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THE DANGERS OF INSTALLING AIR CONDITIONING IN HISTORIC BUILDINGS 
Tim Padfield 
Conservation Analytical Laboratory 
Smithsonian Institution, Washington DC 
When air conditioning is installed in a historic structure a large 
difference in temperature and in water vapour concentration develops across 
the walls and roof during certain times of year. There is then a serious 
danger of condensation and of soluble salt movement within the wall. 
The physics of the process at its simplest is illustrated by the winter 
condensation within the wall of the Hirshhorn Museum in Washington DC.  The 
interior of this building is maintained close to the present informal 
climate standard for museum galleries in the USA: 21?C and 50% relative 
humidity. The building is pressurised so that polluted air from outside 
does not penetrate to contaminate the filtered air inside.  Inevitably, 
some air flows out through the walls. During the winter the air cools 
during its journey through the wall. The moisture content of the air 
remains constant, or nearly so, until the temperature reaches the dew 
point. This is the temperature at which the vapour pressure of the water 
in the air reaches the saturation value. As the temperature drops below 
the dew point water condenses out of the air onto nearby surfaces. The dew 
point for air at 21?C and 50% RH is 10?C. 
CQVMFttf 
vaptwr bttrrirr The night temperature in Washington 
during the three coldest months of the 
year is often below 10?C.  Condensation 
therefore happens frequently in the walls 
of the Hirshhorn Museum. Figure 1 shows 
the result on a particular day in early 
spring. The night had been cold so that 
ice formed behind the outer skin of the 
wall. The morning sun warmed the wall so 
that the ice melted. The melt water 
dripped down and emerged at weep holes. 
On the outside the water re-froze because 
the breeze was brisk and the air was dry, 
so that the water cooled by evaporation. 
This secondary ice clung to the surface 
of the wall until a general warming of 
the air caused it to melt and fall onto 
the concourse below. The impressive 
quantity of shattered ice showed just how 
imuch water had accumulated during one 
night. 
F-iguAZ  I. Th& Hifuhhowi Mu&zum, 
FIQLVLZ 2.    The. National Utuzum o& 
AmeAA.c.an HiAtoiy, 
WditUnQton  PC. 
The National Museum of American 
History (Figure 2) also shows 
streaks of meltwater emerging 
from the limestone facade on 
sunny mornings after cold 
nights. This building operates 
at a negative pressure and so 
there is a flow of air from the 
outside into the building. The 
outside air in winter has a 
rather low moisture content and 
could not possibly be the cause 
of the weeping evident on the 
facade. The most likely 
explanation is that the 
construction of the wall causes 
the infiltrating air to move 
sideways through the cavity 
behind the facing stone until 
it finds a crack which allows 
it to pass through the concrete 
inner wall. During this 
transverse air movement water 
vapour from the inner wall, 
derived from the warm and humid 
inside air, moves diagonally 
across the gap to condense on 
the inner surface of the outer 
wal 1. 
The Museum Support Center of 
the Smithsonian Institution 
provides an instructive example 
of the effect of air pressure differences. This building began operating 
with a positive inside pressure but after two years some equipment was 
installed that resulted, unintentionally, in a negative pressure inside. 
The ice formation inside the walls diminished after this change but still 
some parts of the wall suffer winter condensation through outward 
penetration of inside air. During the summer the outside air penetrates 
the wall and deposits dew on cool surfaces within the wall. An 
investigation of the wall suggested that air flows along definite paths 
through the wall, leaving some parts isolated from the influence of the air 
stream, or even seeing diffusion of moisture from the opposite direction. 
One might assume from these examples that damage to buildings could be 
ameliorated, though not prevented, by dropping the pressure below 
atmospheric in the winter and raising it above atmospheric pressure in the 
summer. This can be done in buildings that have no tall interior spaces. 
In a tall building the natural buoyancy of warm, moist air will exert an 
outward pressure on the roof while cold winter air is being drawn in at the 
door below. 
It is very difficult to prevent the movement of moisture from a humidified 
interior to the cold skin of the building. A vapour barrier only works if 
it is airtight and prevents the flow of air. If it is not absolutely 
airtight it can cause damage rather than prevent it. For this reason the 
necessarily incomplete application of impermeable materials to old 
buildings is dangerous. 
These first examples have been modern buildings. Their walls contain only 
inorganic materials with a limited ability to absorb and desorb water.  In 
this respect they are similar to ancient masonry walls. Many buildings 
contain hygroscopic materials such as wood, particularly in the roof. 
These materials complicate the response to moisture because they absorb and 
desorb water according to the relative humidity of the air that surrounds 
them. 
