Is thinset a must between sheets of Durock

You'll end up with a structure of higher integrity if you go ahead and apply a thinset layer using a serrated trowel, and then screwing the "sandwich" together. It's standard practice. To make it simple, there are tubs of pre-mixed thinset available ready to use off the shelf. Very convenient for small jobs. Rick

Yes you should apply some thinset between the layers.

But a bigger question is just how much do you need to build up your R-value? you only get ~0.20 per 1/2" sheet. I had to go to R1.2 for the Heritage and really didn't like the thought of using 6 sheets stacked up to do it. Steel studs are cheap and easy to work with (wear gloves!) and mortar like Versabond or Flexbond is really easy to mix up and use, even in small quantities. I had great luck using an old dishwasher detergent bucket to mix up little batches. Just used the kitchen or outdoor tap and a mixer rod on a power drill.

Edit: pix attached - I support a 500 lb Heritage on this, despite only a single 1/2" layer of Durock over the steel box sections. Highbeam did something similar but he made much taller airspace and spaced wider. Not sure how he supports the tile/stone he used.

Yeah 0.6 ain't much to do. You're good. Tile is about 0.01 tho - no effective thermal insulation whatsoever.

BTW with only 3 sheets you definitely want thinset - it'll crunch forever without it.

I used the same steel studs as Edthedawg except put them on their sides to support two layers of durock and provide an air space beneath them. The first layer was too flexy by itself so I used a second layer with thinset between them and screwed the sheets together. Durock is cheap and it's not like you'll be reusing it. Thinset is also cheap and easy to mix up. sticking the layers together with continuous thinset layer makes for a laminate that is a single piece. Without the continuous thinset you'll have a stack of individual sheets. You'll need the thinset mixed up anyway for the butt joints.

A 3/4" thick sheet of durock will be heavy. The 1/2" stuff is 96 lbs per 3x5 sheet.

Do you have to use thinset between? No, the durock provides the R-value and not the thinset. You CAN just screw them together.

If you don't put thinset between the layers of Durock, make sure you put screws every six inches or so.

-SF