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The ability of building materials to control indoor air humidity is studied in this paper. First, moisture capacity and transient response of building materials were investigated in small-scale laboratory experiments. Effects of moisture-absorbing interior wall materials on indoor air humidity were measured in a full-scale room under controlled conditions with known ventilation rates and moisture production schedule. The measured interior surface materials included wood, porous wood fiberboard, gypsum board with hygroscopic insulation, perforated plywood board, and, in a reference case, aluminium foil. Second, numerical simulation tools for hygrothermal performance analyses of building envelope parts and for buildings as a whole were used to assess the impact of hygroscopic mass on indoor air humidity. Two levels of testing and simulation were carried out: First, the moisture capacity of building materials in dynamic conditions was tested in small-scale laboratory tests. Second, the materials were placed in a room with intermittent moisture production. Moisture production and ventilation rates were set to correspond to those typical in residential buildings. Mass transfer between the finishing materials and indoor air affects the humidity both in indoor air and in the building envelope. The effect of coatings and their vapor permeance on the moisture exchange was investigated. A sensitivity study looking at the hygrothermal material properties and their effects on the performance was carried out.

The results show that building materials exposed to indoor air can have a strong effect on the indoor air humidity. Potentials, practical applications, and design concepts for utilizing the moisture-buffering effect of building materials are discussed.

Citation: Thermal Performance of Exterior Envelopes of Whole Buildings IX