Be careful of polystyrene! I was looking into using it as a cheaper alternative to HDPE, but searched its MSDS toxicity info, and came across this article:
http://www.ejnet.org/plastics/polystyrene/health.html . Acidic solutions will cause styrene to leach into the solution, which could be having an adverse affect on the plant's physiology. If the roots are able to uptake styrene through the roots, or its off-gassing vapors through the stomata, then you'd be smoking a neurotoxin.
"A 1988 survey published by the Foundation for Advancements in Science and Education also found styrene in human fatty tissue with a frequency of 100% at levels from 8 to 350 nanograms/gram (ng/g). The 350 ng/g level is one third of levels known to cause neurotoxic symptoms.
[5] determined that Styrofoam drinking leach Styrofoam into the liquids they contain. The cups apparently lose weight during the time they are at use. The studies showed that tea with lemon produced the most marked change in the weight of the foam cup.
[1]"
Granted this is talking about drinking cups, but the foam boards are polystyrene as well and we'd be seeing the same effects since we are using a far more chemically complex solution than just tea with lemon in it. It is definitely something to think about. Keep medicine medicine, not poison. This comes as a disappointment to me because I was excited by the potential to save 700 dollars in chamber materials. I think hospital bills are a bit more though, and sorry obama I am not getting health care. Earth is a far less invasive and destructive doctor than any hospital.