VAPOUR BARRIERSA frequent cause of structural damage in refrigerated structures is the formation of ice within walls and roofs. Atmospheric air always contains some water vapour. The total atmospheric pressure is the sum of the pressure exerted by the dry air and the pressure contributed by the water vapour. If progressively more water vapour were injected into the container, a state would be reached where some of the vapour would begin to condense into liquid. At that state the air is saturated, which is a condition shown by the saturation line on the psychrometric chart. The vapour pressure at which condensation begins is a function of temperature, and the saturation curve for water.The next subject to consider is gradientsboth of temperature and of water vapour pressure that exist in walls. Visualize a composite wall of two materials, as in Figure 19.5, with a high temperature outdoors and a low temperature indoors. Figure 19.5a shows the temperature gradient, with the steeper drop in temperatures occurring in the insulation. Figure 19.5b shows the gradient of the water-vapour pressure from the high vapour pressure of the warm outdoor air to the low vapour pressure inside the refrigerated space. Water vapour flows continuously through the wall from the ambient side of the wall to the interior. Figure 19.5c shows a composite graph of the water-vapour pressure distributionthe dotted line being the actual water-vapour pressure line from Figure 19.5b. The other curve in Figure 19.5c is the saturation pressure corresponding to the existing temperature at the various positions in the wall from the ambient to the interior. This distribution was obtained by translating the temperatures of Figure 19.5a through the saturation pressure curve of Figure 19.4. The consequences of the liquid water are twofold: (1) the insulating value of most materials is diminished, and (2) if the temperature is below freezing, the resulting ice will expand and possibly damage the structure. How can condensation of water within walls and roofs be prevented? The answer is by the careful installation of a vapour barrier. A vapour barrier is an impermeable membrane that blocks the flow of air through the building envelope. The vapour barrier, because of its extremely high resistance to the flow of vapour, causes an abruptly steep gradient, as shown in Fig. 19.6a. When applied to the warm side of a wall, as in Fig. 19.6b, the actual vapour pressure drops so low on the left of the vapour barrier that it remains below the saturation pressure throughout the wall.

Films made of such materials as polyethylene and Mylar are available as vapour barriers. For example, a common vapour barrier material is polyethylene plastic film, typically installed in thicknesses from .002" to .008" (0.05 mm to 0.2 mm). This material is inexpensive, transparent, easy to handle, and is available in wide widths. It can be attached by stapling, mastic, and other means. Even though thin films of most commercial vapour-barrier materials serve adequately to retard moisture flow, but a vapour barrier of adequate thickness should be chosen to prevent tearing and puncturing during installation. It must be remembered that water vapour is a gas and it is not sufficient merely to make the outer surface waterproof; overlapped joints, for instance, must be sealed.

Reference:1. Industrial refrigeration Handbook by Wilbert F. Stoecker