Do I need to use a waterproof sealant in my entryway, over the backerboard and under the porcelain tile or is the thinset adequate? Sub floor is above grade, 3/4" plywood. thanks!
You will only need the backer board for your entry way. No other waterproofing needed. If it was your bathroom at the shower entrance, it would be a good idea, but since it will only be an occasional wetting from tracking in water or snow, and porcelain tile is impervious, it will handle it very well. You should seal your grout joints too, so the color remains, not getting changed by conditions over time