In regards to using OSB, I have the following comment: I am a structural engineer in California, and I have specified OSB as a subfloor, roof sheathing & shear wall sheathing for hundreds of homes. As far as an underlayment for hardwood floors, there appears to be two issues. 1) is the floor structurally sound for hardwood, and 2) whether attaching the hardwood directly to OSB will result in nail slippage many years down the road. I have comments on both issues. As far as the floor being structurally sound, I have seen a few posts on this issue & I am not one to automatically say that hardwood should be placed perpendicular to the floor joists... But it would definitely be stronger and may help in easily repairing the hardwood attachment if the nails should slip many years down the road. I myself have placed the hardwood parallel to the floor joist because my subfloor is very stiff. To get a stiff floor, this is my recommendations: joist spacing should not exceed 16"o.c.; floor joist span should be designed conservatively (building code minimum is not conservative), and floor sheathing should be 3/4" nominal and placed with the face grain (long dimension) perpendicular to the joist & staggered. In regards to item 2: some national floor associations have said direct OSB attachment is acceptable as long as sheathing is PS2 standard & 23/32" minimum. That is all we can go by. As I mentioned above, let's say the nails do slip after many years... If your hardwood is perpendicular to the joist, couldn't you just screw it down to the joist below as a repair? That is one benefit I can see with placing the hardwood perpendicular to the joist or adding blocking between the joist. In regards to nail slip, I posted a blog regarding using the hardwood nailers that drive the cleats at an angle in the tongue of the hardwood edge. The cleats are locked in place inside the T & G connection. How is this going to lift up? Also when you drive the nail at an angle, there is a vertical bearing component of the nail that is preventing the nails from lifting. This is versus using a brad nailer that goes vertical. The vertical nail would rely totally on friction between the OSB & nail and I can see this possibly slipping down the road.

glenn, can you explain in detail a little more? Use to fill 1/8" gaps is acceptable? Are 1/8" gaps even acceptable? I have never seen a floor with big gaps like this. in one 10 x 10 area I have about 15 gaps that nickels, pennies, dimes and quarters fit in. I am sure that they will not fix it without a battle, and I can tell DH thinks we are stuck, but gee whiz, if you bought a new $30,000 car, and the dash board worked, but was crooked, and the windows worked, but didn't close all the way... you'd make them fix it. I really feel that they should have to fix this. I need to find my camera

You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

ok, this is an area where I actually know something! I sold and spec'ed hardwood for a custom shop before my mommy-life. The information you are getting here is correct. Engineered floors will give you 1-2 refinishings ON A PERFECT SUBFLOOR. One of the issues that sometimes arises if there are inconsistanies in the height of your subfloor. Because there is thinner wood wearlayer (before you get to the engineered wood) there is the chance that you can damage the floor irreparably when you sand it. Now I only saw this saw this happen twice, once on a loft job and once in an older home. The loft was more subtle as it was over concrete, the old house more obvious as they were DIYers and picked engineered flooring to try to negate their irregular subfloor (they neglected to tell us that that was why they chose engineered, we would have talked them out of it had we seen the floor first) Neither of their claims to the manufacturer were fully settled because both times they did not follow the guidelines for floor prep. The loft job was also a materials only job for us with their GC laying the floors but he insisted he knew what he was doing. A good engineered floor will not be cheaper than most of your nail down options because the production costs are higher. Now having said that, GOOD engineered floors certainly have their place in places where you can't put traditional hardwood due to height restrictions or concrete subfloors. My boss had a great engineered floor in his basement and it survived a flood remarkably well (it was ash, Kahrs brand out of Scandinavia and one of the beefier engineered floors and one of the pioneers) My Grandmother also had a Kahrs floor in her ground suite condo and she LOVED it and I was impressed by the milling. I'm considering it for our basement because it's warmer and less echo-y than laminate There has been a lot of 2nd rate engineered flooring coming out of China and being labeled for all sorts of different companies. You should be able to tell an engineered sample because the layers are sandwiched together and the tongue will be a different wood than the top (showing) layer. Distressed finishes are usually more expensive because of the extra step in manufacturing to make it distressed. That is different than just getting a lower grade of hardwood like a rustic or tavern grade (in solid hardwood). Those grades have more character but they often don't carry a warranty and generally have a higher waste percentage. Another thing to think about---if you ever have to replace part of the floor, you will always be able to replicate 3/4" thick hardwood but engineered floors are more like tile, they change and get discontinued over the years. You could go pre-finished, save the hassle of site finishing but with the thicker wear-layer a 3/4" nail down floor offers. Only you will know if you can handle beveling or not. The changes in milling the last 10 years are huge. The bevel in your better milled wood is tiny. I like Canadian mills, Model or Mirage for pre-finished. Remember that if you ever re-finish, the bevel will disappear and you will, essentially have a site finished floor. One benefit that many people don't know about? With a prefinished tongue and grooved floor you can have a good installer cut out a single board (or more) from your floor if it gets damaged which is why we did a fair bit of pre-finished in new homes since inevitably we would get call-backs at move in to repair trade damage (@#$%^ plumbers and their big wrenches!). Prefinished hardwood deals the best with shinkage and expansion issues because the finish is on individual boards. Site finished floors are more labor intensive but the materials are cheaper (apples to apples). They are what many people consider the gold standard however there are more steps and you don't always know how it's going to look until it's done. There is more chance of finish contamination and trade damage during the rest of the building process. We refinished 100 year old floors. I loved the idea that there wasn't 4 generations of carpet and crap in the landfill. Definitely a forever floor and it can usually be re-sanded for all but the very worst water damage (think entire house under water for a few days) If you are going with a wide plank, keep in mind your species and humidity level (and your ability to keep humidity stable). The wider the plank (with 3/4" thick flooring), the more obvious shrinking and expanding will be. If you take a 2" wide board and it shrinks 2%, it isn't very noticeable. If you take a 6" wide board and it shrinks 2% the gaping will be more noticeable and in the case of a site finished floor *could* pull your finish apart. You'll need to find a flooring contractor in your area with lots of experience. Most of the best hardwood shops ONLY do hardwood. Beware of carpet shops and many of the big box stores in many instances. Make sure they are accredited with the NWFA (national wood flooring association) http://www.nwfa.org/member/ good luck!