Carol, you'd certainly like some the extra ventilation for your greenhouse in summer if you're going to keep any plants in there. Even with shade cloth over it, I'd think the hot sun would 'cook' some plants on a 100+ day.
I have a 6ft. by 9ft. Aluminet sheet that is on its 3rd year and not showing any breakdown. I've had a bad result with some shade cloth from Home Depot which I put over my pergola at the other house, and it was only a year before it started disintegrating. They gave me my money back on that one. Our sun is very strong, with clear air, and it was in full sun all the time, so a really tough test. But . . the Aluminet sure has been more worth the money I spent. It was about twice the price of the regular shade cloth.
You can buy it online in varying percentage of shade from 30% to 80%. Just google Aluminet. I got mine from Shade Cloth Store.com If you don't want to spend that much just be sure that what you buy has a guarantee and says it's UV resistant or treated.
