I see the use of roofing felt mentioned alot for leveling purposes when used over various wood subfloors. My question is can it be used to level lightweight concrete that will have a engineered wood floated over it?

I am also thinking of using this product called redupax+ that is a rigid sheet designed to go under click together floors. Has anyone heard of it and have an opinion?

http://redupax.com/

Yes....
No, and No.