Vertical vs Horizontal hearth surfaces and radiant heat

[voith]

We are building a hearth for our new Hearthstone Phoenix wood burning stove:

4X4s and 2X4s for the stepped up stove platform
3/4 " plywood base on the platform and floor (4' x4' total coverage with close clearance rear shield)

3/4" micore
1/2" cement board
ceramic tiles/grout

We'd like to build a 6 inch stepped platform in the center of the hearth, 2" wider than stove legs in all directions, then make up the rest of the clearance requirements on the floor level. (using the materials listed above on all horizontal surfaces)

Considering that heat radiates out from the stove, do we need to use micore on the vertical sides of the step as well as the horizontal surfaces or are the cement board and tiles enough to protect the verticals. I'm thinking the vertical angles won't get the direct heat radiation that the top surfaces will absorb. (???) What do you think?

 

[fossil]

If I'm picturing this correctly, you'll build a stepped hearth, with the stove sitting on the uppermost surface. In plan view, you'll have all the clearances covered from the stove. In elevation cross-section view, you'll extend the micore, cement board, and tile to cover all the top surfaces of combustible framing materials. You'll use cement board and ceramic tile on the vertical "risers" of the steps you build (all facing away from the stove), and micore, cement board, and tile on the "treads" of the steps you build (the top hearth surface, and on the floor below the riser out to the required horizontal distance from the stove). Your question is whether or not you need to include micore in the "riser" sandwiches. If that's an accurate picture of the installation you're envisioning, then I'd think you'd be OK building it that way...all required clearances from combustibles would seem to be met. Rick

 

[pinewoodburner]

Is the step platform going to be vented or completely enclosed. I am wondering how hot it will be in that area and if you are using wood framing. Jeff

 

[fossil]

The micore, cement board & tile should stack up to an r-value of slightly more than 2.0, which, depending on the specific stove's requirement for the floor beneath might be sufficient even without a ventilated airspace. Need to know the stove and the manufacturer's stated hearth r-value requirement. Rick

 

[voith]

Thanks for your reply!

I think you've pictured it, Rick.

We are planning on using just the cement board and tiles on the "step" vertical sides, without the micore insulation. the vertical sides will be within the clearance zone but will not be facing the stove directly (as the stove will be 6" up on the small "stepped platform") and the fully insulated platform top surface will overlap the less insulated vertical tiled sides.

None of the framing lumber and plywood will be exposed.

 

[voith]

The R value required is only 1.0 so the micore/cement board and tile should be fine on the horizontal plains, however, the stepped corners might not be strong enough where the micore layers meet under the cement board. This is why I'm thinking of skipping the micore on the step sides..

 

[fossil]

I think your plan is a good one...in fact, I think you could decrease the thickness of the Micore, as that's the only part of the sandwich with marginal crush strength. Micore's a great insulator, but not a great structural material, so why use more than you need? Maybe instead of 3/4" you could go to 3/8", and still meet the r-value of 1.0, with less fear of that corner around the edges of the steps being crushed. Like my aviator friends in the Navy used to tell me..."If it wasn't good enough, it wouldn't be the minimum." :lol: Rick