Base for tile floor
I have a bathroom to tile, and right now the floor is 3/4″ OSB with a layer of latex paint. I’m not sure what I need to put down before the tile. Backerboard? Uncoupling membrane? Both? If I were to go with just uncoupling membrane, would the thinset mortar adhere to the paint (and does it really matter)? The subfloor is over 12″ OC joists, so I’m not too worried about rigidity.
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Whatever you end up using is acting to decouple the tile from the wood subfloor to avoid cracking issues. You can use something like hardibacker, which is a type of cement board, or something like Schluter Ditra, which is a membrane. You need one or the other, but not both. Either requires thinset between the subfloor and the board/membrane to make sure everything is evenly supported. BE SURE to use the CORRECT type of thinset (modified or not) that the manufacturer specifies!
My general rule is that "small" projects get hardibacker. I just think it's easier. "Large" projects get ditra. I seem to recall you mentioning you want to put in some in-floor heat, and if that's the case, the ditra system is AWESOME because the heating cable can be woven between the cells of the membrane to lock it in place. The membranes are polyethylene, which is MUCH easier to cut than the cement board materials, and you don't have all the silica dust issues with the membranes, either.
For all tile related advice I'd go to John Bridges: https://www.johnbridge.com/vbulletin/
If I remember right, from another post, your tile floor extends into the shower. If that's right, I know a lot of people, pros and non-pros, like the full line of Schluter products--membranes, prefab corner pieces, drains, pipe seals, etc. It always looked slick to me, but I've never had a chance to use it myself. The feedback I have heard is that it really simplifies a lot of the details.
Fine Homebuilding has covered projects using these projects quite a bit, if you're looking for some step-by-step help.
I use Schluter products on my tiled shower. I have no regrets and would look to them again. 15 years and no problems.
Tile Council of North America (TCNA).
Buy the TCNA handbook. That tells you everything you need to know from between joist deflection to the codes you need to look for on the bag of thinset to waterproofing methods.
Beyond that, the John Bridge forum is great, but is not a substitute for the handbook.
Thank you for that info about the handbook!