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Hearth pad being built, R-value required 1.2, no micore in sight

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Highbeam, Jul 12, 2007.

  1. webbie

    webbie Seasoned Moderator Staff Member

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    Correction - found this old post by Driftwood:

    "Hardy backer is made of cellulose; I called James Hardie technical support 1-888-JHARDIE about the lack of R value off their ½ inch hardybacker product. They told me ½ inch hardybacker has no Rated R value, “it conducts heat”. Hardy backer is not the stuff you want behind your stove or under it. WonderBoard or Durock cement boards are R rated and UL Listed for that application. "

    Uh Oh.....I hope no one has been using this stuff for hearths, cause I'm gonna make 'em rip them all out!

    (Note: They may have other products that we don't know about supplied to HD, etc......there should be a label on them or product info available)

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  2. babalu87

    babalu87 New Member

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    Kamco Supply , 6 or 7 locations in the Northeast.
  3. webbie

    webbie Seasoned Moderator Staff Member

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    OK, yeah they are a drywall type house:
    http://www.kamco.com/

    I'll have to see if Goose put that wiki entry in and we'll work on it....also get this hardie info up, although maybe we should wait for elk to check the board - it seems strange that the factory rep would say "no R value" when Elk obviously got his from somewhere! Something doesn't match.
  4. elkimmeg

    elkimmeg Guest

    hardi backer board is r0.13 per 1/4" for specs ccheck out this web limk to conform fire resistande and r-value

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

    In comercial construction used in fire rated assemblies is fire retrardent wood and plywood. These products are not fire resistant but damn hard to ignite.
    One can use a mapp torch on this material and only produce a charr area but won't flash into combustion .I know I have applied the mapp torch
  5. Gooserider

    Gooserider Mod Emeritus

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    Looks like from the FAQ associated with the page that Elk was pointing to - the official Mfgr spec page for the stuff, like it can be used as a non-combustible. Elk - I'm assuming the numbers mentioned in the web page are the ones that you would look for as an inspector and / or are the ones called out in the codes?

    If so, then I see no real problem, and a possible advantage in that it appears the HardieBacker is available in 1/4" thickness as well as 1/2" - important for those concerned about keeping the total thickness down.

    However this would raise the question as to whether 1/4" HardieBacker over Micore would be stiff enough to avoid cracking under the weight of a stove and human traffic? - once again thinking in terms of the method I've suggested before of cutting out the first layer of subfloor, and putting the Micore over that, then the HardieBacker and tile or other non-combustible hard floor.

    This brings up another sort of relevant question that I asked a while back, and I don't think got an "Elk - level" answer... What about "split level" hearths, where the stove itself is on a raised platform that is smaller than the required "hearth footprint" with an extension at a lower level to get out to the required distances. The extension is at a lower level, thus farther from the stove bottom, and therefore presumably shouldn't need as much thermal protection as it would if it were closer - how much, if any, "credit" does one get for the extra space?

    From the earlier thread where I was asking about this

    Attached Files:

  6. begreen

    begreen Mooderator Staff Member

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    You'll note in the specs that hardibacker board has a mold retardant for wet area use. That is not required to the non-cellulose products. I trust my tile guy, he says don't use it except perhaps for vertical surfaces.
  7. Gooserider

    Gooserider Mod Emeritus

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    Well I'm now working on a LENGTHY "Hearth Design" article - that will include a reference to Micore and the other materials in it, but so far hasn't gotten to that point. Also I haven't been doing a very thorough job of "wikifying" the text. It would probably not be a bad thing if folks were to start looking at the article and "fact checking" it, adding additional content, etc...

    Gooserider
  8. webbie

    webbie Seasoned Moderator Staff Member

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    I will do so when I have time - if it ends up being "timeless", I can then even move it to the main articles page, although we cannot edit from there. Another option is to link the relevant main articles to that wiki entry -

    Amazing how much information there is to be had! It's like that old Buddhist saying "Sentient beings are numberless; I vow to save them all..." In our case "The information is limitless, we vow to attempt to write it all down".

    BTW, speaking of that, I had fun the other day in the Internet Archive looking at the old hearth.com and also some other sites I did. In fact, I'm gonna ressurect some blog entries that I thought had disappeared - but the wayback machine has 'em.

