After seeing the severe day-to-night temperature variations in the board room, I decided to monitor a room in another corner of the same floor. I used a small EnviroMon system, separate from the permanent system that we have installed in the rest of the building. The graph shows that the upstairs spare room behaves in a similar way, suggesting that it's the construction of the outer walls that's to blame rather than any oddity of the air conditioning or building layout.