The Arts and Industries Museum 
to the Hirshhorn Museum. The i 
laths with plaster infill which 
on iron trusses. This roof was 
in the late 1970's. The new ro 
without removing the exhibits f 
made in prefabricated box secti 
stripped and re-covered during 
to modern standards of energy e 
uninsulated roof that it replac 
standards.  All these demands, 
to maintain a warm, humidified 
trouble that was only diagnosed 
conducting a laboratory simulat 
of the Smithsonian 
oof was made of sh 
in turn rested on 
replaced, after a 
of (Figure 3) was 
rom the galleries 
ons so that parts 
a working day. Th 
fficiency, but had 
ed. It had to be 
combined with the 
atmosphere within 
by placing sensor 
ion of the condens 
Institution is n 
eet metal laid ov 
a metal ceiling 
bout a century of 
designed to be in 
below. It was th 
of the roof could 
e roof was to be 
to be no thicker 
fireproof to mode 
power of modern e 
during the winter 
s in the roof and 
ation process. 
ext door 
er wooden 
supported 
service, 
stalled 
erefore 
be 
insulated 
than the 
rn 
quipment 
caused 
by 
FiguAe 3.    Roo-6 design o? -the Atubb and lnduA&U.2A Museum. 
The new roof certainly kept the rain out and the heat in but it soon showed 
one unfortunate defect: rain poured into the galleries during warm, 
cloudless, sunny weather in spring. The reason for this clear weather 
precipitation is sketched in Figure 4. During the winter (on the left) the 
warm, humidified interior air penetrates into the roof sections, cools as 
it passes through the porous glass fibre insulation and deposits dew on the 
plywood that supports the sheet metal roof skin. The wood absorbs the 
water so no condensation is apparent during the winter. The water 
absorption is enhanced by the fireproofing salts in the wood which form a 
saturated solution when the air reaches a relative humidity of about 80%. 
During the early summer (to the right) the sun heats the grey metal of the 
roof to 75?C. The wood below the metal becomes warm and the water stored 
up during the winter is vapourised into the insulation. It diffuses down 
to the vapour barrier and there condenses, because the cool interior of the 
building is below the dew point of the moist air which originates at the 
under surface of the heated wood. The vapour barrier is made of 
polyethylene film, which is hydrophobic so that the water collects as 
discrete drops on the surface. There is still no indication of trouble 
within the roof. After about three days of warm weather so much water has 
accumulated on the polyethylene that the drops begin to run together and 
quickly gather into a torrent which runs down the slope of the roof until 
it meets the lower edge of one of the box sections. Here the vapour 
barrier stops and the water passes over the lower plywood layer of the box, 
dissolving the fireproofing salts. The surge of contaminated water now 
escapes from the box and falls on to the metal ceiling and then into the 
gallery below. The process is dramatic and seemed at first to be very 
mysterious, particularly as eye witnesses described clouds forming in the 
tall rotunda at the centre of the building! 
d 
Winter mmer 
F-cgitte 4.    The zondzmcuLlon pioc&AA -in the fioo^ 0& the. 
A-ttA and Indix&tAj-Z* Muxseum, in winteji and -in ?iiimmeA. 
In all these examples the structure of the part of the building which 
causes trouble is fairly uniform.  Even though the cracks play an essential 
part in the processes, they are evenly distributed within a basically 
uniform section. 
Buildings in mor 
problems. Here 
Institution's bu 
Washington. The 
been crumbling f 
thorough renovat 
durable and the 
search failed to 
match in appeara 
was rebuilt with 
e highly decorated styles of architecture present other 
I will use as an example yet another of the Smithsonian 
ildings, The Renwick Gallery, near the White House in 
sandstone facade on the brick wall of this building had 
or many years. Two years ago the Institution completed a 
ion of the building.  The original stone was evidently not 
original quarries are now inaccessible. An international 
turn up a quarry that could provide stone with a close 
nee and a guarantee of better performance, so the facade 
blocks of cast stone made from tinted concrete. 
The damage was attributed, in part, to movement of salts from the massive 
brick wall into the stone facing, which was not protected by an impermeable 
layer. To prevent salt movement the wall was reconstructed with an air gap 
between the original inner brick wall and the new facing blocks of 
concrete. Stainless steel pins separate the two parts of the wall. Sensors 
were embedded in the walls as they were rebuilt so that the climate within 
the structure could be monitored for several years to demonstrate the 
success, or to warn others against the defects of the design. 
concrete block. 
sfainltss sUtl pin. 
original brick wall 
2.J cm. void 
ict forming m cold parts 
?IQUKZ  5. The waZZ aomttaation  o-? 
the. Renmccfe GaZteJiy. 
