Civil Engineering Reference
In-Depth Information
Figure 14.4: Bourne House, Aberfeldy (interior view). Surfaces with moisture-regulating
properties. Source: Howard Liddell
micro-organisms growing. The deposition and emission cycles of dust on inside sur-
faces will be reduced. Water vapour carries various gas contaminants combined with
water vapour molecules, which also penetrate the wall. Hygroscopic walls will there-
fore have a moderate air cleaning effect for nitrogen oxides and formaldehyde. This is
only effective as long as the gases stay in the material or are broken down inside it.
Hygroscopic materials lose their moisture-regulating properties if they are covered with
diffusion resisting materials such as plastic wallpaper, varnish, etc.
It is also an advantage if sealing and insulation materials in the wall are hygroscopic.
Condensation is no problem when the amount of condensed moisture is low compared
with the material's potential capacity for holding moisture (below the threshold for rot-
ting), as the water that is stored during a damp period can evaporate during the rest of
the year. This applies under normal circumstances to brick, earth, timber and other nat-
ural fibres.
Constructions with insulation materials such as foamglass and mineral wool are not
hygroscopic and should be insulated from internal moisture by a vapour barrier. Otherwise
there is a risk of absorption in the structure, and this can be too much, even for timber. A
wind-proof membrane with a large capacity for moisture absorption and permeability can
compensate for this to a certain extent. Sheets of gypsum or porous fibreboard glued with
asphalt or untreated are well suited for this, as long as their surfaces are not treated with
less permeable materials.
