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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
in their solid form, but ice is lighter (and so
floats). At less than a nanometer in diameter,
water molecules are tiny: smaller and lighter
than the air molecules they displace (oxygen,
nitrogen, argon and carbon dioxide), and with
so little mass that gravity has no effect
1
: humid
air is therefore buoyant and rises. At room
temperature, most other small molecules are
gases, but water is liquid because its boiling
point is very high.
The underlying reason for many of water’s
special characteristics is the structure of
the molecule. Water is a ‘polar’ molecule:
its electron cloud is not evenly distributed;
instead the two hydrogen atoms are twinned
together and form a positive charge, whereas
the single oxygen atom is negatively charged.
As a result it reacts strongly with electrical
fields, as well as with many surfaces and
with other charged particles. This includes
other water molecules, so the liquid has an
unusually high surface tension, and forms a
very strong ‘meniscus’ (the surface separating
the liquid from the air). For the same reason,
liquid water flows, is an excellent solvent, and
it transmits an electrical current with ease.
WHAT THE STANDARD ‘HUMIDITY’
TERMS MEAN
The common terms describing humidity
all refer to water molecules in free air. The
‘absolute humidity’ (AH) is the actual number
of molecules (by weight per area or volume).
‘Vapour pressure’ (Vp) is most easily envisaged
by imagining the air inside a box. The
molecules will be bouncing around at speeds
that depend primarily on the air temperature
(which gives them their energy), and the Vp is
the pressure the molecules exert on the walls
of the box as they hit; more collisions means a
higher Vp. Vp can therefore be increased in two
ways: by adding more molecules to the box, or
by increasing the temperature.
Each time the molecules collide with each
other, it is likely they will lose some of their
energy, and eventually they will no longer have
enough momentum to break away again (that
is, they will ‘condense’). Eventually droplets
of water form (mist; then – when the droplets
are massive enough for gravity to take effect
– dew, and eventually rain). Molecules will
also lose energy when they collide with a
surface; and the colder the surface, the more
energy they will lose, making condensation
increasingly likely. A condensed molecule can
break away again – ‘evaporate’ – if it gains
energy (heat, perhaps, or momentum from
being hit by another molecule), and in practice
condensation and evaporation are happening
continually.
The number of molecules that air can
hold depends on its temperature and AH (that
is, on the number of collisions occurring).
At some point the number of molecules
condensing will be balanced by the number
evaporating, and so the air will be holding
as much water as it can (‘saturation’). Air
will not generally be saturated unless it is
in a closed space together with a source of
water molecules, so the humidity of the air
is commonly expressed as ‘relative humidity’
(RH), which is the percentage of saturation
(the amount of water molecules actually in
the air, compared to the amount it could
potentially hold at that temperature). Air
at 80 per cent relative humidity will be
holding many more water molecules if the
temperature is 25 degrees than if it is 15
degrees. To increase the RH, either the AH
must be increased, or the temperature must be
decreased, or both.
THE IMPORTANCE OF LIQUID WATER
Water vapour is often the principal topic raised
by building physicists, but when it comes to
understanding deterioration, liquid water is
of infinitely greater importance. It is affected
by gravity, so water from a leaking gutter will
quickly find a path through the pores of the
materials and flow down to collect at the base
of the wall; this is a common cause of so-called
‘rising damp’. In pores, however, gravity is only
one of the possible forces driving movement.
The air inside all three sealed bottles is saturated (as
shown by the condensation) and this is independent
of the water level. There are more than a ‘vigintillion’
(10
20
) water molecules in a single drop of water, so the
reservoir needed to reach saturation is tiny.
All the familiar water-driven deterioration (salt cycling, timber decay, corrosion) requires liquid water; even where
water vapour is involved, the problems arise when it condenses.