njkev said:
Elfin...I was mistaken about it being "marble". It is really polished "granite". The product is called "Stone Fusion" by Armstrong, and sold (just recently) at Lowes. The stuff was listed at a EI rating of 5 (at Lowes). The Armstrong web-site has NOTHING about this product.
Now...about my extended hearth. BTW....my Heatilator Constitution fireplace is raised 8 3/4. I intend to raise a 24"-deep extended hearth roughly 8". I had planned to use two sheets of 1/2" Durock, and 1/2" stone/ceramic tile over a 2"x6" frame. However, the manual states that I require a K-value of 0.43! The raised extended hearth will be 11" below the bottom of the firebox door. In addition, I have two 160cfm fans sucking room air below the door at 9" above the proposed extended hearth. I placed a "temporary" structure of block and marble slab at the proposed height for my permanent hearth as a test. I had that fireplace burning nice & hot for over 18 hours, and that marble slab was COOL to the touch!! Why specify such a low K-value (0.43) if the extended hearth doesn't get hot??? I think I would notice a pile of red-hot coals or a flaming log sitting on my extended hearth! If I read the R - K Value Chart correctly on this site, the 11" of AIR should give me an EXTREMELY low K-value! Someone please enlighten me! Are the Manufacturer specs. overkill?? I understand the 1" of non-combustible requirement....BUT not the K-value for my application.
thanks,
kev
If it's really polished (natural) granite, watch it carefully. Granite is mostly an amalgamation of dark, light and "in-between" component materials (quartz, feldspars and ferro-magnesians), which absorb heat at different rates and thus expand and contract at different rates. This expansion differential can cause the stone to crack (as it does in nature from exposure to sunlight, freezing, thawing, and so on) and compromise the stone as a heat barrier. If it's a manufactured "stone" and somewhat homogenous in color, it may resist the tendency to crack, but, as with all unproven materials, careful monitoring is the order of the day.