I'm an all around carpenter with some successful floor installations in my quiver. I'm looking at a job where the floor was more or less leveled with 2x4's 24" on center. "Bubble wrap" insulation was laid between the sleepers, radiant tubing was fastened down and concrete was poured and screeded flush with the sleepers. The concrete finish is not particularly smooth and there are high and low spots. The GC wants 3/4" walnut 4" wide nailed to the sleepers with no underlayment except 15# felt. I'm concerned about flexing and squeaking as well as possible loosening of the boards. I'm wondering if the thin foam underlayment for floating floors would help in this case. This is a remodel of a funky old adobe house in Northern New Mexico and some wackiness is acceptable but really substandard work isn't. Any Ideas?

Well, 24" OC is too far apart to install 3/4" solid wood planking over. That would mean you would only be able to fasten the floor every 24". And the nailing schedule for 3/4" T&G solid plank is every 8". Usually, only strip flooring can be installed directly over sleepers, with those often set 12" OC, and 16" OC max. So, the floor will fail. Not only that, but a solid plank directly over radiant tubes is also a pretty bad idea. I'm afraid a stout subfloor will be needed to support ANY kind of wood floor. I'd recommend a T&G 1&1/8" subfloor panel. That will spec out fine for 24" OC. You will need a type of vapor barrier as well under the plywood subfloor. Fortunately, American Walnut is a fairly stable wood that moves less than oak. But I'd still expect some expansion and contraction over radiant heating. Gluing as well as blind nailing may restrict this movement more than nailing only.