Since we have an indoor pool we tried it when it first came out. It was better then nothing, but not by much. If I had to guess I would say no cover was 0 and the bubble cover was 100% the fish was about 20%. We even actually dropped what was rated for our pool surface area, and then tried doubling it with little added effect. Not enough to allow us to leave the cover off, but again better then nothing at all. The heat loss was still way too much and the humidity never really came back down after swimming until we covered it.

Better then nothing, but not by much and as water bear said if it was outdoors and windy I wouldn't think it would do much of anything.