Calculating a Raised Hearth for a QuadraFire 7100

[farmfitz]

I am ready to install a 20" x 68" x 2" thick bluestone slab for a hearth on a quadrafire 7100. This raised hearth will start at the bottom of the facia. The installation manual reads:

Non-combustible flooring on the raised hearth must be 20 in. in front of and 8 in. to either side of the fuel opening with a minimum thickness of 1 in. and (“k” value = 0.49).

Anybody got any idea how that translates into thickness measurements of Durock cement board or other masonry materials under the bluestone to combustibles? ...didn't know I needed an engineering degree to install this.

 

[Wet1]

I haven't looked into the details for your QF7100, but I noticed you mentioned it needed to have a K value = 0.49. That suggests to me that your hearth needs to have some decent insulation properties. I believe a K value of 0.49 is roughly equal to around R=2. With that said, Micore 300 is R=2.33 per inch and cement board is around R=0.52 per inch. Your 2" of bluestone is probably only around R=0.15 at most (a guess).

I'm sure this is not what you wanted to hear, but hopefully it was helpful.

 

[farmfitz]

Thanks, that did help. You sure know your R values. With some additional research, I now understand. Here's what I came up with:
Quad 7100 Hearth Extension Minimum R= 2.06

2 " Bluestone @ .08 per inch = .16
(2) 1/2 inch Micore 300 @1.03 per sheet = 2.06

Total R = 2.22

And use all non-combustible material from the bottom of the fascia to the horizontal plane that the fireplace sits on.
That should do it.