I have a PDF with a comparison of most of the coverings available. It's on our company website though and Gardenweb doesn't like to see links to company websites, so just let me know where to send it and I'll email it.

It's not the GW dosn't like links to companies. It's that
greenhousemegastore is a company and you can not advertise your own. Or make it look like you are pushing a certain company or product.
I think your GW name implies an advertisment.

Greenhouse Covering Insulation (R) Values
4 mil polyethylene 0.83 4 mm twinwall polycarbonate 1.43
6 mil polyethylene 0.87 6 mm twinwall polycarbonate 1.54
6 mil poly double layer (inflated) 1.43 8 mm twinwall polycarbonate 1.61
11 mil woven polyethylene 0.95 10mm twinwall polycarbonate 1.89
3 mm glass (single layer) 0.95 16 mm 5 wall polycarbonate 3.03
Two layers of glass (insulated) 2.00 Fiberglass or polycarbonate (single layer) 0.83
R value is a commercial unit used to measure the effectiveness of thermal insulation. A larger number represents a higher insulation value and therefore greater heating and cooling efficiency.
Specific, brand-name, product R values may vary slightly from these figures.
Link below has better graphics.

Here is a link that might be useful: covering comparsions

Hi Poppa
What criteria are you using for comparison?

Its worth remembering the R-values will be modified depending on the orientation. Panels installed vertically will have a somewhat lower R-value than horizontal ones ;)

R-value is not an issue (or should not be) as I am building a Solaroof style GH. (A double roof where the cavity gets filled with foam/bubbles at night for insulation. The cavity varies between 24" to 36")

In the past i had seen coverings made of the various plastics... and i wonder if anything new has come out. Tefzel would have been my favorite if i could find it in rolls big enough and if i could afford it.

Light tranmission would be a biggie because i want to try growing through the winter in new england. Obviously i want the longest lasting material i can get. Need to think about the IR and Anti-condensation properties. Any other new additives that enhance the film properties?

I'll probably start going to various websites and building a geneic spreadsheet, and see where that takes me.

Poppa