I heard of ThermalSpan years ago. Others might know of it as well. I had enough polyspan on hand so I never thought about using ThermalSpan. After running low on polyspan and buying more of it I decided to look for the ThermalSpan. I don't know if it is that much better than polyspan but it might have qualities that some prefer.
It is non-directional and it appears to have a slightly denser weave than polyspan. Maybe it will take less dope to fill. It is supposed to go around curved areas much better than polyspan but I can't attest to that yet. I have just started my latest project so it will be a while before I get to try the ThermalSpan.
The package I bought is 14" width X 16' length. $13.60 plus $2.50 postage.
If anyone is interested, it is available from:
Harris Design
5835 Griffith Drive
Brighton, MI 48116
810-599-7319
harrissd@comcast.net