A computer model of a museum store is used to explore the effects of various methods of air conditioning. A room that is filled with an abundance of hygroscopic material can be held at a constant relative humidity by blowing in a stream of air, about one twentieth of the room volume per hour, which has a water content adjusted to be in equilibrium with the desired water content in the hygroscopic materials. This method works even if the temperature is uncontrolled, because of the buffer effect. The room also needs air recirculation at about one change per hour to ensure temperature uniformity and to allow the removal by filtration of internally generated pollutants.