# Using Cement Board as fire retardent walls



## mywaynow

I used 1/4 inch Durarock cement board to provide a fire block on the ceiling above the stove, and on a wall located about 48 inches in front of the stove.  I noticed when I picked these boards up, they had an odor, chemical like.  Thinking it would disappear, I installed it as planned.  Now after heating the stove up a few times, the odor becomes unbearable and literally pushes us out of the house.  I have used similar board before and have no recollection of such an odor.  I had installed a second board that is different, above the stove with a spacer to allow heat disapation at that critical point.  That board is backer board, and is light in color.  It has no odor at all.  I will be returning to the Depot to make a "stink" (pun intended) about this board.  Bottom line, I wouid not use the gray cement board if you are doing something similar.  Stay with the lighter backer board (found in Lowes).


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## SolarAndWood

That's bizarre.  My redneck not so temporary hearthback is Durock and I have never smelled it.  Must be because I used 1/2 inch.


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## LLigetfa

Was that Next Gen or the former classic board?  I haven't worked with Next Gen and have never seen Durock in 1/4" thickness.  For 1/4" backer board, I've used the Hardie backer but not in a high heat application.


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## mywaynow

Like I mentioned, I have used cement board before too, with no odor issue.  This 1/4 inch stuff is newer and must be a different material.  The first time it got warmed up, it actually emitted a hazy vapor into the basement.  You could see the haze when looking across the room.  It was not smoke from the stove either.  It is very annoying.  Granted, it only happens when the board is very warm, when you can barely hold your hand against it.  Regardless, it is going.

The hardie board is the stuff to use.


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## mywaynow

Here is an excerpt from the Hardie backer board page on their site:

Can I use HardieBacker board as a fireplace facing?
Yes, HardieBacker board can be used as a fireplace facing. HardieBacker 1/4'' board can be used as a component in a 1-hour fire resistive construction when installed in compliance with National Evaluation Report NER-405. HardieBacker boards are recognized as non-combustible, when tested according to ASTM E 136.


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## begreen

Hardiboard is not ideal for this application. It has cellulose binders in it. A fire "resistive" fireplace facing is not the same as a fireproof shield like pure cement board. My understanding is that firestop material has to pass a stricter ASTM E814 test. Pure cement board or a sheet of metal is a better shield material. Though without an airspace behind it, neither qualify for a wall shield. 
http://sti.fmpdata.net/ftp/tech_updates/TU-FirestopVsFireblock.pdf

Why is the ceiling shield being used here? Is there a 1" air space behind it or is the material attached directly to the ceiling?


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## Renovation

BeGreen said:
			
		

> Hardiboard is not ideal for this application.



Hey BeGreen, Solar, Et. Al.,

What's your recommendation for relatively strong, non-combustible cement board?  I'll be using it in a horizontal application, and would like crush and bending strength.

Anything from this list catch your eye?

http://www.chimneysweeponline.com/horvalue.htm

For example, the Micore 160 looks like it has great specs, but could suck for all I know...


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## mywaynow

I used all the cement board directly affixed to wall and ceiling beams.  This was to deflect the most intense heat off those wood surfaces.  My belief is that it would also provide additional time to keep a fire from igniting those beams.  The backer board is a single sheet (3x5) mounted on the ceiling directly above the stove, with a 1 inch aluminum grate between the cement board and the backer board.  That specific spot was getting hot with use of the stove.  The single wall stove pipe is 15 inches below the ceiiling there.  My setup is as follows:
Defiant 1a
6 inch single wall up 40 inches from stove top (top exhaust), 90 degree elbow then 12 inches to wall thimble
insulated wall thimble exiting throuigh masonry wall into elbow/cleanout
insulated vertical run of 20 feet straight up
2- 30 degree elbows used to route the pipe off the soffit
4 feet of insulated straight pipe to cap.

I hated to add the 30 degree elbows but chose that as opposed to cutting the roof edge.  It is going to make a more difficult job of cleaning.  I assume I will have to dismantle the top section down to the offsets so I can access the straight pipe.


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## LLigetfa

mywaynow said:
			
		

> This 1/4 inch stuff is newer and must be a different material...


Try as I may, I cannot find any Durock cement board that is 1/4 inch.  The thinnest I could find specs on is the Next Gen 5/16th.

Are you sure it's Durock?


