It's not the air from the cool side that your losing. if your vents are up high, in a 8 inch tub. If the vents are on the lid (I don't know if they are) then it's the warm are that is escaping, as air warms it expands and rises and rolls along the top of the enclosure if it finds a way out up there it's trouble. 150 watt bulb only 8 inches of height with a way out its's a jet stream.