Advice/opinions needed for new hearth plan

[Mike S.]

We are in the process of planning a new hearth for our wood stove due to an unsafe condition that currently exists. Since we want to do this right, we are turning to the experts for help. This will be for a chimney in an interior wall. The chimney is constructed of 16” square concrete chimney block with a 6” terra cotta flue. The terra cotta thimble extends about 10 inches into our living room, where the stove will be located. The other three sides of the chimney are in a closet. Our plan is to “box-in” our chimney using steel studs and Durock with the studs being 2 inches from the chimney and the Durock about 5 ½” from the chimney (2 inches plus the stud width). Hopefully, this will give us a non-combustible wall behind the stove and the proper clearances around the other sides of the chimney.

Our questions right now are these:
Does this type of framing sound acceptable?
Is it necessary to use Durock for the 3 walls in the closet or can they be regular drywall if they are behind a non-combustible wall and are the recommended distance from our stove for a combustible wall?
What should we do around the terra cotta thimble? Can the Durock go right up against it? Would we able to use single wall stovepipe going into the thimble with this type of construction?

Just for info: We are planning to replace our old Scandia stove with a new Hearthstone Craftsbury. We like the looks of it and it is in our price range. We will be heating about 1000 square feet in a ranch style log home.
Thanks!
Mike

 

[Mike S.]

Bump. We really could use some help! Anyone?

 

[begreen]

I'd go over the plans with the local inspector. But first review the code. Here is a recent thread on the subject of thimble design and clearances. About halfway down there is a picture of thimbles from the code book posted and a link to the public version of the NFPA code book. You'll need to meet or exceed requirements to pass code.

https://www.hearth.com/talk/threads/11876/

 

[Mike S.]

We called our township and there doesn't seem to be an inspector for this.
If we frame the wall with steel studs and cover it with Durock, at some point the Durock will come into contact with the thimble since it protrudes through the wall.
Is it OK for the non-combustible Durock to be in contact with the terra cotta thimble?
If not, is there something that goes in between the thimble and the Durock for a finished wall?
We are planning to use double walled stove pipe for this installation.

 

[begreen]

Do you see this option in the code provided?

 

[Mike S.]

I'm sorry, but I don't see anything in the drawings pertaining to the finished wall. Please excuse my ignorance.
Thank you for the help.

 

[Arlo]

this is what I did: 2 inch air space, 3/4"plywood, sheetmetal, durarock, mortor(thick), tile, spectra grout (easy to clean). Notice the cross supports in hearth are measured to where the stove legs will rest. You dont want the hearth to sag, it will crack the tile.

 

[tradergordo]

Just curious - is an air gap really helpful when its built using wood studs (as opposed to metal studs)?

 

[bmstove.com]

OK, the wall behind where you plan to add steel framing, durock and tile;

Your work will ONLY be considered a heat shield if you allow for air circulation. That means leaving a means in the top and bottom for natural convection. Close the top and bottom and the heat will transfer through. Additionally, reduced clearances are only allowed if this is included in your design. Of course if you are not attaching to a combustible surface, this would not apply.

Your hearth construction looks ok as long as your stove only requires a non-combustible floor protection. Some stoves have tremendous underbelly heat and require much more protection. That should be in your manual. I believe most stoves only require non-combustible and the ones that require more often include a bottom heat shield (Vermont Castings Aspen or Dutchwest) just make sure you use it if it came with your stove.

Jack

 

[mikeathens]

this is what I did: 2 inch air space, 3/4"plywood, sheetmetal, durarock, mortor(thick), tile, spectra grout (easy to clean). Notice the cross supports in hearth are measured to where the stove legs will rest. You dont want the hearth to sag, it will crack the tile.

Arlo: this is basically what I have (less the sheet metal); I have concrete floor, 2X4 studs, plywood, concrete board, thinset, tile, more thinset, another layer of tile...based on other posts and the manual, this only comes to something like R-0.42, which is less than the 1.2 R called for in my Hearthsone Heritage Manual. What R-value does your stove call for call for? You only have durock, thinset, and tile over your combustibles (your plywood is combustible; so is your 2X4 framing). From what I understand, the dead air isn't included since you have combustibles between your heat source and your air space. Is this correct?

