The paper presents a study on the thermal behavior of a solar greenhouse set against one apartment of a residential eco-sustainable building in a temperate climate. Particular attention was given to the analysis of the contribution of solar radiation in winter heating in relation to natural ventilation. This study makes use of a transient energy simulation system named ESP-r (Environmentals System Performance - research). The results so far have revealed that the preferred orientation of the greenhouse to maximize the solar collection in January and February (South) can cause overheating in March, as the temperature inside the room exceeds 25°C (limit of the comfort zone) for 5% of the hours, and in April when the discomfort is for 60% of the time. The final step of the study focuses on the dimensioning of the openings for the reduction of the above mentioned percentage through natural ventilation, identifying a design methodology that integrates low-energy concepts in the design process.