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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 6
T W E N T Y T H I R D E D I T I O N
1
PROFESS IONAL SERV I CES
water-vapour molecule to travel right through
a permeable material of any thickness. Water
vapour is most likely to be a problem if it
forms enough condensation to drip or to run
down as liquid. (Condensation on glass, for
example, can lead to the decay or corrosion of
window frames.)
THE CRITICAL IMPORTANCE OF
MOISTURE HISTORY
Exactly what happens inside a permeable
material as the environmental conditions
change will depend on the existing distribution
of water within the building materials: in
other words, the ‘moisture history’ is critical
to behaviour. This is the key to understanding
puzzling observations such as the fact that
although, in theory, driving rain cannot “get
through” a wall, in some cases it can be seen
to cause moisture problems on the interior;
or that in the laboratory permeable materials
buffer humidity much less well than they do in
real buildings.
To take the example of driving rain, the
supply of liquid to the pores is by no means
continuous (as it is, say, with a Karsten tube);
even in the most violent storm it is highly
unlikely that two consecutive raindrops will
hit the same pore. This means that the rain
will not generally penetrate beyond the first
millimetre or so. If, however, a good number
of capillaries are already filled with water from
some other cause (such as a leaking gutter), and
the raindrop stands a good chance of hitting
the mouth of a wet capillary, it can be drawn
in; and if that capillary is filled with liquid and
connects to the interior surface of the wall,
then evaporation can quickly wick the rain
right through.
Another apparently confusing observation
is that rapid drying after flooding tends
to fail: after a few weeks the surfaces are
once again wet and covered with mould.
This comes down to the difference between
liquid and vapour movement, which causes
drying to proceed in two stages. In the first
stage, liquid-filled capillaries connect the
water inside the material to the surface,
so evaporation can quickly draw it out.
Eventually, however, the speed of evaporation
will exceed the speed that water can travel
through the capillary, and the liquid stream
will be broken. Across the break, water must
travel as a vapour rather than a liquid, so
immediately the drying rate plummets. This
second stage of drying will continue until the
water inside the material has moved around
enough to once again establish a liquid ‘flow
path’ to the surface. Trying to dry a flooded
building too quickly is counterproductive;
effective drying is slow and steady, staying
in the first stage for as long as possible.
PUTTING KNOWLEDGE INTO PRACTICE
Analysis of building moisture problems
does not mean checking to see that standard
water-handling features are in place, so much
as looking for the signs of failure. Water cares
nothing for our intentions, and will never flow
into a drain if the drain is higher, just because
it is a drain, nor enter a drying vent only when
dry, and then leave again conveniently as soon
it has picked up moisture.
It is critical to keep an open mind about
the source of the water. It is all too easy to
develop complicated scenarios, when in fact
on closer consideration the cause is quite
straightforward. Common sources that are
often overlooked include plumbing leaks and
routine cleaning. We use much more water for
cleaning than in the past: for example, decay on
the inside of a window frame is now more likely
to be caused by routinely washing the window
than by condensation.
The construction techniques used
in the past are also often overlooked or
misunderstood by those repairing them,
sometimes with disastrous consequences.
Today most construction is based on
‘waterproof’ materials down which rain
flows rather than being absorbed. If we look
at traditional construction in the light of
water behaviour, it is immediately evident
why it has worked so well: it is the result
of refinements by generations of observant
builders, who understood how to make
optimum use of the many advantages of
permeable materials.
ROBYN PENDER
is a physicist with a degree
in wall-painting conservation from the
Courtauld Institute of Art, a commissioner
on the Cathedrals Fabric Commission
of England, and works for the Building
Conservation and Research Team in Historic
England’s Conservation Department.
Together with Brian Ridout and Tobit Curteis,
she edited the Building Environment volume
of Historic England’s
Practical Building
Conservation
series.
Water will not run uphill into a drain, and yet poorly-
positioned drains are common.
Traditional building features such as wide overhangs and hood mouldings were designed to keep water from entering
the wall at weak points, keeping the walls dry and allowing them to resist driving rain.