R values on a hearth build?

I'm building a 7" elevated hearth for the new Woodstock Ideal Steel.

Their manual states:

IF YOU CHOOSE TO BUILD YOUR OWN HEARTH PAD
1) Start with a plywood base or subfloor.
Over this apply:
2) a layer of insulating board with an R-Value of at least 0.41. Depending on the material
you choose, the insulating board can be as little as 1/2” thick. For additional
help with material specifications, contact Woodstock Soapstone Company at 1-800866-4344
or [email protected].
Over this apply:
3) 1/4” or greater of a decorative, non-combustible material such as tile, slate, stone,
or brick. Use mortar or grout to set the material in place, then grout the seams.

Once alternate materials have been converted to R-values, the values of multiple layers can be added to
determine the combined protection.

If the overall R-value meets or exceeds the specified .41 then the materials are acceptable.

R-values of common hearth materials:

Ceramic Tile 1/4” 0.020
Granite 1/4” 0.020
Slate 1/4” 0.025
Cement Mortar 1/2” 0.025
Cementboard 1/4”-1/2” 0.20-0.39
Common Brick 2.25” 0.450
Common Brick 4.00” 0.800
Mineral/Ceramic Fiber Board 1/2” 1.10-1.470​

I built the platform using 2"x6" studs with 23/32" plywood on top.

One of my patients just gave me 4 sheets of one inch thick 2ft x 3ft slate. I'm going to use that on top of the hearth. (I had bought 13"x13" ceramic tile on clearance, only paid $20 for 18 pieces, but I just wasn't quite satisfied with the color.) This slate will match the stone on the walls, and I can put the same Airstone on the front of the platform now to complete the appearance.

The way I'm reading Woodstock's manual, one inch of slate should be more than adequate with a total R-value of 0.1. And I can put it on top of the plywood without the need for cement board.

Correct?

 

I think you need over 4 inches of slate to get up to .41 Brian.

 

Oh, you're right! My decimal was off.

I bought 1/2" cement board to use with the ceramic tiles and I thought I could get away without it. I'll just go ahead and use the cement board under the slate as originally planned with the tile.

(And that's why I asked - something didn't seem right...)

 

You just got the stove Brian? That's gonna be a nice looking set up once completed.

 

Actually I've had the stove sitting in my living room on a pallet for several months. Being too busy with work and then some health troubles kept me from getting it installed. Frankly the delay was providential. I wouldn't have stumbled on these awesome pieces of (free!) inch thick 2'x3' slate slabs for the hearth if I had finished the install earlier this fall.

 

My procrastination has proved beneficial at times too

 

Progress:

Test fitment. Gotta clean all the slate after cutting the pieces. Forgot one cut on the piece under the lamp. Next we mix the thin set and set the slab pieces, then finish the bottom front of the hearth platform with the same Airstone as on the walls behind the stove. We will treat the slate with boiled linseed oil to seal it a little but to also bring out the colors.

 

These are the slate slabs we started with. They had been in my patient's front yard under an arborvitae for the last ten years and they had a lot of green (algae?) and red (red soil) discoloration. They cleaned up nice, the dirt came off with Clorox water and elbow grease.

 

I'm glad you went with the cement board. The plywood isn't as strong of a bond as the backer board. Even 1/4" backer would have worked. It should hold for a long time.
My install was more towards the center of the floor so I had to also install two lolly's in the basement to support the additional weight, it looks like you are ok being on the outside wall in the corner.
Is the project finished yet?

 

Yes, the rear wall is an outside wall. The left hand wall on the corner install has a beam under it in the middle of the basement with a post about 7' from the outside wall under the beam so everything is well supported.

My oldest son and I are still working on it. Sunday is supposed to be a day of rest so we're just cleaning the stone and double checking everything today and applying some of the Airstone to the platform, before we set the slate tomorrow.

 

Using up some old spray paint to help hide any gaps in the slate and stone work (the bare wood and white trim sticks out like a sore thumb where the slate meets the Airstone):

 

Is airstone the same as backer board or cement board?

 

No, it's the stuff we put on the walls behind the stove in the first photo in this thread. Lowes sells it. Its a blown cement product with recycled materials in the cement and it does have some insulating value. It's a quarter the weight of real stone so it can be applied directly to existing walls and surfaces including drywall, plaster, wood etc. We're using Loctite PL Construction Adhesive to glue it to the walls.

Here's the first couple rows on the front of the hearth platform (note the fashionable FHC shirt being worn by my Airstone technician, otherwise known as my youngest son):

 

The whole build so far can be viewed on my Facebook page (even without a Facebook account) at https://www.facebook.com/media/set/...073741831.100006765817519&type=1&l=4d7168fe09

 

We finally got the slate slabs installed on the new hearth tonight with thin set mortar. Tomorrow, the new IS wood stove takes its proper place upon its pedestal.

 

Looks good Brian. Nice job. It will look even better with the new IS sitting on it.

 

2012, with the Woodstock Fireview:



2014, all done, just letting the thin set mortar cure before putting the new stove on the hearth: