Crap. After calling every tile store in town, and also Lowes and HD an hour away, none of them carried or had heard of "next gen" durock. I looked up the R value on the USG/Durock website and it listed the R value of 0.39 for "Durock brand cement board". I've read elsewhere that regular Durock is only R 0.2. Not sure where that came from though. Here's the product info from USG. R value listed on page 3 under "technical data".R=.39 is for Durcok NexGen. Screw down the second layer too. The mortar between the Durock layers is not necessary.