Shade Cloth

We used shade cloth purchased at a home improvement store that is used for patio covers.  (Greenhouse supply shops also carry a product that allows more light and less heat through.)  After building a frame and stapling the shade cloth to in, we put it on the roof of the greenhouse and screwed it staple side down to prevent the wind from catching the cloth.

Those with aluminum frame structures will most likely need to create anchor points either on the structure or in the ground next to the structure to keep the shade cloth on the roof.

Although the shade cloth can be suspended inside by the rafters, I suggest this as a last resort as it is a lot less efficient means of keeping the greenhouse cool.  With the shade cloth outside, less sunlight is allowed inside the greenhouse.  With the shade cloth inside, the sun is still heating up the air inside.