In what part of the country is this home located? What is the construction material? These homes work best in the Southwest, or where there is a lot of sunshine. Also, is the wall in the atrium some thick material that will make a good "heat sink" -- that is, it will absorb heat during the day, and release it at night. Adobe is excellent for this. Brick or possibly concrete would work OK, too. A standard framed wall does not do this. The atrium floor should also be a material that will absorb heat by day and release it at night. I've lived in a couple of passive solar homes and they are delightful, as well as cost / energy efficient. Good luck.