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A project initiated by TC 4.4 looked at the nature, significance and control of solar-driven water vapor diffusion in wall systems. The project combined experimental and simulation work to provide an in-depth characterization of the phenomena occurring during inwards vapor diffusion in insulated wall assemblies. Small- and large-scale laboratory tests provided data under controlled conditions, indicating that porous claddings that absorb rain become the source of moisture when subjected to solar radiation. The vapor permeance of the interior finish layer is a key parameter leading to moisture accumulation in the gypsum board. Field studies were performed over a period of 2 years and occurrence of solar driven diffusion was documented for different wall assemblies. Once the capacity of computer models to reproduce the observed behavior was verified, a parametric study was performed for 18 different wall assemblies in seven locations in USA. It was found that the design of wall assemblies should include the evaluation of behavior under conditions leading to inwards diffusion in warm and mixed climates.

Units: Dual

Citation: ASHRAE Transactions, Vol. 116, pt. 1, Orlando 2010