Although ventilation is of considerable value in reducing the severity of condensation problems, it is not a substitute for preventing leakage of moist air into a cold space. The cause of failures at the tops of solid masonry walls is often obscured by the fact that several phenomena in sequence are required to produce the damage. The primary factor is frequently air leakage, which results in condensation and wetting of materials. Under freezing conditions water will migrate to growing ice lenses, causing excessive expansion and displacement of masonry units. If the outward leakage of air with a dew-point temperature higher than the outside air temperature is prevented, such problems may not occur.
Severe problems resulting from air leakage can occur in metal and glass curtain walls.
Following the normal practice of sealing curtain walls against rain penetration at the exterior surface, designers consider that they will also be air tight and do not attempt to seal them at the interior. Realizing, however, that rain penetration does occur in most curtain walls sealed at the exterior, intricate drainage systems are incorporated within the wall, with openings provided to permit water to flow back to the exterior. During cold weather condensation occurs and accumulates as frost. Here it can freeze, producing icicles, which may be large enough to present a real hazard when they fall to a busy street below. The need to prevent the flow of moist air from a building through any cold space within walls cannot be over-emphasized.