Does the roof vent have a cfm rating?

The purpose of any roof vent, powered or passive, is to release hot air from the GH and as hot air rises naturally, the obvious place to release it is from the roof. The speed at which the exchange is made is what makes the difference in GH temps., so the more CFM, the faster hot air will be released.

However, regardless of the exhaust method, you still need to bring fresh, cooler air in and for that, your best bet would be a powered intake vent set as low to the ground as possible, preferably on a north facing wall, with the shutter motor wired to the thermostat on the exhaust fan.

Most GH's set up with powered intake vents also have wall mounted exhaust fans, because most kit GH's are equipped with roof vents that open with wax filled controllers, which are fine if the structure is high enough to keep the hottest air well above average human head level. Meaning; the higher the ceiling, the cooler it will be in the strata in which we work and grow plants.

MY GH has a 12-1/2 ft. ceiling height and the worst mistake I made was in not going up higher with the knee wall, to take the ceiling up to at least 16 ft.

I have come to believe that roof vents on most garden GH's are included in standard designs more because they are traditional than practical, because in a lower roofed structure, no opening in a roof will pull cooler air up from a floor and out with the speed and force of a standard mushroom capped roof exhaust fan.

My GH is a typical kit, prefab type, in that it's aluminum framed polycarb glazed. It's 12 X 16 ft. and came with four, 48" x 22" roof vents, set facing on either side of the roof ridge, 4 ft. apart.

But they are passive air releasers and would not be very effective if they had not been turned into active ones by a pair of 52" ceiling fans, mounted direct below each pair of vents, so they serve the same purpose in forcing air up and out as would a single, far less costly, standard mushroom cappped roof vent.

If not for those ceiling fans, and the ability to power the air up and out and keep my GH below 82 deg. on the hottest days, I'd have as been as well off simply sealing up the GH and installing an AC unit.

As for sealing a roof vent for winter, I'll repeat from another of your posts: just a piece of foam insulating board shoved into the frame will seal it for winter. You could get into a more elaborate hatch cover if you had the time and inclination to fabricate and install one, but just a few inches of foam would do the job.