One of the members here, DrDawg (Ken Ramsay), used solar pool covers over his greenhouses in Mississippi; even if you still needed to use a heater, it wouldn't have to work nearly as hard. Ken is active on the orchids forum, you could send him a tree-mail if you want to ask him about it.
PVC pipe can also be painted to eliminate the issue of breaking down the plastic. One thing I learned the hard way is that using the foam pipe wraps is a very bad idea - the plastic self destructs in less than a season when in contact with those things.
