R vs K value question. I'm puzzled

[jocam]

This site is great! I could read these forums all day and never get bored. I want to quit my job.
I have a question.

I have a Morso 5660 installed with my fireplace face reworked. I printed off the manufacturer's installation manual months ago. At that time, the hearth width requirement was 18 inches in front of the door with no instructions about fire resistance.
I have not yet completed the hearth because of other priorities for this reno we just did.
Now, I rechecked with Morso on line and the installation requirements have changed and become much more stringent and detailed. If the bottom of the door glass is 12.5 inches or higher, the hearth must extend 25 inches in front of the door. The hearth material must be 1.5 inches thick with a K-value of 0.84 (BTU/IN FT2 HR F).
My hearth is solid concrete for 10 inches out then hardwood floor. I can remove the hardwood and subfloor down to the joists to rebuild the hearth with a plan for a tile surface. I have read about Micore and cement board from other posts but have a few questions.
1- If I need 1.5 inches of fiber board with a K value of 0.84, is 0.75 inches of fiber board with a K value of 0.43 just as good? This would save some height.
2- How do I calculate the R-value? Do I need to?
3- What is the order of installation? Micore- cement board-tile?
4- Who comes up with these scientific recommendations? I have checked with several fireplace distributors and everyone is mystified by these requirements. Especialy the K factor.

I don't want to burn my house down, but I don't want a hearth that sits 4 inches off the floor when the original intention was to be flush.

 

[Delta-T]

well, i think if you are going to be working with multiple materials you may want to convert everything to R factor, as they can be added together, K values cannot.
R=1/k x inches.

is that .84 the actual K or the K per inch of that material?

I dunno who comes up with this stuff, but I'm willing to bet I wouldn't like his jokes at a party.

 

[Delta-T]

I could be calculating thing wrong, but i think you'd need .767857" of fiberboard with K of .43

can anyone confirm or just point and laugh at me?

 

[vvvv]

well, i think if you are going to be working with multiple materials you may want to convert everything to R factor, as they can be added together, K values cannot.
R=1/k x inches.

is that .84 the actual K or the K per inch of that material?

I dunno who comes up with this stuff, but I'm willing to bet I wouldn't like his jokes at a party.

thermal conductivity K vs resistance R?

 

[summit]

k and r are the same only different... k is conductivity of heat, r is resistance to it. either way you compute it, the needed material will be the same.

 

[jocam]

I could be calculating thing wrong, but i think you'd need .767857" of fiberboard with K of .43

can anyone confirm or just point and laugh at me?

That is what I thought too. I was looking for a confirmatory response. This thickness would make the hearth height closer to the floor level once I add cement board and tile.

Can anyone confirm this calculation with authority?