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