This article concerns the determinants of relative humidity in a room. Temperature, vapor production, and ventilation are the chief factors. Variation in relative humidity may bear some relation to that of the air shut in a box with hygroscopic materials, which is used as a model for experiments discussed in this article. An unpainted wood surface is one of the optimum hygroscopic materials to be used as a wall surface for the purpose of reducing the humidity variation in a room by its absorption and exhalation. Porous surfaces made of inorganic material, such as glass wool and plastic foam, perform much more poorly for this purpose.