Sceptical to start with as I thought separate top to the side would’ve been a better design to keep heat in while you’re using the Spa but after using  it now for a few weeks it’s been a revelation, instead of Spa temp dropping to 20~22 degrees overnight it now only drops a few degrees from set temperature 33 to around 28degrees, which is excellent. The more we put it on and take it off the thicker it appears to be, don’t know what’s in the filling but it’s great, we’ll worth the money as it not only keeps Spa warm but protects it from the weather also 10/10