1 INTRODUCTION Solar XXI building is a prototype of low energy office building located in Lisbon (38º46’N, 9º11’W) where passive and active solar strategies have been applied to reduce the use of energy for heating, cooling and lighting, combining also an extensive photovoltaic (PV) façade for electricity production (Rodrigues et al. 2008). Lisbon climate is characterized by monthly average temperatures that swing between 10.6ºC for the coldest month (January) and 22.6ºC for the hottest (August). During summer solar radia- tion is high, more than 6.5 kWh/m 2 per day, and maximum average air temperature is around 28ºC; however extreme values, air temperatures higher than 35ºC, can be observed during a se- quence of days. Winter is less severe because the average minimum temperature is between 8 and 10ºC and precipitation days are not frequent. Solar XXI was built in 2006 and it has been intensively monitored ever since (Gonçalves et al. 2008). It is considered a high efficient building, close to a net zero energy building (NZEB), because the difference between the energy consumed and that produced is 1/10th of the energy consumed by a standard office building. Solar XXI building design includes a large number of energy efficient concepts, such as a high insulated envelope, south sun exposure, windows shading, ground cooling or stratification and cross ventilation. Considering that Solar XXI’s design is based on the concept of integrating passive solar strategies for heating and cooling in a single building, in this paper, such integration is explored and analyzed in order to assess if the results obtained so far constitute a proof-of-concept, or else further research is required to improve it. Solar XXI building: proof of concept or a concept to be proved? M. J. N. Oliveira Panão & H. J. P. Gonçalves LNEG, Lisboa, Portugal ABSTRACT: Solar XXI building is a prototype of low energy office building where passive and active solar strategies have been applied to reduce the use of energy for heating, cooling and lighting, combining also an extensive photovoltaic façade for electricity production. Solar XXI was built in 2006 and is considered a high efficient building, close to a net zero energy building (NZEB), because the difference between the energy consumed and that pro- duced is 1/10th of the energy consumed by a standard new office building. Its design includes a large number of energy efficiency concepts, such as a high insulated envelope, south sun expo- sure, windows shading, ground cooling or stratification and cross ventilation. The windows solar gains and the effectiveness of shading devices were proved correlating solar radiation, external and indoor air temperatures. It was also verified that ground cooled air has a temperature close to that theoretical expected. Chapter 6: Case-studies 801