I have never left the top/two sides (one piece) on. The ends stay on year 'round but one end has sliding doors and looks onto the north , so the blanket material doesn't contribute to heat retention. In fact, yesterday I removed that top/sides blanket and replaced it with shade clothe. One reason I don't leave the blanket over the top and sides is because of vents. I want as much hot air to escape the greenhouse as possible and having the solar blanket material covering the vents, greatly restricts that air movement.
Covering the solar blanket with good shade clothe might just work but if I were doing this, I would use 80% shade clothe to cut down as much sun penetration as possible. I use 80% on the highly sun-exposed greenhouse and 60% on the other one that gets less afternoon sun.
