Yea that surprised me too on the PVC, I found that to be true on more than one web site when they mention the chemical dissimilarities will cause this problem.
http://westsidegardener.com/howto/hoophouse_faq.html

I want to go with green house grade polycarbonate next time.

The light wave reflection inside a green house is more involved than I ever knew and it was what made me go with plastic instead of a glass structure.

Interesting the north side of green house you would think tomatoes would grow mostly on the south side of the plants but they grew as well on both sides north and south yet these bushy plants were so tall and thick they shaded the ground yet the north side of plant developed as many tomatoes as all others no matter where they were located in green house.

Also the north side (outside of greenhouse) we had 10 foot high sunflowers growing and I thought it would be way too shady for suns light waves to bounce around for uniform growth yet they did well. I was thinking they might end up like a bush next to a house that tends to grow out on one side...quite amazing.

Heat was a major factor that caught me off gaurd as it was much more intense than I was prepared for so next time I will be venting with better care and planning.