Heater tube insulation

[stans4@ix.netcom.com (Stan Schaefer)]

I managed to get ahead of the weather for once and got my old rusty,
crusty heater box-to-body tubes replaced before snowfall.  The manual
shows some insulation material around them, I'd like to replace it.
Any ideas for some suitable material that wouldn't soak up too much
water( or rot)?  I managed to get some stainless steel spiral wrapped
2 3/8" flexible exhaust pipe that I cut to length.  This is a tight
fit, but shouldn't give any more problems from coming unwrapped or
rotting out.  Any and all ideas for replacment material cheerfully
considered.