You got a lot of desert up there in Richmond? :p
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I had my BC tarp up last weekend in the backyard in full sun. I could definately tell there was a lot of heat radiating down under the tarp.
I saw somewhere a material that was ripstop nylon, I don't remember if it was coated or not, but it was white/silver on one side, and black on the other side. the black would be used as the lower side, with the white/silver on the upper durring the summer. The idea behind this was to reflect the sun's radiant heat while blocking the light from comming through the tarp, leaving you with a darker shadow.
I don't remember where it was that I saw this material.
Here it is.
http://www.seattlefabrics.com/nylons...z%20Reflective
A bit on the pricey side though.