OSB is not a "good" subfloor material for hardwood. The "good" professionals won't go anywhere NEAR OSB under hardwood. You need SOMETHING for the second layer (called "underlay" for a reason). You cannot go over the 1/2" substrate (the thing sitting on your joists). You NEED the 2nd layer. Good side plywood is your "go to" product for hardwood....and many, many other floors.

The 2nd layer is there for 2 reasons: rigidity for the hardwood; and something for your glue/nails/cleats/staples to grab on to. Carpet is allowed a lot of cr@p underneath it...so don't go on what was there. OSB does not like glue, nor nails, nor staples. It will "work" with cleats and screws...but not as well as plywood.

Spend the extra money for the plywood. You will be screwing the plywood into place as per National Wood Flooring Association Guidelines (you can purchase the down load for $$$...or see if someone will send you the spes for "subfloor" building).

There is no glue between the two layers of subfloor...just screws. You do NOT screw the second layer to the joists. You have to "miss" the joists...so it is best to figure out how to document (a chalk marke on the wall where the joists are centred will work) where they are.

You do not mention how you plan on installing the 3/4" hardwood. You do not mention if it is engineered or solid. You will also want to look at what you are putting in between the plywood and the hardwood. Again, this depends on whether this is a glue down or a nail/cleat/staple in place. Are you working with craft paper? Or did you plan to go straight over the plywood?

Have you checked your HVAC system to ensure it can handle the humidity requirements of your new hardwood? Do you have the ability to adjust humidity up and down or to "balance" it around 45% (ideal = 45%-65%...but MANY hardwood floors have their own requirements...and you must STICK TO THEM or you lose warranty)?

So many things that need to be dealt with before you lay down the hardwood. So many things to learn/be taught. Are you doing this yourself or have you hired someone to do this for you?

If you have the choice go with the plywood. Wayerhauser gold osb is a decent quality. Most big box stores sell econo/Chinese quality osb. Yes you will need a second layer over the original 1/2"(that actually seems too thin-do you mean 5/8"?). Solid or engineered,species is also a consideration. For example you don't install Jatoba or hickory where it's bone dry.. Please do PL400 and screw the ulay after first making sure the original subfloor is sound.

Thanks for the recommendations. I'll look into adding plywood onto what I have. The existing subfloor could very well be 5/8"...I'm not sure how to tell. I definitely did not realize I wasn't supposed to screw the new layer into the joists.

I am hoping to install 3/4" prefinished solid, so it will either be nails or staples (I don't yet really know why you use one or the other). Here is the floor I'm hoping to use: http://www.lowes.com/pd_186617-88302-17057_1z11oh9Z1z11ajwZ1z0ymadZ1z0yoqnZ1z0ykwoZ1z0ymacZ1z0yl4rZ1z11ajwZ1z0wx61Z1z0wx6jZ1z0wx6cZ1z0wx6bZ1z0wx66__?productId=3121769&pl=1

What material would be appropriate to go between the new plywood and the flooring? I've seen some simple papers, and some more expensive "cushy" pad like options. The price difference is substantial. I don't want to pay for something if I'm just ruining it by nailing through it. Some have said that what you choose can make a big difference in the sound of the floor.

I don't think our heating system can have any control over humidity. It's about 30 years old, so it's just a standard heat pump. Hadn't heard mention of that before.

I am hoping to do as much of the work as I can by myself, but this is my first flooring project (except tile). I may have to hire someone to help with transitions.

Thanks!!

Everything you need to know about your floor is listed on the Lowe's website. Primary form of installation = staples; Secondary form of install = staples. I guess you are stapling.

Type of Underlayment = Plywood only. I guess you are working with plywood. You can find this information by clicking on "specifications" off of the link you sent us.

As for subfloor prep and product that goes underneath the hardwood (craft paper vs. underpad) you will need to contact Mullican directly to get a full set of installation instructions. That includes type of staples and what the staple gun needs to be set at to achieve a proper install (not too loose, not too tight...just right).

Go out and buy a $10 hygrometer from Lowes/HD so that you can start taking humidity readings throughout the home...especially in the areas where the hardwood is going to sit. This simple purchase could tell you if you are "OK" for humidity...or if you have to spend $$$$ to upgrade your HVAC system.

$10 means you can buy 3 of them...I would say it is the best "pre-installation investment" you can make. Without humidity knowledge you cannot correct humidity issues. Ten bucks...go pick up a few of them. Start taking readings for the next 2 weeks. Document, Document, Document. Your warranty depends on it.