I am currently reconstructing an 1880s rock house that was 80 percent leveled when I bought my farm in 1983. The walls, windows and roof are in place and I had a contractor install a pier and beam structure with joists 16 inches on center. The subfloor material is 3/4 inch T&G Advantech. The ball is now in my court to put down a 3/4 inch red oak hardwood floor. My problem is I realized the flooring is going to run parallel to the joists so I will be adding another 1/2 inch of subflooring to make this work.

This is all being done in a building that will not have AC and the heating will be a wood stove, only when needed. Average temps here in South Texas are now in the high 90s.
I have several questions:
1). OSB or plywood for the second layer on the subfloor? I've seen posts pretty much saying this doesn't matter. But, if plywood, what grade?
2). My understanding is whatever I use should be screwed (and not glued according to an Advantech inquiry) to the existing subfloor, but not to the joists. Agree?
3). Does the second layer of the subfloor need to be acclimated before securing to the first?
4). What length screws should I use when adding the 1/2 inch layer of subfloor? Total width will be 1 1/4 inch. I ask because I've seen posts saying the screws should not go through the bottom of the OSB because it weakens the bond.
5). Should the cleats I use for the hardwood floor go all the way through the subfloor?

I have the feeling if I just go for it with CDX plywood, 1 1/2 inch screws for the subfloor, 24 hours acclimation for the subfloor and use a moisture meter and 2-inch cleats for the flooring I'll be fine. But, this is my first dance with hardwood flooring and I'm sure there are gotchas I'm not aware of.

Thanks for any help.

Be sure to check the sub-floor for flatness with a short and long straight-edge. A four,six and ten footer will be good. A six footer should be within 3/16ths and a ten footer should be within a quarter inch. The four footer, if it is made of aluminum will help mark the high spots as the aluminum will rub off while sliding across a high area. An edger with 12 or 16 grit floor paper will flatten the floor after you set the nails with a quarter-inch punch. You can also use an electric hand plane, but hitting a nail will chip the blades.
Build up the low areas with roll roofing or three tab asphalt shingles before you add the extra layer of half inch ply. Check for flatness after you fasten the new layer down with sub-floor adhesive and nailing schedule of 6" on edges and 8" in the field. You can glue the asphalt paper down with latex sub-floor adhesive and then use a hammer tacker to keep the paper in place, or just glue where there are natural gaps between pieces of paper. You will be able to figure where you will need glue as you go. You will want to glue the two pieces of plywood together to get the full value of adding another layer of plywood. If you are using a bostich stapler for fastening the flooring, take the base off and use it to fasten the plywood together with 2 inch staples into the floor joists.

Thanks, Pete.
The team that put in the pier and beam with the Advantech was great. The floor is super flat and extremely solid. I have checked it all over and watched them check it as they did the work. I just wish I had discussed which way I was going to lay the floor when they did it. Then, I wouldn't be adding the extra half inch to enable me to lay the flooring parallel to the joists. I did not know the flooring should be perpendicular to the joists when they were doing the work.

I don't think that a second layer of plywood is necessary every time that flooring is laid parallel to joists. Some sub-floors are only half inch plywood, which was OK in the past. Now ¾ is the norm. If it makes you feel better, then add the extra half inch underlayment. I think that a lot depends on what kind of floor you are installing. I think that a super solid floor is not as comfortable as a floor that will give a little when it is walked on. In other words some deflection is OK in my opinion.