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Cement board research....

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Gooserider, Sep 11, 2007.

  1. begreen

    begreen Mooderator Staff Member

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  2. Renovation

    Renovation New Member

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    I believe you, and this is confusing. Right on the page it says "fire-rated". That seems misleading.
  3. Blazin

    Blazin Member

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    Well, this is definitely confusing. I found where the data sheet says the shouldn't be exposed to temps over 125F.

    Just above that is this: Fire Resistant These panels offer superior fire resistance and demonstrate exceptional surface burning characteristics.

    With such a low temp listed in the data sheet, I hope no one tries to use them in Arizona...
  4. Renovation

    Renovation New Member

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    Yep, that sure makes things easy for the consumer, who's trying to be safe and researching the product. :roll:
  5. LLigetfa

    LLigetfa Minister of Fire

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    5/8" drywall is "fire rated" so you just need to understand the context of what its use is.

    As for Durock and their NexGen product, does anyone have details on its makeup WRT high temp exposure compared to the old product? From what I gather, it looks like they phased out the old product.
  6. RNLA

    RNLA Minister of Fire

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    Does it change anything if you are putting all the stone or tile or whatever on the board. Is it only rated as the bare board???? Someone PM me we are about to do an install and was going to do hardibacker....
  7. begreen

    begreen Mooderator Staff Member

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    There's a difference between a fire resistant barrier intended to resist the spread of fire and a totally non-combustible product that also is an excellent substrate for stone or tile. Durock can't burn, neither can unpapered drywall, but the drywall would make a poor substrate because it would soak up the moisture from thinset or mortar. Would Hardibacker work on a low heat hearth as substrate? Probably, especially if the stove bottom is well shielded. But if the hearth is built in as part of the house infrastructure, who is to say what stove will be on the hearth 10 or 20 yrs from now and what the hearth requirement will be. IMO, this is a good place to overkill a bit.

    That said, I've spent hours trying to get some definitive answers on the subject. USG is clear about Fiberrock, Hardie is not so explicitly clear for a similar product. There doesn't seem to be a consistent answer here. On one building inspector forum I even found a couple inspectors suggesting Hardibacker and then getting corrected.
  8. Blazin

    Blazin Member

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    Well, I'm convinced that I'll have to drive to Lowes or HD and pick up some Durock. The problem with small towns and big states is readily apparent when either of the big DIY retailers is over 200 miles away.
  9. OrpingtonManor

    OrpingtonManor Member

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    Really glad to see this thread. After reading it, I went back to the pics I had taken when I renovated the sunporch where I will be installing my Morso 1410. The floor is slate, but the underlayment is USG Fiberrock. I could have sworn we used Hardiebacker, but not so. Good thing I still have a bunch of slate. I will now be building a hearth.
  10. LLigetfa

    LLigetfa Minister of Fire

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    Ja, but when you get there, will they have the classic Durock or NexGen? I think the jury is still out on NexGen.
  11. Blazin

    Blazin Member

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    I'll be sure they have what I need before I go. Thanks for mentioning the nexgen, I wasn't aware there was such a thing.
  12. begreen

    begreen Mooderator Staff Member

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    Yes, we have the same issue. Call local lumber yards. With lower volume sales they may still have a stash of the original Durock or Wonderboard.
  13. OrpingtonManor

    OrpingtonManor Member

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    I used to get my Hardiebacker at HD, and didn't think anything of it when they switched their stock to Fiberock. I wish I'd known then what I know now.
  14. Intheswamp

    Intheswamp New Member

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    A little off topic but...how are those buff orps doing? :)

    Ed
  15. OrpingtonManor

    OrpingtonManor Member

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    The Buff Orps are mostly freeloading at the moment. Only one of them is laying. The rest are either molting, taking a winter vacation, or on bug/weed patrol. Good thing they're entertaining.
  16. The Dude

    The Dude New Member

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    I have two related questions for my own install. I am about to install my vented clearance reduction board on the wall behind and beside my wood stove, and I just dug up this thread by Googling "Is hardibacker combustible?" (It's funny how all important things along the way point me to this forum).

    My first question is regarding strength of hardibacker and durock nexgen, in which I heavily favor hardi. I will be facing the cement board with thin stone. I worry that the added weight combined with it only being supported from behind every 16", that someone could lean on it or bump something into it and the board could crack. I bought both types of board, and the durock breaks MUCH easier, with only slight bending. It is still held together by the outer mesh, but I can't risk it breaking and having a wobbly, crumbling spot in my wall. Hardibacker seems to hold up much better. I thought about the theoretical risk with the cellulose fibers, but my stove manual (Harman TL-300) specifies non-combustible and this is listed as so. And I'll be creating a thermal airspace behind it. Does anyone advise against me using hardibacker?

    My other question regarding heat transfer to the studs. I cut 3" wide strips of hardibacker to use for spacers between my painted drywall and the cement board shield. In order to support the weight of the cement board and stone, I figure my only option is to screw the board and spacers into the studs. What I can't figure out is if this will defeat the purpose of the vented spacing by transferring heat right through the spacers and into the studs. If I understand correctly, the studs are the most critical combustible material that I'm trying to protect in the first place. Any advice? Thanks!
  17. tfdchief

    tfdchief Minister of Fire

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    Edit
  18. mhrischuk

    mhrischuk Guest

  19. mhrischuk

    mhrischuk Guest

    On edit. I called USG Engineering. They said do not use Durock Next Gen for high heat applications over 200 deg. It is not tested.
  20. The Dude

    The Dude New Member

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    Thanks for the info! I actually tried using Durock but one board crumbled, so I tested the other one to see if it could take pressure when being suspended off the wall with spacers, and just by pressing a little on a space in between studs the board cracked. That would SUCK if it happened after I mortared my natural stone veneer onto it, built the mantle, and put the stove back in place. So I went with Hardibacker even though it has the cellulose fibers. Aside from that theoretical concern, it seems superior in workability and strength to me.
  21. mhrischuk

    mhrischuk Guest

    I'm not sure if this is what you are talking about. It does crack normally when handled but when properly installed and supported, once it's covered in tile it should be a non-issue. After all, it is a sheet of cement. But after what you described, I don't blame you. I might have done the same. My application is laying flat on the floor.

  22. The Dude

    The Dude New Member

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    I think that is the crumbling I was speaking of. But when a whole board cracked all the way through and only held together by the fiberglass in the faces, that was as broke as a I am (really broke!). The thing is, I was using it in an unusual way, suspended an inch out from the wall, and pretty much unsupported according to it's intended purpose (see picture). I was so worried that if someone ever leaned against it in between the rows of spacers, the whole thing would crack. I think if it is used solidly against something, this would not be an issue. And considering how you showed me that those little white balls on the interior are not the flammable styrofoam that I thought they were, I'd say it's the most cement-based looking board out there. I had two issues for my project, the combustible one which you proved wrong, and the fragility, which probably isn't an issue in most people's projects. So it looks like we are in two different boats here!

    [​IMG]
  23. mhrischuk

    mhrischuk Guest

    Are those spacers designed for this application?
  24. The Dude

    The Dude New Member

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    Yes, ceramic spacers. I initially planned on using durock strips for spacers (see pic), but got too hung up on the idea of air circulation being limited to within the channels, while solid strips were placed directly over the studs, allowing heat transfer to the main material that the shield tries to keep heat away from.

    [​IMG]
  25. The Dude

    The Dude New Member

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    Completed wall with heat shield and stove installed. Thin creek stone to be installed over cement board soon.

    [​IMG]

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