I too have been looking at shade cloth, the aluminum kind.
I am wondering if I can use it alone over my GH frame and what the results would be.

GHerser, Since there two HFGH's near me, one owned my my landlord and the other by his son, I have looked at them. I think it would be quite simple to make a U type arrangement of PVC pipe to place at the open end of your vents. Something to hold the shade cloth "OFF" the open vents. From what I have read, Aluminet streches (SP), but only one way. You would want to orient the s???? the right way.
In hot sunny places I don't think anything can our preform Aluminet.

Let's hear it from you guys and girls that use it.

1eyedJack and the Dawg

I don't use my roof vents as I have installed an exhaust fan and intake. However, shade cloth is durable but you can easily trim it. I'd just cut a flap for the roof vents. I don't know if the aluminized shade cloth can be easily cut though.

I don't use my roof vents anymore either, but I agree with troykd, I've seen posts mentioning just cutting flaps (in regular shadecloth) for the vents, so the vents can raise up normally.

As Jack says, Aluminet does stretch in one direction, and you can even request which way you want the stretchiness to go when you order it (thanks to birdwidow for this tip.) I believe birdwidow has it installed on the outside of her roof, and the stretchiness of the material allows the roof vents to raise up under her Aluminet without tearing it. (I think she added padding to the vent corners, to be sure it didn't snag the Aluminet, if my memory is right.) Aluminet is very tough and forgiving stuff in my opinion! Also, you can cut it, and the cool thing is, it doesn't even unravel where you cut it.

I tried to go the lazy way and I currently have mine installed hanging just inside my roof (I didn't want to battle our winds) but I think our heat will force me to figure out how to install it on the outside later this summer. Everyone says it works most efficiently that way, if you're trying to reduce heat gain.

You can order Aluminet hemmed with grommets, or you can order it raw. I'm usually looking for the cheap route, so I ordered mine raw. I've found it easy to roll over the edges and sew by hand with a big needle and nylon thread, but my hand sewn edges have not been subjected to our 50 mph wind gusts yet...so we will have to see! :-)

Mud,
I would think that you have it anchored close and tight to your GH wherre the wind could not get under it you would have no problem?

1eyedJack and the Dawg

Yes, I think that's the key, Jack. Some of our big gusts come from the West, where the open edge of the shadecloth will be, so we'll just need to give some good thought to securing that leading edge in a snug way...close and tight, as you said.

Gardenerwantabe secured his solar pool cover on the outside in a VERY secure manner, and I might take a page from his book!

Also you might want to consider rolling the edge. When I so finally buy the Aluminet I plan to attach the long edges to either wood rounds or jointed PVC so that it will enable me to spread the stress over a longer length rather on just a few grommets.

1eyedJack and the Dawg

I just hunted up a thread I remembered that had a nice way to apply shadecloth, but unfortunately the photos have been removed. Rats! Anyway, this person sewed a sleeve in the bottom of the shadecloth, and inserted a piece of electrical conduit, with glued on caps. He filled it with water for extra weight. (Seems like I've also heard of folks using PVC in the same way, and filling it with sand for weight.) Something to consider, anyway.

119 degrees? In April! What will June be like when outside temps are over 100? Tiny seedlings can't stand that.

My experience has been that once the Georgia sun gets hot, there is no cooling a small greenhouse in full sun without tremendous expense.
I'm moving everything outside and direct seeding where future plants are to grow.
Nell