Hi,
I am located in zone 4a in the Mid-Atlantic.  I am doing a basement renovation that involves pouring a new 5″ slab (~600 sqft) after gutting a previously framed floor that was destroyed by termites over the last 90+ years.  (Floor joists were laid directly in the dirt when the house was built in 1930.)  The basement is a walkout although the house is on a hill so the floor is partially at grade or slightly above at the downhill side and below grade uphill.  Perimeter stem walls have been poured where there were originally concrete piers on the downhill side so the entire slab pour will be enclosed.  

I have set down comfortboard 80 vertically along the interior of the stem walls down to the footers, a crushed gravel base and passive radon mitigation pipes and am now focused on horizontal insulation.  My plan was to use 3 inches of comfortboard 80 horizontally as well because the aforementioned termite issue makes me reluctant to use foam even though it is cheaper.  I did try to source borate treated eps foam but had no luck ordering less than a truckload in my area. 

I am planning on having radiant pex installed in the slab, which I will tie to rebar mesh that will be elevated on chairs/bolsters.   Even with thick stego wrap as a vapor barrier on top of the insulation I am concerned, having had some firsthand experience with it now, that the comfortboard 80 could be damaged/vapor barrier punctured during the pour with foot traffic weight concentrated on the chairs/bolsters.  At the very least the give of the insulation seems like it could make walking on the elevated mesh more difficult for the installers.

I did price out the higher strength commercial grade comfortboard 110  as an alternative, but it is significantly more expensive ($1200+), partly because I can only get it in larger skid amounts of 240 sqft each, rather than 24 sqft bags like the comfortboard 80. Also, according to the specs the compressive strength is only ~30% greater (439psf vs 584psf @10% compression).

Today, while considering my options I thought of an idea to put down a layer of 1/2″ durok as a protective layer on top of the Rockwool and under the stego wrap.  This would be considerably cheaper than buying the higher density comfortboard, and probably much more durable during the pour.  My gravel base is currently a little short so adding the extra 1/2″ of material would not negatively affect the target slab thickness.

Can anyone think of a reason why this would be a bad solution?  Or perhaps have a better one?  

Thanks in advance,
John