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T W E N T Y T H I R D E D I T I O N
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PROFESS IONAL SERV I CES
WATER IN PERMEABLE
BUILDING MATERIALS
ROBYN PENDER
T
HE CRITICAL
links between water
and almost every form of building
deterioration do not need repeating
here, nor does the fact that porous materials
interact closely with moisture. On the
other hand – as the various claims for
miracle remedial products make clear –
the mechanisms by which water moves into
and through building materials are not so
well understood.
Does this matter? The problem is that,
when it comes to dealing with moisture
problems, correct identification of the source
is vital: treating symptoms simply does not
work, because very different problems lead
to almost the same symptoms, and ‘feedback
loops’ are common (moisture causing damage
that lets in more moisture). To complicate
matters, the same words are used by
laboratory scientists, building investigators
and practitioners, but they are used to mean
subtly different things, and this can lead to
considerable confusion.
Although at microscopic level, the
behaviour of water and contaminants such
as salts is likely to occupy researchers for
many years to come, it is not difficult for
practitioners to develop a picture of the
processes involved at building level that is
good enough to underpin effective surveying
and remediation. This paper outlines one
such picture, based on the more detailed and
wide-ranging explanations in the Building
Environment volume of Historic England’s
Practical Building Conservation
series.
WHAT DO WE MEAN BY ‘PERMEABLE’?
It seems sensible to begin by disentangling the
words ‘permeable’, which is used technically
to describe any material through which a
particular liquid or gas can pass, and ‘porous’,
which in this context simply means that the
structure of the material includes voids or
‘pores’ (porosity being the ratio of voids to
solid matter). Not all porous materials are
permeable – if the pores are bubbles cut off
from each other, then nothing will be able to
pass through – and some may be permeable
to one liquid or gas but not another, or
permeable only under certain conditions.
The speed at which a liquid or gas can travel
through a permeable material depends to a
great extent on the shape and size of the pores,
and on how they connect with each other and
with the surface.
For materials scientists, ‘permeability’
is measured by packing a column with the
material of interest, and passing the liquid
or gas (its ‘mobile phase’) through under
pressure. For building conservators, the
mobile phase is always water (although it
may well be mixed with salts and other
contaminants), and the behaviour of single
materials is of little interest: we are concerned
with systems of multiple materials having
differing densities and porosities, with many
joints and voids; in movement of water in
every direction, not just one; and in what
happens under ordinary pressures, rather than
the extreme pressures imposed in laboratory
tests. When we speak of the ‘permeability’ of
a wall, it is this complex transfer of water that
we actually mean.
THE BEHAVIOUR OF WATER
Which, then, of the many factors involved
in moisture movement is it critical for
practitioners to understand?
Water is so familiar to us that its
strangeness is often forgotten, but its
behaviour both physically and chemically
really is peculiar. Most materials are densest
Failed remedial treatments are often based on wrong notions about the behaviour of water. The operation of Knapen tubes, for example, is based on the idea that humid air is
heavy and so will fall. In reality, humid air rises.
The structure of water molecules allows them to cling
together strongly, forming a strong ‘meniscus’.