    Now don't spend too many hours there, but type any url into the wayback machine and you can imagine how big of a repository the archive is!

    http://www.archive.org/web/web.php
  9. webbie

    webbie Seasoned Moderator Staff Member

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    I'm with Begreen. I would stay away from Hardi board now that we know this - unless we have some exact proof from the manufacturer which refers to that use. The cellulose and the conversation with the factory (Driftwood) is enough for me. Yes, I agree is does not ignite and has a low flame spread and smoke index, but when alternate UL materials exist which do not have the cellulose....well, I'd use 'em.

    As far as just vertical surface, I do believe that their instructions show use on floors and counter tops. I think it would function fine if used as directed.

    Wow, this is really some detail that normal people (only a crowd could research this) would have a tough time with.

    To be clear, I'm with Elk in that the use of it is probably more than safe. But safe is not the standard......Well, time for perhaps a little more digging.
  10. Gooserider

    Gooserider Mod Emeritus

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    I think this is an application that I would consider safe and suitable if Elk is willing to buy off on it. Hardie does say it is fire resistant, and acceptable for fireplace walls and heat shielding. They also specifically list is as suitable for floor underlayment if it is glued to the substrate with thinset or equivalent. The two should combine, I would think. HardieBacker may contain SOME cellulose, but their spec sheet says it is "90% Portland cement and ground sand." and later that it's surface fuel contribution is ZERO - That would cause me to assume that any potentially combustible cellulose is buried inside the material, and not present in enough quantity to be a problem

    However to be sure, it might be a good thing to have Elk write to them, waving his building inspector credentials, asking them to clarify the use in their FAQ - I'd ask, but I figure that if Elk puts his inspector hat on, they would be more likely to give him a thorough and complete answer...
    I'd ask things like:
    1. Is it acceptable to use HardieBacker underneath a woodstove as part of a non-combustible hearth?
    2. If it is acceptable, is the R-value of .013 per 1/4" still correct?
    3. Is it acceptable to stack multiple layers of HardieBacker to achieve required total R-values? If so, are there any special recomendationd for fastening the layers together?
    4. Is it acceptable to use HardieBacker over other types of insulating sheet materials (i.e. Micore) as part of a sandwich design? If so, is 1/4" acceptable over a somewhat soft material such as Micore or is 1/2" needed?
    5. If building a raised hearth with metal studs supporting it, how many layers of 1/2" HardieBacker would be needed to obtain sufficient strength to function without combustible underlayment? (or suggested alternative to the combustible underlayment (plywood)
  11. webbie

    webbie Seasoned Moderator Staff Member

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    Goose, caution would say that we should work the other way around. Unless Hardi says it is OK for this use, then it isn't. Again, the question is not whether it will work, but whether it meets any listing requirements. Here are the exact words from Dura and WB:

    Wonderboard
    "UL Rated Report No. 7L30 for floor protector and wall shield."

    Durock
    "also adaptable for high-heat areas like fireplace fronts and UL-listed wall shield/floor protectors. DUROCK"

    I have not seen any information of this type on the hardie site. There is a big different between fire resistant and approval for hearths.
    I think some of your questions are already answered - some of the sites have guides to the weight bearing capability. R values can always be added together, that is the way it works.

    Hardie does show a smoke generation index of 5, which I assume is because of the cellulose burning.

    Maybe we will hear from them...but will they have a test report like the others and specifically mention it in their instructions? Or will some factory phone person tell us something as silly as the last one "no R value, heat passes right through it".

    Since the other two products are available almost anywhere.....I guess I am questioning why use something without this specific listing? As to use on fireplace fronts, firewalls and such, that is a different application.

    Also, Durock has 30 year interior warranty....I though I saw 10 on the Hardie.

    My guess on this whole thing is that it would pass the tests, but they have not submitted it for such. Another one of those thousands of situations that fall between the cracks.
  12. elkimmeg

    elkimmeg Guest

    Goose that Donor stove in march we installed and ended up 2" shy of 54" above I have James hardi fax my office all the flame spread and asked the
    very questions and concerns you guestioned. sufecient evidence was produced and acccepted by a very differcult inspector in the neighboring town.
    In the end there was little to question. Not only is it easier to cut but I in full belief it is superior to cement boards reinforced with fiberglass webbing.

