durock multiple layers

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I just got off the phone.
I was told to follow the picture. One sheet of durock and tile will get it done.

The metal that highbeam is talking about is only if you decide to not use grout between the tiles.

I'm still confused about the .84 k value. I wonder if I have the wrong numbers or equation. I'll have to look it up.
 
I just took this from the Englander website -While my simple mind can't comprehend all the numbers, you might.

EXAMPLE:
The specified floor protector should be ¾” thick material with a k-factor of 0.84. The proposed
alternate is 4” brick with a C-factor of 1.25 over 1/8” mineral board with a k-factor of 0.29.
Step (a): Use formula above to convert specification to R-value.
R = 1/k x T = 1/0.84 x .75 = 0.893
Step (b): Calculate R of proposed system.
4” brick of C = 1.25, therefore R brick = 1/C =1/1.25 = 0.80
1/8” mineral board of k = 0.29, therefore Rmin.bd. = 1/0.29 x 0.125 = 0.431
Total R = Rbrick + Rmineral board = 0.8 + 0.431 = 1.231
Step (c): Compare proposed system of R of 1.231 to specified R of 0.893. Since proposed
system R is greater than required, the system is acceptable.
Definitions:
Thermal conductance = C = _____Btu____ = ____W____
(hr)(ft2)(deg F) (m2)(deg K)
Thermal conductivity = k = __(Btu)(inch)__ = ___W____ = ____Btu____
(hr)(ft2)(deg F) (m)(deg K) (hr)(ft)(deg F)
Thermal resistance = R = (ft2)(hr)(deg F) = (m2)(deg K)
Btu W
 
Well I think it all boils down to this statement in the durock pdf mentioned a few posts back.


Durock cement board may also be used as a floor protector in place of one layer of 3/8” thick millboard.
(Having a thermal conductivity of k ≤ 0.84 Btu in./(ft. ² h °F) in the minimum dimensions specified by the room
heater/stove manufacturer.)”

The tag on the back of the keystone says floor protection should be a minimum of 3/8 inch and k value of .84

I think my problem was that the 1/r =k is an over simplistic equation.

I'm going to stick with what Woodstock told me today. And put the slate on this weekend
 
Highbeam,

It is my understanding, by the woodstock information, that you only need to use sheet metal as a ember/spark barrier if you have open gaps in your stone work or tiling (i.e. no grouting between them).

I sure hope the drawing shown in an earlier post is still correct because that is what I have spent SEVERAL hours building for my fireview. 1/2" cement board (wonderboard) and then 1/4" porcelain bonded with thinset and with grout between the tiling so no need for a sheet metal plate between or underneath.

Also, my understanding is that hardie board is NOT a viable option for a hearth anymore. I still have 2 unused sheets at home that I need to return to HD, I purchased them first and then decided not to use them after discussion with the helpful group here on this forum.

Hope some of that helps...

Kenny
John 3:16
 
Trouble is that the picture doesn't say IF no grout then use the sheet metal. My inspection went very well but I would hate to have a picky inspector looking for that sheet stock.

I don't think hardi ever was a viable hearth product since it was always combustible.
 
Looks like this thread is all set. But I don't want anyone to get the wrong Idea. The below is cut and paste from the Hardi website. I had to check again because I used Hardi based on this info. ( On top of micore, and unfortunatetly below -oops! )


Non-Combustibility
HardieBacker® cement board is recognized for use in non-combustible construction in NER-405.


Surface Burning Characteristics
When tested in accordance with ASTM test method E-84:

Flame Spread 0
Fuel Contributed 0
Smoke Developed 5

Thermal Resistance
(Approximate value) 1/4'' thick: R:0.13

http://www.jameshardie.com/homeowner/products_backerboard_halfInch.shtml
 
HB,

I know what you are saying about inspectors but I'm not to worried because it is in black and white just below the picture that was previously posted. My inspector told me they would be using the manufacturers info as their guide.

From Woodstock:

"Brick, flagstone, or stone should be mortared at all joints to prevent sparks and embers from falling through cracks."

"If the finishing materials are simply fitted together “dry”, with no mortar or grout, the NFPA requires that a piece of at least 24 gauge sheet metal be placed under the cement board in order to prevent embers from making their way down to the plywood base."


murph,

I think it's more of an emissions thing than a burn/catch fire thing.

Look here: https://www.hearth.com/econtent/index.php/forums/viewthread/45188/

Kenny
John 3:16
 
Interesting - clearly wonderboard - durock seem to be better choices - or atleast I see nothing to say otherwise. For my part thought, I think I'll take my chances with off gasing, only becasue it's already done, And I can't see anything in the hardi information that scares me. But If I were to build another hearth I would use the older "cement board".
 
Our Inspector specifies the 1/2 inch listed Durock . . . the sheets that actually have a sticker on them saying they are approved for hearths. That said, we don't make the home owner show us the actual label or dig up the tile or stone to verify it's Durock and not Hardiboard or another cement board underneath.
 
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