Complex sentence: In order for a building to be ventilated adequately via buoyancy driven ventilation, the inside and outside temperatures must be different so that warmer indoor air rises and escapes the building at higher apertures.
Split into simpler sentences: In order for a building to be ventilated adequately via buoyancy driven ventilation, the inside and outside temperatures must be different. When the interior is warmer than the exterior, indoor air rises and escapes the building at higher apertures.