Hello I live in Houston TX where the EMC (equilibrium moisture content) is rather high 8-13% and the weather is warm year round. I am planning on nailing down 1/2" solid stranded bamboo (with hand-scrapped finish) on my upstairs which has wood subfloor. I am wondering if I can use Fortfiber #15 Felt paper as my underlayment (see link):
http://www.homedepot.com/p/Fortifiber-500-sq-ft-AQUABAR-B-Tile-Underlayment-Roll-70-195/202592567

Aquabar B is a good vapor retarder. It has asphalt sandwiched between layers of kraft paper. The literature says there might be a faint asphalt odor but I could never detect an odor.

What does the manufacturer call for?

That's the first place to look. Otherwise if there are problems even if the cause is not the underlayment many come back and say "...see it's on page five of our warranty...you didn't use our recommended underlayment."

Be careful out there, some manufacturers always wiggle their way out of responsibility. Hard to believe bamboo is still popular here and there. It's been getting ripped out of homes like crazy in south Florida. Of course Florida was very early to bamboo floors. I recall it being very popular in the mid 90's. Hand applied finish in China. You could see the brush/bristle marks in the finish.

But it's a matter of what you like best. Best wishes!

Thank you for the input, I've looked at the manufacturer recommendations, they list a whole bunch of different options (15 lb felt paper was one of them). I was leaning that way and so I wanted to see what different people recommended for underlayment given my climate and conditions.

Felt paper has been used forever. The nice thing about Aquabar B is that you will be able to draw layout lines on the kraft facing, the new flooring will slide into place easily over it, it won't mark/marr walls, it's clean to kneel and work on. If you go to their web site you can check permeability rating etc.
I do not work for the company that manufactures Aquabar B.
Home depot carries it if you want to take a look-see-feel.