livewireoak is 100% correct...engineered hardwood is real wood, and if your floor is being installed over a slab, you have to go with engineered as solid wood is too unstable. Flooringbro is also 100% correct in stating that a floor is only as good as the installation. We paid a lot of money to upgrade to an 5" wide engineered natural maple floor installed on a slab in our newly contructed home. The floor is beautiful, but just weeks after the install, planks starting coming up all over the place. The problem was that the subfloor was not made level prior to installation. So whatever you choose, please make sure you have good installers who properly prepare the subfloor. Our expensive upgraded floor is an excellent example of that. Good luck to you.

I really appreciate your response. The builder took a look at the floor yesterday and readily admitted that they've never encountered this degree of gapping. He further stated that it was probably due to moisture in either the oak or the subfloor during installation. I'm sure they are going to want to try to repair, but the poly is not going to blend into the rest of the floor. The raised joist runs right through the middle of the kitchen into the great room. As far as using slivers in the other gapping, can you use a sliver when the gap extends the entire length of the house? In other words - you'd have to use really long slivers. Is this doable? I'm going to see if I can find a floor inspector. I really appreciate any help. Thanks.

Many possible reasons why you are seeing gaps; wood species, moisture differential between subfloor and flooring at time of install, HVAC system in the house, new house or old...lots of variables...even the general climate of the region you live in can have an effect.

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.