I don't know but you might want to post your question on the Greenhouse and Garden Structures Forum. Perhaps they can give you some ideas about the vagaries of this stuff.

Nancy the nancedar

Here is a link that might be useful: Greenhouse & Structures Forum GW

somebody posted on this forum last year with a whole bunch of references that you could get the twin-wall locally. let me dig and see if i can find it. i remember because we are considering using it as well. no discussion of what kind to use, and i don't know, either, so i'll be curious what you find out.

here it is:

Here is a link that might be useful: twin-wall

There is also www.FarmTek.com - they have twin-wall panels made by Palram, custom @$5.95 per 4' sheet and 4x8' sheets @ $41.95. Shipping seems to be $7.00 per linear ft per package up to 12' - to a maximum no. of sheets per package of 50. They also have a roll of twin-wall corrugated plastic: 8' wide; each @ $7.39 per linear ft, and a 250' roll @$6.95/linear ft. They don't say what the shipping/freight charges on that would be. You can also call them - 1-800-327-6835. While I am not sure how much they would be price-wise compared to local sources, their prices for the water filters I have bought from them seems to be competitive ($5-7 against what I priced at Lowe's a few years ago at over $15!) - they have stuff for agricultural growing and storage, so they carry everything that can come under that heading - the specialty catalogue I just got, for that only, runs to 352 pages.