I agree with the PVC pipe causing degrading of the plastic film -- I painted mine before covering it (5 years ago) and have had no issues in that respect.
As far as using the pipe insulation - a few years back I had the "brilliant" idea to do that on the steel frame of my smaller hoophouse - and the plastic film was toast within that one season... so don't do that, either.
If at all possible, I would recommend constructing your GH using a steel framework; much sturdier and more durable.
