Flooring Tile - moisture barrier between backerboard and subfloor?

I am installing ceramic tile in my bathroom, and I'm preparing the floor by removing the plywood underlayment and laying 1/2" backerboard on the subfloor. The subfloor is 1 x 6 tongue-in-groove and rather worn (house built in 1942). There are open knot holes and some small gaps. Before I put down the thinset should I lay some roofing felt directly on the wood to hold the thinset and then lay the backerboard on top of that, or should I just lay the thinset on the wood directly, and the backerboard over that?

thanks,
m

I'm not a backer board pro, but I think you should replace the plywood and then install the backer board. I don't think the old floor boards are going to give you enough support. Get the little instruction booklet for the backer board you are using.