    That webbing can melt in less than 800 degrees, the normal melting point of common fiberglass. In reality the heat exposure should never approach that level. especially faced with ceramic tile of cultured stones. Did you know there are two micore products. One should also check into accostical ceiling tiles rated for fire assemblies.. I believe they are made of micore

    I must look into fire rated treated wood and plywood to see if their heat range will allow their use in hearth extentions I have to find out the heat range required for the extentions. Knowing that might open up more alternatives It may be possible to use common fiber glass packed between layers of hardi backer boards One inch of hvace fiberglass duck wrap is r4.5 and 1.5 " can go as high as R5.5 given the density of the fiber glass
  13. Gooserider

    Gooserider Mod Emeritus

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    From the HardieBacker FAQ
    . How ASTM E 136 compares to UL 7L30, I don't know. The FAQ also says it IS acceptable for fireplace fronts. The message you pointed at earlier in this thread saying it wasn't OK was also talking about BBQ's, and from the context of the quote it appeared the issue was the exterior application, not the non-combustiblity.

    Perhaps, but if you say the other application data is not relevant, then I'm assuming this should be confirmed as well.

    Possible, the question is what does that number mean in comparison to other products - what is the SGI for Durorock and Wonderboard for instance?

    Hardie appears to be a lighter weight product, and be easier to work with (or so Elk says) It is available in 1/4" or 1/2" thickness, the others appear to be only availale in 1/2". It has a slightly higher R-value (0.260, vs 0.200 for the others) it claims higher strength than the others.

    HardieBacker's Website says it has a 20 year transferable warranty, although reading it I would not consider it a highly valuable sales point - it's only good for the cost of the product, and is pro-rated so the amount drops every year, and (like lots of warrantees) is loaded with tons of exceptions and loopholes. I'd expect the others to be similar.

    [/quote]My guess on this whole thing is that it would pass the tests, but they have not submitted it for such. Another one of those thousands of situations that fall between the cracks.[/quote]

    Perhaps... They also may not have realized that this is an important enough market segment to put the data on their website.

    Additional data - pulled from the Donor stove install where Elk failed the first inspection because of a missed clearance and fixed it with HardieBacker - Granted, this was overhead and not underfoot, but...

    "I must say Hardibacker board is much easier to work with that cement board lighter and cuts better

    Thermal Values (ASTM C177)
    1/4” HardieBacker and 1/4” HardieBacker EZ Grid® cement board Thermal Conductivity: k-value 7.80 Btu/ft2 x h x °F

    1/2” HardieBacker cement board Thermal Conductivity: k-value 20.07 Btu/hr-ft2 - °F

    1/4” HardieBacker and 1/4” HardieBacker EZ Grid® cement board Thermal Resistance: R-value 0.13 ft2 x h x °F/Btu

    1/2” HardieBacker cement board Thermal Resistance: R-value 0.05 hr-ft2 - °F/Btu

    Non-Combustibility
    When tested in accordance with ASTM Method E-136, HardieBacker cement board is recognized as a non-combustible building material in NER-405.

    Surface Burning Characteristics
    When tested in accordance with ASTM method E-84: Flame Spread – 0, Fuel Contributed – 0, Smoke Developed – 5.

    Fire Rated Assemblies
    Hardibacker cement board may be used as a component in one-hour fire-resistive wall construction; consult NER-405 and Intertek-ETLSemko website listings for recognized assemblies, or contact James Hardie’s Technical Services at 1-800-9HARDIE (1-800-942-7343).

    At any rate, I'm sending an e-mail to Hardie Tech support, will see what they have to say. In the meantime I've added a blurb to the Wiki Article saying that there are questions about the suitability of HardieBacker and it is not currently reccomended. (BTW, I need some help getting the table of materials in it right - looks fine when I'm in edit mode, but looks like CHIT in the display mode...)

    Gooserider
  14. DonCT

    DonCT Minister of Fire

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    If you don't mind the raised hearth look, the way I constructed my "Pad" is solid as a rock.

    There are sooo many ways to meet R requirements that you can get pretty creative. Don't have Micore? Do a Durock sandwich. :)
  15. Gooserider

    Gooserider Mod Emeritus

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    This sounds excellent, Elk, I would love to see more research into alternative methods of getting non-combustible hearth structures. However I don't know enough of the code's "alphabet soup" standard references to tell what is meaningful and what isn't.

    A couple of suggestions I'm still not sure about is something I asked about a while ago, and you never got back to me on, is whether it would be structurally acceptable to replace part of the 3/4" subfloor decking with a thinner non-combustible alternative. I know you had suggested replacing the decking w/ 2x? blocking flush with the joists but that sounded like a difficult job. What about replacing the wooden decking with something like 1/8" steel or aluminum plate? I would think that would be as strong or stronger, do the same job of stabilizing the joists, and act as a heat dissipating shield under the pad...

