Thus in many instances there are a number of air passages either leading right through the wall or connecting with spaces on the cold side of the insulation. Airflow outward from the building into the wall in winter may deposit considerable quantities of water in the wall. This water runs down the back of the panel to emerge at the outside face of the wall or runs through various passages through the insulation and emerges at the inside face of the wall. On the inside, it runs down the wall or collects above suspended ceilings to cause the problems so often reported. On the outside, it forms icicles, leaves calcium carbonate stains, or etches the window glass.
Water vapour that moves by diffusion through the wall material is not normally a serious problem, since the rate of water movement by this mechanism is slow. Considerable time would be required to move a significant amount of water and few buildings are located where adverse weather conditions on the outside persist long enough for troubles to develop. Nevertheless it is desirable to analyse the vapour diffusion characteristics of a wall and assess the need for a vapour barrier in relation to the amount of water which may collect and also in relation to its potential to harm the wall (CBD 57). Dense concrete in the thicknesses normally used for precast panels, i.e., 3 in. or more, has a high resistance to vapour diffusion. In conformity with the principles set out in CBD 57, it should be used on the high vapour pressure side of the insulation.
Thermal Conditions. Items connected to a cold panel will have the same temperature as the panel at their point of contact, modified only insofar as the item concerned can feed heat into the panel and so raise its temperature locally. In view of the massive proportions of the panel and the high conductivity of dense concrete, it is easy for heat fed into the panel to flow away. As the panel is the structural element of the wall, items mounted on it do not have a compensatory easy path for heat to flow into them and so they tend to be cold.
Window frames are such items. They are cooled by contact with the cold concrete panel and in turn cool the edge of the window glass. The centre of the pane of glass may be warmer for various reasons. The higher thermal resistance of a multiple glazed window will keep the inner pane warm; heat supplied by the building heating system can have a similar effect, even with single glazing; and heat-absorbing glass will be warmed by the sun. Thus, a situation is produced in which the edge of the glass is colder than the centre and so is in tension. If the temperature difference is great enough, the glass will break. The cold frame and edge of the glass will, in any case, suffer from condensation.