We are in the middle of updating our guest bathroom and while removing the vinyl flooring we found that it was sitting on a 1/2" particle board underlayment. Given that particle board in a wet environment is a no-no and we are looking to put down a tile floor, we pulled up the particle board underlayment (staples, galore!). My question is, I now have about a 1" gap from the plywood subfloor and the start of the wall (as well as the carpet in the hall) and am debating on how to best build up the floor height. We did take off the baseboard trim around the floor and will be putting that back in place so we don't need to be completely flush with the wall. I am interested in the Ditra underlayment, but given it's low profile (1/8"), I am thinking that I need to put down something between the Ditra and the subfloor. Would another layer of plywood be the best bet? I suppose I could also go with the Ditra XL, but that is still less than a 1/2" thick. My other option is to use 1/2" backer board, but I don't love the thought of hauling and cutting the board when Ditra is so much easier to work with.
Thanks for any insight provided!