hi MR.Fork you seem to be new here so welcome,

as for shade cloth % we need to know what you are growing? also you don't need to cover the whole green house. just about 1/2 of the top.
is your greenhouse facing north and south or east and west?
is there anyway that you can make the sides roll up at least 3 feet ? that makes all the difference in the world and you wont have to take of the plastic in the summer...

In cooler months it would be greens, bok choy, spinach, cabbage. In summer with plastic off I thought of starting some tomatoes to hopefully go into December. I want plastic off in summer to hopefully extend life. Its set up so I can take off the top plastic relatively easily.
Greenhouse is 20x20 so its facing all four directions.
I've though maybe I'd just get enough for 1/2 first to see what the results are.

I use an shade cover (60%) made with 'Aluminet', which is a high quality metalized knitted screen. It has stood up well to both the strong sunlight & the winds that we have here on the Texas gulf coast (it is going into it's 4 yr. of service).

Have you checked to see how much the temp is lowered? That is what I'm looking for but don't know why I should get cloth with 30% or %60. I saw the price for the "Aluminet" and looked elsewhere.

I have never checked to see what the temp was before and after. reason why my shade cloth is 100 by 40.. at "40% if I remember right" Polyethylene Knitted..

don't know how much you want to spend on a shade cloth? but it will be between $100 and $200 for your app.

shade cloth store.com A aluminet '14x'24 finished $ 159.00 or Unfinished - 21' x 14' $97.00 @ 40%

I put shade cloth over the pens to my chicken coops about 3 1/2 years ago. It is now becoming brittle and has holes where chewed by mice. I doubt mice chew Aluminet. ;-) However, I suspect that Aluminet would not block the rain. I used tarps for the rooster pens and will probably buy Aluminet for my greenhouses after I finish building them.

Does rain fall through Aluminet when it is at a roof angle? How about when vertical or nearly vertical?

Rain will go through any shade cloth at any angle except maybe vertical. Aluminet is very effective but no where near as durable as black woven polypropylene. The former lasted about 3 years, the later 10+ in same application, ie, outside on top of a greenhouse. I'm still using some woven polypropylene after 30 yrs of intermitent use.

To original poster: design your greenhouse and accessories like shade cloth around the crops you grow. There are cooling systems that are more effective than shade cloth and allow more light inside. Evaporative cooling is the most effective unless you have a very humid climate. And I'm not talking misters although they are effective for crops liking humidity. A wet wall opposite exhaust fans will cool my greenhouse as much as 15F below outside temperature. Shade cloth can never cool below outside.

For greens and tomatoes 40% shade cloth would be about right. That might lower temperature 5-10F. But you still need good ventillation or it will still be much hotter inside than out.

This post was edited by fruitnut on Wed, Mar 26, 14 at 12:27

Has anyone tried using a white plastic tarp for a temporary shade cloth. I'm thinking about the ones that the big box stores usually sell for about $7 for a 10 ft X 12 ft tarp. This one is the thinnest of 3 different weights so I would think that a fair amount of light would pass through. It would be going on top of a 6 X 10 ft steel framed green house. The green house itself has a reinforced opaque plastic cover.

I got some %40 shade cloth that covered 80% of the top. I think maybe it reduced the temps on a sunny day by 10degres. Hard to tell since I've got two fans going now. I did not have near enough vent area at first and have so far 4x my original up high exhaust area and am adding 2x original low intake area.
I'm thinking to somehow put some black plastic bags under the shade cloth to get some additional cooling. Going to look at evaporative cooling next

I used landscape cloth. I just made it like a roll up shade operated with cords. I took it off as soon as weather cooled down in the fall.
Using this on the south peak, it kept the temperatures inside that same as out. This was with my vent open, no fan.
I had to sew a couple of lengths together to get the right size but that only took minutes to do.