The comfort and facility that a building offers can be
adversely affected by the presence of moisture on internal surfaces
or within the construction. The fabric of a building can be a
contributory factor in the accumulation or presence of moisture in
two ways: either by not protecting from or contributing to the
ingress of liquid water either from precipitation or ground, or by
allowing the formation of condensation.
The prevention of liquid water penetration
from the outside through the building fabric is a critical task for
both the design and construction team of any building. The designer
must select appropriate materials and provide details that will
enable their correct installation in line with the details provided
and maintaining a high standard of workmanship. The positioning of
and integrity of membranes often next to insulation is critical as
is the correct specification of materials next to layers which are
intended as rainscreens rather than waterproofing layers.
Condensation
Condensation occurs most commonly where warm
humid air meets a cold surface. Harmful condensation can occur both
on inner surfaces and within construction elements. The former
typically occurs where a specific area of the inner surface is
significantly colder, either caused by poor detailing or
workmanship that lead to the omission of insulation. Condensation
occurs within the element of structure where water vapour is able
to migrate through a structure by diffusion, or more seriously,
transported by air movement through gaps, until it meets a barrier
which prevents further diffusion/transport and causes the vapour to
condense. This can be overcome by installing effective vapour
control layers on the warm side of the insulation. In all
circumstances, air movement into the construction through gaps must
be prevented.
The Law
Approved Document C (England
and Wales only) sets out requirements for resistance to moisture
via floors and walls (from the ground and outside), and roofs (from
the inside and outside).
Approved Document F (England
and Wales only) sets out the natural and mechanical ventilation
requirements of dwellings and non- dwellings.
In Scotland, Technical Handbook,
Section 3 and in Northern Ireland, Technical
Booklet C covers site preparation and resistance to
moisture.
Key guidance
BS 5250 Code of practice for the control of condensation in
buildings.