The void that separates 
stone from brick does not 
exactly follow the contours 
of the outer surface, so it 
is at a different 
temperature at different 
points. The air in the 
building is humidified in 
winter. This air 
penetrates the wall, which 
has no vapour barrier.  In 
the colder parts of the 
void it will deposit dew. 
Air diffusing into the 
warmer parts will be able 
to circulate through the 
void to supply more dew to 
the cold points. There is 
therefore an accumulation 
in a few cold spots of 
water that has penetrated 
the wall over a large area. 
A small, uniform leakage of 
air through the wall can 
therefore result in serious 
damage in a few places 
(Figure 5). 
These case histories show that it is very difficult safely to install air 
conditioning that provides adequate relative humidity for the display of 
artifacts, in buildings which are set in climates that impose a real need 
for air conditioning. The fundamental problem is the large difference in 
air temperature and in air moisture content between the inside and the 
outside. Only a rather unusual and expensive design can provide a new 
building with adequate defences; for an old building it is hardly ever 
possible to add an adequate barrier to air flow. 
Clearly we cannot just adapt an old building to museum use by installing 
the standard industrial air conditioning plant.  We have first to survey 
the building and study the local climate records to estimate the likely 
danger from condensation. Then we have to draw up a specification for the 
absolute limits to variation in temperature and relative humidity of the 
interior. The two are linked by the need to keep the dew point of the 
inside air low in winter. The outside temperature and relative humidity 
determine the minimum summer temperature that can be imposed inside. The 
specification for the interior climate will also be influenced by the 
nature of the collection and it may not be the same for all parts of the 
building.  For a general collection the limits would be a relative humidity 
between 35 and 65 percent with a slow drift within these limits. The 
temperature should usually be held below 28?C but above the weekly average 
dew point of the outside air in summer. 
For people the relative humidity can be quite low without discomfort. The 
minimum temperature for comfort is about 18?C. The winter climate is 
therefore set by the need for a relative humidity above 35%, for the 
objects, and a temperature above 18?C, for the people. The summer 
temperature is set by people's discomfort working above 26?C.  If there are 
no people permanently stationed in the building the problem is solved 
because the temperature can be allowed to drift down in winter to maintain 
a low dew point and to drift up in summer to stay above the outside dew 
point.  It is the demand for a comfortable winter temperature for people 
with a comfortable relative humidity for objects that is the main cause of 
trouble. 
The best solution is surely to draw on the experience shown in local 
adaptation of architecture to climate and add to this the extra means of 
control conferred by modern technology, using not just the mechanical 
devices but also such innovations as accurate medium term weather forecasts 
so that the air conditioning can anticipate cold weather and cause a 
controlled fall in the indoor temperature.  In this way we can enjoy the 
satisfaction of an intelligent response to the variability of the natural 
world. 
I was helped in this research by Robert Ridgley and Gary Brenner of the 
Smithsonian Institution's Office of Design and Construction and by John 
Frieman and Deborah Spenser, both formerly at the Conservation Analytical 
Laboratory. 
THE DANGER OF AIR CONDITIONING IN HISTORIC BUILDINGS 
Tim Padfield 
Conservation Analytical Laboratory 
Smithsonian Institution, Washington DC. 
ABSTRACT 
Condensation within walls and roofs of buildings in a cold 
climate is an inevitable consequence of warming and humidifying 
the air within them because the dew point of the air inside the 
building is, for several months of the year, higher than the 
temperature of the outer surface of the building. The vapour 
barrier which is often installed to prevent diffusion of 
humidified air into the walls is not usually able to prevent 
transfer of moisture through air flow caused by the buoyancy of 
warm humid air in a tall building or by the pressure difference 
generated by the air conditioning equipment. The museum 
buildings of Washington DC, both old and new, testify to the 
difficulty of maintaining a humidity within that will preserve 
the collections without at the same time damaging the building. 
Sensors installed within the structures have clearly demonstrated 
the complexity of the patterns of air flow and of moisture 
transfer. Condensation, ice formation, corrosion and rot in 
walls and roofs may show no symptoms but still cause serious 
damage over the years. For historic buildings that house 
historic collections the specification for the interior climate 
must be a compromise based on a study of the construction of the 
building, the local climate and the nature of the collection.  In 
particular it may be necessary to change the interior climate 
slowly through the seasons to minimise the difference between the 
inside and outside temperature and water vapour pressure.