This paper discusses the thermal analysis and economic performance of an attached sunspace, used in a conventional dwelling environment. The sunspace is a modified greenhouse with special geometry of glazed inclined roof and inclined glazed waifs and is capable to operate only as a completely passive system. New solar radiation and ambient temperature data for the area of northern Greece is presented. The calculation of thermal loads using the degree-day method and the annual auxiliary energy requirements of the residence are estimated using the unutilizability method (UU method). The economic analysis is performed using the Life Cycle Savings (LCS) method, modified to comply with the Greek tax policy, and the cost-optimum collector＇s area determination using the P-chart method. Also, the payback period of the initial investment is estimated, after taking under consideration economic factors applied to Greek energy policy. (C) 2000 Elsevier Science S.A. All rights reserved.