Im building a hearth for a Quad 4300 ST and the Installation instructions for floor prtection is stated as ” For US installations, it is necessary to install a floor protector
of a minimum of 3/8 inch (9.5mm) thick metal clad millboard
or equivalent a minimum of 16 inches (406mm) in front of
glass and 8 inches (203mm) to both sides of the fuel loading
door. ”
Im interested in the equivalent as I cant find metal clad millboard anywhere around here. Im hopeing that 2-  1/2” layers of Hardibacker over 2x4 metal studs over a 1/2” layer of Hardi will do what I need. All of this will be capped with 3” non combustable cultured stone. I am aware that the front protection now needs to be 18”. Any thoughts would be helpfull. Thanks guys and gals.

I just recently discussed this with the wood product specialist over at Quad Technical. He told me any non combustible material at least 3/8” thick will work. That seems a little off to me but that’s what he said. I should request an official letter stating this, then they will have to actually make sure its OK by the engineers. The metal clad part he said is in there just to make a solid continuous surface so no embers could possibly pass through it.

Hardibacker I think is non-combustible, but the MFG website recommends NOT using it for fireplaces. There are a couple of threads about it in the last couple months. Most people suggest to use durock instead because it is cement board.

I’ve never heard of the metal clad millboard either but can tell you that the planned layers will give you extremely good insulative value from the charts that I have seen. I used the metal studs, 2 layers of 1/2” durock and then ceramic tile. The air gap created by the studs was the biggest value in R factor.

He is wrong to say ANY non-combustible, as there are big differences between various materials…..but, your setup will do vastly more in terms of R-value than this. In fact, I would say that you could do a simpler job by building it from wood (frame, and plywood top), using perhaps three layers of dura-rock or wonderboard, and then the cultured stone - make certain the stone is completely high temp, as it is possible some products are made from plastics, etc.

Well I already have the materials so I plan to build as stated. The cultured stone is Owens Corning and non combustable according to their web site and printed material. So based on this info Ill proceed as planed.