be sealed off, and we control
the environment with ‘conservation heating’. This is the amount of heating
required to reduce the relative humidity to below 65 per cent. The good
news is that, in Britain, indoor temperature has to be only around 5 Degree
C above that outside, all year round, to achieve this target. Indeed the
air must not be overheated during the cold winter months, or the relative
humidity will fall below 50 per cent. In that case, organic materials dry
out, become brittle and, in extreme conditions, shrink, crack and split.
Fortunately, Calke is closed to visitors during the winter months, so we
do not have to take into account the comfort of visitors, which might require
us to lift the temperature by 20 Degree C. This would reduce the relative
humidity to below 40 or even 30 per cent.

Our aim is to keep the relative humidity between 50 and 65 per cent.
In a good summer, the Sun can provide the 5 Degree C increase in temperature.
In winter, solar radiation is much less, and we have to make up the difference
by heating the house. The relatively low level of heating required, though,
has allowed us to reassess several conventional assumptions. Insulation,
for example, is much less cost-effective than we expected. This is convenient
because the National Trust has to preserve the fabric of buildings as well
as the contents, and fitting insulation normally involves modifications.
The low level of heating also helped to solve the perennial problem of where
to site modern heating equipment in historic rooms. Water-filled radiators
are ruled out, at least above the K K ground floor, because of the risk
of damage from leaks. At Calke, we opted for electric heating. Where possible,
we have concealed the heaters in cupboards and under floors; in others,
we use portable radiators that we can remove easily during the summer. The
system provides enough heat to raise the temperature 10 Degree C above that
outside when people are doing conservation and cleaning work in winter,
but normally runs at half that capacity.

To keep the relative humidity at the right level for conservation, the
heating system needs to be controlled, with humidity and temperature monitored
and recorded. These requirements helped to justify the cost of an energy
management system. Our system uses an IBM-compatible computer that communicates
with a series of local controllers around the house. Each room is wired
for temperature and/or humidity sensors, and these cables and the power
supplies to the heating units are fed back through the controllers. The
computer program allows us to set temperature and humidity. The system then
interrogates the sensors at regular intervals, switching heaters on or off
as required. The humidity sensors are expensive, so a sensor in one room
controls the heaters in a group of rooms with similar thermal characteristics.
At intervals, the system records all the values registered by the room sensors
and stores them on floppy discs. These provide a permanent record that helps
us to understand how to continue to improve atmospheric conditions.

Some of the contents at Calke are so sensitive or important that a higher
standard of environmental control is needed than is usually possible in
a country house open to the public. The state bed is one example. It dates
from the early 18th century and the silk hangings of Chinese embroidery
are in almost pristine condition. It is displayed in a glazed and sealed
compartment in one of a series of three ‘museum’ rooms. These rooms and
two storerooms below are served by a local dehumidification system. A large
dehumifidier is hidden in an original linen cupboard in one of the rooms,
and ducts are arranged to draw air from and return it dried to each of the
five controlled spaces. Sensors in each area control fans in the duct circuit,
passing air through the machine to remove some of the moisture when the
humidity is above the set point. The state bed itself is sealed within an
enclosure to achieve a very low exchange rate of air. The bed, therefore,
is buffered against variations in the external atmosphere, and changes in
relative humidity will be smaller and more gradual than elsewhere.

The first phase of the conservation of Calke Abbey and its contents
is now complete. Since April the house has been open to the public. The
extensive conservation programme gave us an unusual opportunity to install
systems to control temperature and humidity. Calke, though, is but one of
many buildings in Britain that the National Trust is responsible for. More
often, we are confronted with a house with a central heating system that
is operating to a greater, or more often lesser, extent for the benefit
of the contents. In these circumstances we can still make improvements to
allow the heating to operate for conservation rather than for people. As
our properties are closed during the winter months, we can separate heating
circuits, so that showrooms are on different circuits from occupied parts
of the house. Humidistats, or weather-compensating controllers, can then
manage the temperature of the circulating water to achieve a differential
of 5 Degree C between indoor and outdoor temperatures in the showrooms.

The contents of houses where we have made improvements for environmental
control will deteriorate at a slower rate, and we aim to introduce these
measures in all the houses. The experience at Calke Abbey, and we hope its
success, will help us to fulfil this ambition.

* * *

1: The relative costs of controlling moisture in the air

TRIALS at several National Trust properties have confirmed the advantages
of using dehumidifiers instead of heating to control the relative humidity
in rooms that can be draught-proofed.

The picture gallery at Upton House, Oxfordshire is a converted squash
court, which houses part of one of the Trust’s most important collections
of paintings. Some of these are on wooden panels and, therefore, particularly
susceptible to damage from high and variable humidity. Monitoring shows
that damp penetrates through the floor and roof of the gallery, which adds
to the moisture content of the air indoors and artificially increases humidity
levels. Some heating is necessary in spring and autumn, for the sake of
visitors, when it can be introduced without reducing the humidity below
50 per cent. The question was whether to use heating alone to control humidity,
or whether to install dehumidification as well.

The Trust had to remove paintings temporarily for building work to be
done in the gallery, and this provided an ideal opportunity to compare the
cost and effect of dehumidifiers with those of electric heaters. The trial
showed that dehumidification equipment, either of the refrigerative or of
the desiccant type, was needed to gain an acceptable standard of control.

The heaters used an average of 4 kilowatts, but were unable to bring
the relative humidity down to the set level of 45 per cent (deliberately
set low to prove capacity): they maintained an average level of 51 per cent.
This was partly because the additional moisture in the air (from damp penetration)
required temperatures to be kept well above 20 Degree C to try to bring
the humidity down to the set point.

The dehumidifiers maintained the set level of 45 per cent to within
2 per cent, using an average of 1.3 kilowatts of electrical power. And dehumidification
would pay for itself by reducing heating bills in less than three years.

Sarah Staniforth works for the National Trust as a conservation adviser.
Bob Hayes is an engineer involved in the care and maintenance of historic
houses and their contents. Deborah Thornton is a science writer.