I'm right on a slab and i am above grade, but i dont trust the
concrete. It is probably a waste but for $30 for 200sq/ft.. I spend
it for piece of mind

In your first post it just seemed like you were concerned about the
price. If $30 is in your budget, go for it.

Also, I was reading that cork may crack if installed above heating
ducts in cement floor/subfoor. Cracks can install when over 84
degrees.

http://www.diyflooring.com/inst_165/installation.html

Well it doesn't say above heating ducts, it says if you have under floor
heating. From the web site:

"If the room in which the panels are to be installed has under floor
heating, the surface must not exceed 82.4 degrees. Under-floor heating
can lead to the formation of cracks in the cork during long heating
periods."

Thus if you don't have under floor heating you don't have to worry about
it. If you do, don't exceed that temperature. That's 28 degrees Celsius
and I read on another site that 26 degrees Celsius is the usual
temperature of underfloor heating.

It's possible it also depends on the cork manufacturer. If you have
underfloor heating check with the cork's manufacturer before you
purchase it to verify the maximum temperature it can withstand.

very last line on the install instructions..

Anyone ever heard of this?

c_kubie