We have torn up the tile in the kitchen to put down Brazilian Koa hardwoods. In the process I came to find out the previous owner nailed and mortered the hardibacker to the wood sub-floor. Can we use an 1/8 inch foam type pad under the Koa wood verses using a felt paper to compensate for the uneveness? There is about 300 square feet of removed tile with patches of morter/thinset.
There are several types of underlayments that are available for use in hardwood floor installations. You can check them out at this web site: http://www.hardwoodinstaller.com/hardwoodinstaller/reviews-flooring-underlayment.htm. We have used the foam type underlayment you have described for use over concrete floors in multi-level housing situations. They have performed well.
If it’s really uneven, you might consider using a ¼” underlayment board, that you glue and nail to the surface, then install the hardwood over it.