Two things to mention here. The vent(s) only need to open when it gets hot in the GH, which means the sun is intense enough to heat the GH at or above 90 F. That sun's heat softens the solar blanket material and it stretches. My GH with a root vent has corrugated polycarbonate panels, and that allows air-flow down both sides, since only the sides of the solar blanket is screwed down. The two ends at the ground-level are free, with only cross-wiring to hold it in place. Certainly there won't be a great amount of air-flow, but there is some. During the fall/winter months, the GH's just don't get all that hot anyway. I simply open the doors when necessary.
I also have auto-vents on the upper ends of the GH's, and those louvers open quite easily, allowing air to flow out of the GH's.
