It is possible to devise suitable joints for these locations, although there are special difficulties mainly related to the thermal conditions to which the panel is subjected. As it is insulated on the inside, a panel will be colder in winter and warmer in summer than would be the case if it were not insulated. As this leads to increased expansion and contraction, the sealant materials or gaskets are subjected to greater strains. Alternatively, the joint width must be increased to allow for the deformation while keeping the strain constant. With a wider joint more sealant is required, there is a greater risk of the sealant's sagging, and, in some cases, it is not acceptable aesthetically. Furthermore, the final portion of the extension takes place when the temperatures are lowest and the sealant or gasket is coldest, and so, stiffest. Thus it can be seen that although joints can be devised to meet the principles set out in CBD 40, there may be some question as to whether, as the sealant materials deteriorate with the passage of time, they will retain their ability to control both rain and air leakage.
The foregoing discussion presupposes that the panel is not cracked. If it is cracked, the situation can be changed considerably. Some hairline cracking of concrete panels is not uncommon and such cracks are usually accepted provided they do not exceed 0.005 in. in width on the exterior face or 0.01 in. on the interior face. The large movement of panels under temperature variations must be allowed for by the way in which they are fastened to the structural frame. If this is not done satisfactorily then the cracks could open further or additional ones be induced. Once there is a crack through a panel, capillarity, gravity, or air pressure can move water into the wall and the principal defence against rain penetration has been breached.
Vapour Movement. Water vapour in the air within a building can move outwards through the wall by two methods - transportation by air currents, or diffusion through the wall material. Of the two, the former is by far the more important, as has already been stressed in CBD 72. The mechanisms causing the pressure differential through the wall have been discussed in CBD 23. If air that contains water vapour moves from a warm to a cold location, there is a danger that it will be cooled to below its dew-point and deposit water vapour by condensation (CBD 1). For this to occur, the air need not pass right through the wall. It can enter it, be cooled by contact with the colder parts of the wall, and return by convection to the inside of the building. In order to stop this convective air flow, either an air barrier is required on the warm side of the insulation or there must be no air spaces in which it can occur. The precast panel is on the cold side of the insulation and thus can control through-wall leakage only. The interior finish of a wall is seldom applied in the form of a continuous air-tight shield, and would be difficult to make air-tight because of the various pipes, ducts and electrical conduits that pass through it and the inspection panels that may be required in it. Thus, there is no effective air barrier to control convective air flow unless the insulation itself can form one or be applied so as to eliminate all air spaces.
The inside face of a precast panel is not usually finished to a high degree of smoothness and it is not uncommon for air spaces to be formed between the insulation and the back of the panel. Many of these spaces will be connected one with the other and with the inside of the building. Because of difficulty in applying insulation to the panel where it is opposite the structure, it is sometimes specified that the structure be insulated before the panel is erected, and that for the remainder of the area the insulation be applied to the panel after erection. Not only is it difficult to make a good air seal at the point of changeover, but relative movements between the two will open a crack at this point. This crack leads directly to the air space between the back of the cold concrete panel and the insulation that has been applied to the structure.
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.