Civil Engineering Reference
In-Depth Information
20°C only holds 3.8 g/m
3
, it can pass through the wall to outside air at 0°C without any
condensation being formed, but if the air is saturated with 14.8 g/m
3
then there will be
condensation within the wall of 11 g/m
3
. In a normal situation, a room contains about
5-10 g/m
3
water vapour, while a bathroom, in short periods, can reach almost
14.8 g/m
3
.
Big condensation problems can occur with open-air leakage or cracks in walls
and roofs. At the same time, moisture diffusing through materials normally
occurs without large amounts of condensation being formed inside the wall. A
wall completely free of small cracks is unrealistic, so it is necessary to take cer-
tain precautions using the following principles:
• Vapour barriers
• The absorption principle
• The air cavity method
Vapour barriers
The use of vapour barriers has become the most widespread method in recent
years. The main principle is that water vapour is totally prevented from entering
the wall by placing a vapour-proof membrane behind the internal finish. The air
and its vapour is then ventilated out of the building. This method has certain
weaknesses. The only usable material for this purpose is plastic sheeting or metal
foil. How long plastic sheeting will last is not really known. During the building
process, rips, holes and such like will inevitably be caused. At these points small
amounts of vapour will creep through, and after a time condensation will occur
in the wall.
A more moderate and less vulnerable solution is a vapour check that limits
vapour diffusion. This is not as absolute as vapour-proofing, but reduces pene-
tration considerably. Materials used for this are high-density fibreboards and dif-
ferent types of sheeting. The choice of material is determined by the type of
wind-proofing used on the outside of the wall. A rule of thumb is that the resis-
tance to vapour diffusion on the inside must be five to ten times higher than the
wind-proofing layer on the outside to give the vapour a direction (NBI, 1989). It
is important to note that the windbreak's resistance to diffusion is often heavily
reduced if it is damp - down to 10 per cent of its original value in the case of a
porous wood fibreboard. It is therefore often possible to use the same material on
both sides of the wall.
The absorption principle
Some materials used for walls are very hygroscopic and resistant to rot, and can
absorb vapour. As the condition of the building changes in terms of its tempera-
ture and vapour content, the stored moisture is, after a while, released back into