    The other is the "split level hearth" question I mentioned earlier in this thread - If dead air space is worth R-0.970 per inch, does the same thing apply to open air? If so, it would seem like almost any flush extension to a raised hearth would be compliant if it was non-combustible - even just tile over ply, or maybe tile over one layer of cement board...

    Gooserider
  16. elkimmeg

    elkimmeg Guest

    goose the ASTME 136 standard materials must with stand 1200 degree heat before the break down or failure. This is the same standard of draft stop caulking.

    Boss 136 has been tested to 3000 drgrees Dap tested to 2300 degrees and the 3m brand 136 1200 degrees
    To meet that standard it with stands more heat than ever exposed to wood stoves. If there is 1200 degrees the floor around it would burn threw first
  17. webbie

    webbie Seasoned Moderator Staff Member

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    Ah, Goose, I did a bit of spell check and got that table looking good the only easy way.....took a screen shot of it in text editor and now it is a graphic - looks pretty good, though. Thanks for the effort - I will definitely link to it from our existing clearance articles.

    http://www.hearth.com/econtent/index.php/wiki/Hearth_Design/

    I'll eventually put some pics and scans in there also, to dress it up a bit. We can even link it to some hearth construction pic and threads that are in the forum pictures areas.
  18. begreen

    begreen Mooderator Staff Member

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    Goose, given that the cost difference is negligible, why would one choose anything but the best for the job, no questions asked?
  19. ansehnlich1

    ansehnlich1 Minister of Fire

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    3/4 inch advantech subflooring, then 4 sheets durock, then brick, then Jotul F-500 :)

    Attached Files:

  20. Highbeam

    Highbeam Minister of Fire

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    As you did Don, I intend to borrow heavily from a design on this site. Yours. Not so much in shape but I intend to steal your construction method with the studs and durock. I won't use any sheetrock though.... I had planned and desired a low hearth to keep it out of the way but once I concluded that I must elevate the hearth, then I may as well take advantage of the extra structure of the metal studs. Heck, we may even enjoy the stove being a bit higher from the ground. More at eye height, easier to load, etc.
  21. begreen

    begreen Mooderator Staff Member

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    There ya go! Have fun and take some pictures!

    What are you going to use for the top surface of the hearth?
  22. webbie

    webbie Seasoned Moderator Staff Member

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  23. Gooserider

    Gooserider Mod Emeritus

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    I agree, the table does look MUCH better now - the only question is how hard will it be to edit the table in the future? I'm hoping that over time we might be able to add more materials to the table, or get better sources and "official" values for the materials listed - I might trust Tom, or the folks on the forum, but numbers credited to them are not going to be as convincing to Elk's buddies as a Mfgr website or published standard listing.

    Agreed, but define "best"? I'm seeing glowing reccomendations for HardieBacker from Elk, and claims from the Mfgr website (which I do take with a grain of salt) about how HardieBacker is superior to other forms of cement board, so one could argue that it IS the best... I'm trying to resolve that.

    One of the constraints that I'm operating under in designing my hearth extension is that raising the level is NOT acceptable, as I want to keep that part of the floor useable for other things when we aren't running the stove. Now examining the manual for the Defiant Encore 0028, it does not have extreme R-value requirements, but I do believe in following my own advice on overbuilding to accomodate possible future stove upgrades, and want to make sure the extension has as high an R-value as I can reasonably get. That pretty much means Micore, some form of cement board, and tile or stone. I'm not sure if I'll want to remove the decking to get the extra space or not, I think I will make that decision if I'm ever able to figure out what is under the bricks on the existing hearth - I get no benefit from exceeding the existing hearth's construction with the extension.

    Gooserider
  24. Highbeam

    Highbeam Minister of Fire

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    The wifey and I are choosing between slate and slate looking tile. The same tile will be used for the doorway to the front door and maybe also some countertop backsplash so I am leaning away from slate for fear the dark "dust" of it will be tracked into the house as is discussed on the John bridge tile site. Plus being a first time tiler I would prefer the ease of installation of plain old ceramic tile. The wall behind the stove is plain painted drywall at least 7" away. Maybe in the future we'll stick tile to the wall but for now just paint.

    I will try and provide a similar step-by-step as Don did. Same stove, similar hearth design, and similar tile. The top front corner of the tile is going to be a challenge. Some folks like Don let the horizontal tile hang over but I want the corner to be a plain 90 degree one. Anyway, I'll fumble through it. I look forward to learning this skill.
  25. begreen

    begreen Mooderator Staff Member

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    Avoid Home Cheapo slate. It will flake regularly and make a constant mess. There is a fabulous world of tile and stone out there. Lots of choices.

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