figuring out r value

[Blacktop]

I'm installing a sbi enerzone 2.3i wood stove insert. I was going to build a hearth that is 4" concrete block with 1" bluestone on top. This is going to be built on top of my existing hearth which is 1" bluestone on an unknown amount of cement that is flush with my hardwood floors. Is my r value higher going to be 1.0 or higher with the added height?
Should I leave air gaps under the blue stone? or do it solid? I'm worried about the weight of the stove crushing the blue stone when installing if I leave airgaps.

 

[webbie]

Various values are assigned to masonry depending on the exact units (concrete, brick, block, mortar). My feeling based on your description is that you will be in the ball park and OK. However, here are two ways that you can add a little (or a lot) to the value.

1. Use hollow 4" blocks....they have a small air space inside them. That should serve as a good heat break.
2. Use 2" block and then 4 layers of Durock cement board - that will then far exceed R=1 on the entire assembly.

As mentioned, I think you are close anyway, but either of these methods will add extra margins of safety.