local home store has two bags of 50lb. thinset mix. one is called a "contractor's blend" at 5 bucks a bag, the other said premium blend, at 15 bucks a bag. I wonder what the big difference is? I need to lay a 1/4" of thinset down under the hardi-board for a tile project and I'm thinking I can go with the cheaper stuff??
Cheap stuff should be fine. You'll probably find the more expensive stuff has additives to improve adhesion and workability for tough materials like porcelain tile. Sticking backer board down on the floor doesn't call for anything special unless you are going over a tough substrate like old tile or vinyl.
The aim is a very flat very stiff floor so there is no movement that can crack the tile or joints between. By using thinset under the backer board, the underlayment (plywood) and backer board become one solid assembly that is stiffer than it would be if you just screwed the backerboard down. Tile is pretty unforgiving in terms of movement of the subfloor; the stiffer the better. The thinset may be overkill for one situation and not for another, depending on the floor framing, whether or not the plywood is glued to the joists, etc. I assume they recommend a notched trowel, so you don't end up with 1/4 of thinset under the board, more like 1/8 once it spreads out. This is enought to bond the board and fill minor irregularities. Given the cost and amount of time it takes to do a nice tile job, it's pretty cheap insurance.
Good luck,
Paul
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