Stochastic analysis of moisture buffering in rooms

This paper presents a stochastic analysis of moisture buffering in a room. The relative humidity in a room depends on the ventilation rate (or supply of air with a different RH and temperature), the moisture production rate and the moisture uptake/release rate by finishing materials, also called moisture buffering. The ventilation rate, the outside vapour pressure and temperature, the moisture production rate are essentially random and can be considered as stochastic variables. In this paper, the moisture transport properties of the surface materials and the surface transfer coefficients are considered to be deterministic. As a measure for moisture damage risk, referred to as damage indicator, we consider the difference in maximal and minimal relative humidity during a day. Since the inside relative humidity variation can be considered as a stochastic process, the damage indicator is also random and can be described by a cumulative distribution function. As performance criterion in a reliability approach, we can then define a maximum risk of exceeding a limit value for the daily variation in relative humidity. The cumulative distribution of daily variation in relative humidity depends on the moisture buffering capacity of the surfaces. We found that the moisture buffering capacity of a room is related to the reduction of the moisture damage risk, when comparing a room with and without moisture buffering capacity. As long as the average moisture production rate and ventilation rate are maintained, the moisture buffering capacity seems not to be influenced by the randomness in the stochastic variables. Using a penetration depth model, an average effective surface mass coefficient for the room is defined, taking into account the vapour transport from air to the surface, through paint layer and finally to the active surface material layer. A relation between moisture buffering capacity and effective surface mass coefficient has been demonstrated for a given room, with geometric characteristics, such as the total surface to volume ratio, with a given average ventilation rate and moisture production rate.