Vikram,
It seems to me that your test building just has a lot of thermal mass. That would explain the thermal lag and also the small temperature swings during the day. The sunshine first heats the floor, walls, furniture, etc., which does not heat the room air very much right away. After the sun goes down the stored heat in the floor, etc. is released and warms up the room air late into the night.
Keith Swartz, P.E., LEED AP
