Creative hearth ideas needed to meet required R value

[TheTodd]

Our current fireplace hearth is 1 3/4 - 2 inch thick fieldstone installed directly on the wood subfloor, and provides nearly no insulative value. I'm looking hard at a Regency HI400 that requires an R value of 2.13. Micore 160 sounds too soft so I'm looking at 2 sheets of Micore 300 under Durock Next Gen, then mortar and porcelain floor tile. Assuming the floor tile is 1/2-3/4 inch like most, the hearth would be 1/4 - 1/2 inch too high and not flush with the bottom of the firebox. I'm not seeing any combination of materials that gives the appropriate R value and stays under 2 inches total. Any creative ideas?

The fireplace itself is a metal Heatform unit, so I'm not sure if I'm allowed to add another inch of firebrick to the floor to raise it up and give me more room to play with. The metal comes down to just above the existing floor, so new floor would have to be mortared up against it, or have the metal cut an inch higher which might reveal more surprises. Could the existing floor even be cleaned enough after 30 years of fires for the new mortar to adhere to? This whole option seems questionable to me.

Other worse ideas:

• Use thin porcelain wall tile instead of thicker floor tile, which we probably break while installing the insert
• Go with one layer of Micore 300, lie to the insurance company, and hope the adjuster doesn't notice after I burn my house down.
• Live with the hearth being slightly higher than the firebox, and hope the insurance company doesn't notice, the installer doesn't care, and that the angle doesn't affect performance of the unit
• Just pick a different f-ing insert...

I've spent a few weeks reading through lots of threads here already, which has been very valuable in getting this far. Thanks for your help.

 

[begreen]

There's a lot of good 1/4 and 3/8" tile on the market, porcelain is tough. 1" of Micore 300 is R=2.06. Put a layer of 1/2" Durock NexGen (R=.39) on it and you'll be fine for R value.