Museums are increasingly becoming aware of the need for good environmental control in order to slow down the effects of environmental instability on objects. In theory, air conditioning can provide this level of control, but at a price. For example, many museums are housed in historic buildings where it is very difficult and expensive to install air conditioning, and maintenance and operating costs are high in terms of energy and personnel. Furthermore, the introduction of air conditioning in an historic structure is questionable. Using monitored data from historic house museums with and without air conditioning, the authors examine the level of environmental control that air conditioning provides in practice. Analysis of monitored temperature and humidity data suggests that air conditioning does not always provide tight control or a more stable environment. A new attitude to environmental control is required among both building designers and museum professionals in order to achieve agreement on an acceptable level of environmental control at an affordable price.