A study was conducted to determine the cause of interior moisture damage at selected Florida hotel-motel sites generally identified by (1) peeling of the vinyl wall-covering material, (2) high moisture content in the gypsum board, resulting in crumbling or material structural failure, (3) fungal growth with a red, yellow, or black stain, (4) paint delamination, (5) cold, clammy, uncomfortable rooms, and/or (6) surface mildew on some interior furnishings. The primary cause of the "condensation problems" is a direct result of warm, humid outdoor air infiltrating into the wall cavities followed by condensation of the water vapour. Since the hotel-motel sites are in the region of high relative humidity and corresponding high temperatures during a major portion of the year, air conditioning of the occupancy space is a necessity. The condensation of the moisture in vapour form into a liquid occurs when the temperature of the surrounding surfaces is below the temperature at which water vapour condenses, or the dew-point temperature. With vinyl wall-covering material in the guest rooms acting as a vapour retarder, moisture is trapped inside the wall cavities, resulting in "concealed condensation" when the wall temperature is below the dew-point temperature. Moisture migrates from a place of high vapour concentration to a place of lower vapour concentration by two methods - diffusion through materials and by air flow carrying moisture with it. An air conditioner, by lowering the temperature within a guest room, lowers the partial pressure of the water vapour as measured by the vapour pressure. As the vapour pressure is lower in the guest room than the exterior vapour pressure, moisture will flow toward the room interior. The moisture flowing as a result of diffusion is a function of the difference in vapour pressure between the two sides of the supporting walls, the permeability of the construction materials used in the structure, and the exposed surface area. The amount of moisture accumulating as a result of air infiltration is a function of the mass flow rate, the humidity ratio difference between the flow stream and the interior space. In general, air infiltration will have a greater effect on the deterioration of the interior materials than vapour diffusion.

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