The aspects of a system of local humidity control for museum display cases are discussed: a description of the methods employed to condition and to install silica gel at the Sainsbury Centre for Visual Arts is given. The monitoring process is also outlined, the results of which are illustrated to indicate the stabilizing effect of the humidity buffer. In an experimental section, it is shown that diffusion is the chief mechanism for air exchange between the interior of a display case and its surroundings. The results of the experiment are used to calculate the number of air changes per day which occur.