This paper reports on a series of tests that were performed at the National Bureau of Standards to investigate the thermal behavior of externally insulated masonry structures. A one-room test structure was built within an environmental chamber and various climatic conditions were simulated while the transient thermal response of the test structure was monitored. Three schemes, night mechanical cooling, night ventilation cooling and simulated passive solar heating were studied with regard to energy conservation. The results indicate that the latter two schemes, when applied to this type of structure, have the potential to substantially reduce and, in some cases, even eliminate the need for mechanical cooling and/or heating energy.The measured response of the test structure is compared with those predicted by an analytical model with excellent agreement.