First of all, you still haven't specified whether your floor is "not flat" or "not level." Whatever else you have (or have not) done to get where you are, it would help a lot if you could say whether your problem is level or flat. Also, just to be sure here, if I am understanding you correctly, you installed the 2x4s ACROSS the 2x6s that make up the underside of the house? (Notice the very very NON-technical terms...so be aware that I'm not a builder or knowledgeable in this stuff like some of these people that post here...just sympathetic to rising note of panic in your posts....) The only reason I'm trying to clarify that is because I am wondering whether you installed the 2x4s "upright" (so the "4" dimension is vertical) or "laying down". I'm not trying to pile on to what you have *already* done, but I don't know what the proper distance for SPANNING that 2x4s have, and I don't know whether they can span two full feet if they are flat. This makes a difference because you want to install tile. If your subfloor is bouncy or bends ("deflects" I think is it), then all the tile you install will just crack and plague you and your heirs until the end of time...until you rip the whole floor and subfloor out again and re-do it. Or put in linoleum. Whichever comes first. You say in the top post that the cracks are like 1/4", but in your last post you write that they are up to 1". That's a big difference? Which is it? Does it vary from 1/4"-1"? Have you actually measure or are just eyeballing or going from memory? IF YOUR CRACKS ARE VERY SMALL: IF it were me (and it isn't), I'd probably just put down a very tight layer of Wedi board (thinset and screws/washers), seal the seams so it can hold the next layer, then put down a thin layer of self-leveling compound (though you still haven't said whether your floor is wavy or slanted), and then tile. IF YOUR CRACKS ARE 3/4'-1": Maybe use a layer of plywood (which would be cheaper but harder to cut), maybe tape and thinset the seams (ask someone on this board about this because I'm just brainstorming here), then use the SLC, then the tile. Only plywood will actually help form a floor (which I think you need to do with 1" gaps), not backerboard of any kind. Instead of Wedi, you could use probably any backer board (ask someone here what to do for the seams). But I only know about Wedi because that is what we went with, and I like it because it is light weight (good if you are a female working alone). I don't think your solution is in one step. It feels like you are going to have to create a tight subfloor, THEN level or flatten. I think you have a non-standard thing going on there, and that might not be addressed so easily with just one step before tile. One more thing, just out of curiousity, WHY do you think your floor must be level? Oh yeah, and I would start making your peace with a threshold. Given how non-standard your set-up is, AND the fact that this is DIY (or at least the subfloor), I'm not sure it is realistic to think you can get away with no threshold.

I too have severe asthma and allergies. One source for good information about anything to do with a house and potential problems is the American Lung Association. (For example, after we remodeled some years ago, I had horrible reactions. After six months of increasingly desperate searching for answers, an ALA booklet for health care professionals finally alerted us to the pilot light of our gas stove, of all things.) I too have to stay away from all carpeting. We are currently building a house with all floors in tile or wood for that reason. We recently started looking at vinyl for some of the storage areas, and we have found some fantastic products. I would definitely consider them for a remodel. They are far easier to install than wood, not as costly as wood, they are as easy or even easier to maintain and keep dust-free, and they are easier to redo if down the road and with more time to act, you may want to try for wood. I would check with the ALA about off-gassing of different materials, glues, etc. to make sure your son can tolerate the installation process or how long after installation he may be vulnerable. You have my empathy.

I was interested in this, too. However, if you read the FAQ, it only claims to alter the tile's coefficient of friction when wet. It says it will not change the slipperiness of the dry tile. I am debating using some epoxy grout, and just not cleaning it up off the surface totally. Any thoughts on that harebrained idea?

John, I tried your boiling water trick and it worked! (As a bonus, all the water should help prevent asbestos fibers from flying around.) @ci-lantro, I may try your trick too to see if it pulls the tile up all in one piece. So far I'm still pouring the boiling water on the stuck-on backing that won't come off the hardwood. So John, what did you do once you pulled up all your tile? Did you feel okay about sanding your floors? We'll definitely need to sand and that's the only part of this process that worries me health-wise.