I am trying to avoid the plywood subfloor. It will be nailed down to the existing hardwood. Is there anything else I can use as an underlayment?

Perhaps we're having a terminology problem, again. It helps if the posters read the stickys. But at any rate, when one says underlayment, it can mean many things. Usually, it means some form of a manufactured wood panel in a 4 x 8 size, but sometimes in a 3 x 5 or 3 x 4 or whatever. I think in your case, you are asking what type of paper product to install between the existing hardwood and the new engineered hardwood. I would use either Fortifiber Aquabar B or Fortifiber HD-15.

http://www.fortifiber.com/aquabar_b.html

http://www.fortifiber.com/hwd-15.html

As mentioned earlier, to make your project go smoothly:

* Make sure the existing wood floor is flat. Re-nail any loose places.
* Install the new floor at either 45 or 90 degrees to the existing floor, IF you are not going to use a plywood underlayment OVER the old hardwood floor.
* Use the Fortifiber paper of your choice between the two floors.
* Follow all the instructions that come with the flooring to the letter.

Perhaps we're having a terminology problem, again. It helps if the posters read the stickys. But at any rate, when one says underlayment, it can mean many things. Usually, it means some form of a manufactured wood panel in a 4 x 8 size, but sometimes in a 3 x 5 or 3 x 4 or whatever. I think in your case, you are asking what type of paper product to install between the existing hardwood and the new engineered hardwood. I would use either Fortifiber Aquabar B or Fortifiber HD-15.

http://www.fortifiber.com/aquabar_b.html

http://www.fortifiber.com/hwd-15.html

As mentioned earlier, to make your project go smoothly:

* Make sure the existing wood floor is flat. Re-nail any loose places.
* Install the new floor at either 45 or 90 degrees to the existing floor, IF you are not going to use a plywood underlayment OVER the old hardwood floor.
* Use the Fortifiber paper of your choice between the two floors.
* Follow all the instructions that come with the flooring to the letter.