New Hearth

[Woodsroad]

We are about to embark on the last part of our fireplace restoration/woodstove installation project: The Hearth

Right now, we are planning on using bluestone, but I wonder if 2" of bluestone is enough insulation under our Jotul F3. Is there an insulation board that can/should under the bluestone? Right now, we are planning on having the bluestone sit on 3/4 ply topped with mesh and wet set. All this sits over the main beam, a 6x8 old growth who-knows-what.

Thanks, Folks.

z

 

[webbie]

I'm sure the F3 calls for a certain R or K value in the manual - let's look it up.....

OK, this is just for interest - the same stove in Europe requires nothing (well, anything...like a piece of glass or sheet metal or 1/4" tile), but here in the US and Canada they call for R=1.1

That is a pretty low R-Value.

We have various data on the site, like:
https://www.hearth.com/econtent/index.php/articles/k_values_what_does_it_all_mean
or
https://www.hearth.com/econtent/index.php/wiki/Hearth_Design/
(scroll down for table)......

Based on these tables, your stone has almost no R-Value - even with the mortar I would say maybe .25 or so.
So, you need to add something.....like 4 layers of Hardibacker (2") or just 1/2" of Micore.......assuming you can find it. Do a forum and wiki search on micore. My technique would be to glue and screw the micore to the wood, lay some mesh over it and put the wet bed, then the stone down...or something like that.

Or, you can simply use 4 layers (2") of hardi or durarock then some thin slate or tile.......total thickness would be about as you planned first.

Certainly does seem like overkill when the same stove tested with nothing in Europe, but those are the breaks!

 

[webbie]

It's interesting to note the text about the UL or WH approved board, since Durarock and other ARE approved. I have also seen many UL boards that are not R=1.1, so this contradicts the other part of the instructions!

 

[Woodsroad]

My feelings as well. I have relatives in Denmark. They have people, paid by the local community taxes, that come by and clean and inspect your wood stove and chimney.. Not on a fishing expedition, not on the lookout for fines or violations, they jsut come out and clean the flue and answer questions.

I'm inclined to go with what I have...but I WILL look in to Micore....

(broken link removed)

Interestng that "heat insulation" is not among the listed applications.
Thanks, I'll keep y'all informed.

-z

 

[webbie]

It's really the AIR that gives the insulation, and micore and mineral boards have a lot of air spaces within them. If you had one big piece of bluestone, perhaps you could put strips of 1/2" cement board under it every foot or so, and that would provide more than the needed value (the air, combined with the stone).

 

[Woodsroad]

I'll bet thet you are 100-% correct, Admin. Get the slab of bluestone up and air circulating underneath it and all is good with the world. 2" angle iron would have the same effect as 1/2" cement board strips, eh? All this would be sitting above the basement, so it would be easy to keep an eye on it all, heck I could set thermocouples into the the whole shebang, and have a web-monitored model...

-DanZ

 

[Corie]

I'd say angle iron would be a no-no because the point of contact between the angle and the stone would just be the thin edge of the angle. If you want to use metal rather than strips of cement board, I'd highly recommend some steel channel.

 

[Highbeam]

"I’ll bet thet you are 100-% correct, Admin. Get the slab of bluestone up and air circulating underneath it and all is good with the world."

It's "dead air space" or "horizontal still air" that offers the insulation so no worries about air vents or gaps to allow the air to move in and out from under the slab of Bluestone. I consider that R of 1.1 to be quite high. My stove required 1.2 and it was a bugger. Air space to the rescue.

 

[webbie]

Easy way is just cut some strips of wonderboard - will bear against a larger surface on floor and on stove, cost less, etc......

just score it and snap - maybe 2" wide strips or so. The space can be dead or open.

 

[Woodsroad]

This fireplace is weird.
It has a small opening, lined with firebrick. The firebrick looks like it was recycled. Lots of smaller pieces. Now, behind that firebrick, there is very nice blue granite, 18" thick, at least, all around. So, I removed all the original fire brick from the fireplace today and found that, despite it having been wet set in a good bed of cement, there were heat leaks all over. A floor joist and the main center beam are charcoal where they run under the hearth. So, I got a floor jack on the main beam, chisled out all the concrete around the the beams, and prepared to sister in some new wood.
I found a local supplier of Micore and bought a sheet. I'll be able to cut a piece without any seams to fit the opening. So, a little concrete to level the hearth, Micore, thin set and Bluestone. Should be nice. I'll spend tomorrow sand blasting the stone where the fire brick was. I'll post some before and after photos when it's finished...

Thanks.

 

[webbie]

Wow, should be a warning to folks with old fireplaces....charred beams! Might just be heat radiation over the years - not "leaks".

 

[Woodsroad]

The beams run 4" under the hearth, and there wasn't a solid surface between the two. When I dismantled the firebrick, it was quite apparent that heat and smoke had worked it's way through the joints. Some were small holes, some larger, but the contributing factor here was the failure of the original builder to seal the cavity between the firebrick and the fireplace itself. There must have been a good draft back there, sucking the heat out from between the bricks.

Either way, yes, a lesson to be learned: Solid surfaces are your friend, be sure that cavities are dead air spaces, when in doubt, don't burn.

I'm glad that I never atempted to used this fireplace!

 

[webbie]

Just FYI, 4" is WAY short of what is needed. There is actually NO thickness that is enough as per the paragraph below:
2111.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. No combustible material shall remain against the underside of hearths and hearth extensions after construction.

The only reason your house exists is that there was not enough air leaking into this area from below - which would have fed the fire.

Masonry fireplace side walls fail the UL tests at 8" thick - the bottom gets vastly hotter because of the continuous presence of hot coal and less air flow over it. I suppose if frame HAS to be somewhere below, a person might be able to work out a sandwich of various materials and have it stamped by an engineer, but it would need an HIGH R value at elevated temperatures.

 

[Woodsroad]

Agreed about the lack of O2 saving this house (before I bought it!). That beam is CHARCOAL, not ashes. Good, solid charcoal.

OK, next question: Will this hearth be OK for a woodstove? I'll be adding an inch of mortar, a slab of Micore 300, thinset, then 2" of bluestone. A Jotul F3 will top it all off...
The beam that burned will be cut out. The main beam will remain (8" below the floor joist).

Thanks...

 

[webbie]

That hearth sounds fine.....the only reason I replied with the other jazz was so that information was here for folks with regular fireplaces (who search someday)....

In the end, AIR is the best insulator, and the legs of the stove plus the built-in heat shield are going to keep you safe. The Hearth is mostly for the spilled logs, a bit of radiant heat from the glass (forward and down) and for that one in one hundred people who decide to burn particle board full time in their wood heater (the glowing stove boy).