Method for Vapor Diffusion Behaviour Determination of the Building Envelope

Abstract

This paper is concerned with a design method, considering vapor diffusion and condensation caused by dew point temperatures on or within building envelope construction. Condensation in building envelope can be either a winter or summer phenomenon, although winter condensation has been given much greater at tention in building standards and guidelines in North America and Europe. But, generally, the potential for summer condensation in building envelopes is related to humid climates and to tropical or subtropical regions. The current guidelines for preventing summer condensation in building envelopes in humid climates are not as specific and comprehensive as those for control of winter condensation. In this paper is analyzed a method for winter condensation control but which can also be used for summer conditions in buildings located in extreme climates: both cold and hot, and humid, where vapor condensation represents a major problem. Winter condensation control in buildings is more important for roof and wall constructions than for floor and foundations constructions. Low-sloped roofs present an especially difficult design problem since the upper surface on the roof must be waterproof and highly impermeable to water vapor and water can be trapped in the insulation, causing deterioration and excessive heat losses. The ambient temperature and relative humidity conditions on which heating will operate during winter determines the "dew point" at which condensation should occur on a cold plane "condensation surface," while building envelope construction. So, the first design criteria is to establish the exterior air temperature at which the vapor condensation is starting within the envelope. For each type of building envelope results a specific starting condensation exterior air temperature. This value must not exceed the average annual temperature, to avoid a year by year progressive humidification of the envelope. The second design criteria is to establish the growth of the envelope humidity content through condensation in winter climate parameters: duration and average temperature, statistically determined, in function of the starting con densation exterior air temperature. This value must not exceed the saturation capacity of the envelope insulation. By means of this criteria can be realized the correlation between interior and exterior climate parameters (temperature, relative humidity) and the hygro thermal properties of the building envelope (vapor diffusion and thermal resistances, vapor saturation of the envelope insulation). It also gives the possibility to justify constructive measures (vapor barriers, ventilated air layers, etc.) avoiding condensation phenomena.