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## begreen

RenovationGeorge said:
			
		

> BeGreen said:
> 
> 
> 
> 
> Hardiboard is not ideal for this application.
> 
> 
> 
> 
> Hey BeGreen, Solar, Et. Al.,
> 
> What's your recommendation for relatively strong, non-combustible cement board?  I'll be using it in a horizontal application, and would like crush and bending strength.
> 
> Anything from this list catch your eye?
> 
> http://www.chimneysweeponline.com/horvalue.htm
> 
> For example, the Micore 160 looks like it has great specs, but could suck for all I know...
Click to expand...


Micore is soft and quishy. You'll want at least one layer of 1/2" pure cement board on top to distribute the load. Durock or Wonderboard cement board. 2 sheets are better than one if this is for a heavy stove.


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## mywaynow

The name on the product is Wonderboard.  The size is 3'x5'x.25".  I don't know if that is the same as Durarock or not.


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## Renovation

BeGreen said:
			
		

> RenovationGeorge said:
> 
> 
> 
> 
> 
> 
> 
> BeGreen said:
> 
> 
> 
> 
> Hardiboard is not ideal for this application.
> 
> 
> 
> 
> Hey BeGreen, Solar, Et. Al.,
> 
> What's your recommendation for relatively strong, non-combustible cement board?  I'll be using it in a horizontal application, and would like crush and bending strength.
> 
> Anything from this list catch your eye?
> 
> http://www.chimneysweeponline.com/horvalue.htm
> 
> For example, the Micore 160 looks like it has great specs, but could suck for all I know...
> 
> Click to expand...
> 
> 
> Micore is soft and quishy. You'll want at least one layer of 1/2" pure cement board on top to distribute the load. Durock or Wonderboard cement board. 2 sheets are better than one if this is for a heavy stove.
Click to expand...


Thanks so much for the information, BeGreen  Two layers of 1/2" Durock or Wonderboard it is, mortared together, I assume?


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## begreen

RG, best to move the hearth concerns to another thread this is getting noisy and confusing.


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## begreen

mywaynow said:
			
		

> I used all the cement board directly affixed to wall and ceiling beams.  This was to deflect the most intense heat off those wood surfaces.  My belief is that it would also provide additional time to keep a fire from igniting those beams.  The backer board is a single sheet (3x5) mounted on the ceiling directly above the stove, with a 1 inch aluminum grate between the cement board and the backer board.  That specific spot was getting hot with use of the stove.  The single wall stove pipe is 15 inches below the ceiiling there.  My setup is as follows:
> Defiant 1a
> 6 inch single wall up 40 inches from stove top (top exhaust), 90 degree elbow then 12 inches to wall thimble
> insulated wall thimble exiting throuigh masonry wall into elbow/cleanout
> insulated vertical run of 20 feet straight up
> 2- 30 degree elbows used to route the pipe off the soffit
> 4 feet of insulated straight pipe to cap.
> 
> I hated to add the 30 degree elbows but chose that as opposed to cutting the roof edge.  It is going to make a more difficult job of cleaning.  I assume I will have to dismantle the top section down to the offsets so I can access the straight pipe.



OK, the plan for shielding is a good idea. The installation with single wall was too close. Minimum clearance to combustibles for single wall is 18". 

For the shield to be kosher it's supposed to have free air circulation behind it. The correct way is to mount the sheet of wonderboard on 1" spacers so that it floats off the wall or ceiling. That ventilated air space makes a huge difference vs direct mounting. When directly mounted the heat is actually transfered through the board. The air space acts as a great insulator to prevent this transfer.


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## begreen

LLigetfa said:
			
		

> mywaynow said:
> 
> 
> 
> 
> This 1/4 inch stuff is newer and must be a different material...
> 
> 
> 
> Try as I may, I cannot find any Durock cement board that is 1/4 inch.  The thinnest I could find specs on is the Next Gen 5/16th.
> 
> Are you sure it's Durock?
Click to expand...


It's Wonderboard backer board: 
http://www.custombuildingproducts.c...nch_WonderBoardConcrete.aspx?user=arc&lang=en


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## LLigetfa

mywaynow said:
			
		

> The name on the product is Wonderboard.  The size is 3'x5'x.25".  I don't know if that is the same as Durarock or not.


I did a search on Wonderboard and could not find anything on its use as a heat shield.  It could very well be that it has binders in it that might off-gas if heated.  All the specs only mention using it as a moisture resistant tile backer.


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