graymtn, I had the same problem, and in my very windy climate, I gave up after a year or so. The constant up-and-down vibration of the roof vents in bad winds (even when in the closed position) made me worry about an eventual failure, and it did cause the poly panels to wiggle out of the frame a bit, until I glued them in place with silicone caulk. I finally wired mine tight shut, and years later, removed them entirely when I had to replace the roof panels because of deterioration.

I can only get away with this because I remove four panels on the south side in warm weather, and replace them with screen panels. I also have an exhaust fan to help with ventilation.

Before giving up though, I've thought I probably should have tried a vent opener, like those sold by Bayliss or Univent. Seems like these might be able to snug the vents down more tightly, but I really don't know. (The vent openers sold by Harbor Freight have a pretty poor reputation online.)

Maybe other HF owners will chime in with other ideas...?

I'm not a HF owner but for my greenhouses I use 2 methods to keep air from leaking- both involve use of a piece of plastic. The easier method, if the vent has an internal or external lip, is to use a string and bungee cord combination to fit around the lip circumference holding the plastic in place. The other option is to screw fast a square of wire lock base around the opening and then use wiggle wire to secure plastic. Whenever you cover any powered opening you want to first turn off power to that site and use a sign at the switch to alert you that the vent is covered.

I saw on a video a guy use a piece of wire (hanger maybe). He bent one end into hook shape and wrapped the other end around the neck of a water bottle. He hooked it into the vent and the weight of the water bottle held the vent closed.

Love some of these creative ideas, especially the water bottle trick.

Our 6x8 HFGH is wrapped in a clear solar pool cover November-March, which negates any problems with wind pulling the vent open. The rest of the year it is uncovered with the automatic vent opener engaged and we've not had any problems since installation in 2005 ... except when Hurricane Sandy hit, and all the panels blew out and were scattered like snowflakes in our and neighbors' yards (with surprisingly little damage, BTW).

Thanks for the feedback, agardenstateof_mind! I do wish now I'd tried installing automatic vent openers. In fact, I wish I'd also tried the water bottle trick!!!

A water bottle is a weight. Most all vents have an adjustable setting spring but over time even power louver may leak air. Thus a piece of plastic and better if it can be easily fastened and taken off, even on sunny days like today in the northeastern part of the country.

Barrie, I'm afraid the vents on the Harbor Freight greenhouse are probably more bare-bones than on other greenhouses, so no adjustments. The only photos I have are from our build, many years ago. Since I eventually removed my roof vents, this is the only horrible photo I have to share. The handle is the part hanging down with the three holes. To the left is a small piece with a peg, to catch in the holes.

Also there's a much better picture of a harbor freight roof vent open on this blog, about 2/3 of the way down the page:

good pic of Harbor Freight roof vent in open position

The simple handle is hinged and has three slots, each of which can be captured on a peg. That's the entire summation of the adjustments, and even in the tightest closed setting, the wind can pick up the frame, lift it a fraction of an inch, and let it bang down again. The repeated banging vibration is what caused me to abandon my roof vents eventually.

That's why I'm wondering if mommafixit's simple suggestion of a weight, to make it harder for the wind to lift up the vent, might have been worth a try. (Too late for me...)

For me it wasn't an air leak problem, as might be solved by wrapping plastic over the vent. It was a problem caused by the wind actually picking up the vent and dropping it back down, over and over, with each good breeze.

The vent door sits on the outside of the opening in the roof, so no internal or external lip as you suggested. I suspect the HFGH vents are not very well thought-through (compared to other brands of greenhouses.)