Because of the wide range of climatic conditions and variety of buildings used for collection storage, the National Park Service (NPS) relies on the ability of museum cabinets and prefabricated modular structures to provide quality collection storage space for NPS museums. Specialized museum storage cabinets and prefabricated modular structures contribute to the preservation of museum collections by enclosing museum objects in a beneficial microenvironment, one free of ambient light, dust, insects, and rodents and, most important, having stable relative humidity (RH) and temperature conditions. Two tests which recorded continuous temperature and RH readings for 12-month periods compared the storage environment for museum specimens in rooms of historic buildings within which insulated, prefabricated modular enclosures had been assembled. In one case, no supplemental climate control was present; in the other, both the room and enclosure were equipped with air conditioning. The tests monitored three levels of enclosure: the room, the insulated enclosure, and a tight, steel specimen cabinet placed inside the latter. Similar cabinets in the room but outside the insulated enclosure were included in one test. The report summarizes the degrees of control achieved. -- AATA