We use Phifer SunTex 80 and 90 exterior shading frabric.

HEAVY-DUTY SHADING FABRIC The diagram shows a window with an Apple Custom Screen installed using SunTex shading fabric. In midsummer as much as 230 BTUs (British Thermal Units) of heat energy can fall on each unprotected square foot of glass.
With the SunTex 80 or 90 exterior shading fabric used in your custom screens, up to 80% or 90% of the sun's energy is reflected, absorbed, and dissipated before it strikes the window.
Available in a variety of colors.