 

[JimWalshin845]

.... {snip/} our questions right now are these:
Does this type of framing sound acceptable?
Is it necessary to use Durock for the 3 walls in the closet or can they be regular drywall if they are behind a non-combustible wall and are the recommended distance from our stove for a combustible wall?

Welcome aboard Mike,

We just redid our hearth floor area, this link for Durock should give you their ANSI/UL certified spects.

(broken link removed to http://www.usg.com/USG_Marketing_Content/usg.com/web_files/Documents/Prod_Data_and_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf)

Good luck with the project,
JIm

 

[mikeathens]

.... {snip/} our questions right now are these:
Does this type of framing sound acceptable?
Is it necessary to use Durock for the 3 walls in the closet or can they be regular drywall if they are behind a non-combustible wall and are the recommended distance from our stove for a combustible wall?

Welcome aboard Mike,

We just redid our hearth floor area, this link for Durock should give you their ANSI/UL certified spects.

(broken link removed to http://www.usg.com/USG_Marketing_Content/usg.com/web_files/Documents/Prod_Data_and_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf)

Good luck with the project,
JIm

Jim:

I can't quite figure it out, but based on the diagram provided in the literature, durock is claiming that mortar, durock, and then thinset/tile is adequate floor protection? The R-value calculated from the K-value given in the pamphlet is R = 0.59 (R = (1/K) * Thicknes). My stove requires 1.2 R value, and the hearthstone manual provides R=0.02 for 1/4" tile.

this stuff is giving me a headache. Good thing I get to start drinking in a couple hours...

 

[Arlo]

From what I understand, the dead air isn’t included since you have combustibles between your heat source and your air space. Is this correct?

I thought the dead air was considered part of it. My hearth value is .59 and the stove requires .50
The hearth gets warm but not hot. I can put my arm under the stove whith a roaring fire thats been going for a while and not have fear of radiant burns. It is a jotul however and from what I have heard they have some of the minmal clearances of any stove.?

 

[JimWalshin845]

Again, the Durock Tech Doc is a reference. It is not the end all depending upon your stove specs and the R/K values. For many/most (but not all) applications it will work fine and I would suggest a layer of metal directly under your stove if it is just a plywood underlayment.

 

[begreen]

this is what I did: 2 inch air space, 3/4"plywood, sheetmetal, durarock, mortor(thick), tile, spectra grout (easy to clean). Notice the cross supports in hearth are measured to where the stove legs will rest. You dont want the hearth to sag, it will crack the tile.

Arlo: this is basically what I have (less the sheet metal); I have concrete floor, 2X4 studs, plywood, concrete board, thinset, tile, more thinset, another layer of tile...based on other posts and the manual, this only comes to something like R-0.42, which is less than the 1.2 R called for in my Hearthsone Heritage Manual. What R-value does your stove call for call for? You only have durock, thinset, and tile over your combustibles (your plywood is combustible; so is your 2X4 framing). From what I understand, the dead air isn't included since you have combustibles between your heat source and your air space. Is this correct?

You are correct. For some stoves this would be adequate as they only require a non-combustible surface. But for stoves that have high R value hearth requirements, the insulation barrier starts at the combustible surface.

Arlo, in your case you are covered. The F600 only requires a non-combustible surface. Your installation is fine for this stove assuming you are not using the right side door and it is locked closed.

"A hearth pad measuring 45” wide X 53” deep can be used.
However, use of the right side load door is prohibited when this
size hearth is used; because access to the firebox is only allowed
through a door that has a minimum of 18” (460mm) of hearth
protection."

 

[Arlo]

Thanks for the reasurance. After further research, I discovered that my Cat F-600 only requires only a floor protector or non combustable surface like you said. I designed the hearth going off the info for the CB-600 which requires a a highger value and more of a specefic hearth. I guess I over built my hearth! Oh by the way, mine is a top loader, no side door. I luv the top loader when me arms are full....arrrgggg ggg gg! I only wish I had warming shelves for the kettle!

 

[begreen]

Improvise! Build in a non-combustible shelf or maybe a metal plant stand or small metal table with a tile top adjacent to the stove?