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Question about seams in the raised hearth I'm building

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by siddfynch, Jan 1, 2013.

  1. siddfynch

    siddfynch New Member

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    Hi all,

    So I've done my research and have decided to build a raised hearth that has two layers of 1/2" Hardibacker. The hearth measures 48" by 60"; since Hardibacker ('HB") only comes in 35" x 60" sheets, that means each of my layers must have a seam in it where the full 36" sheet butts against a 12" sheet.

    Do I lose R-value from the resulting joint? If so, is there a tape or any other kind of material I can use to fix this?

    Thanks in advance. I have scoured the search engines and not found this issue addressed. The two main hearthbuilding documents on this site's "Resources" sections were very helpful for all my other questions.

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  2. WhitePine

    WhitePine Feeling the Heat

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    Stagger the seams.
  3. ansehnlich1

    ansehnlich1 Minister of Fire

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    yep, stagger the seams, that's what we did.
  4. dorkweed

    dorkweed Guest

    Use Durock Next Gen instead, and then you can seal the joints with latex modified thinset mortar and fiberglass mesh tape. Stagger the seams.
    jeff_t likes this.
  5. siddfynch

    siddfynch New Member

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    Yes, I plan to stagger the seams. If each seam still les a little through though, does it still reduce the r value of each layer?

    I'll try the latex mortar and fiberglass mesh tape. Any reason it won't work on Hardibacker?
  6. siddfynch

    siddfynch New Member

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    My plan was to leave an airspace between the two layers of cement board, which is why I'm wondering if the seams will erode my R value. From ground up, the hearth layers will be:

    3/4" plywood
    1/2" layer of backerboard
    2 1/4" brick "joists", mortared into place, creating streaks of air space between joists
    1/2" layer of backerboard
    1/4" slate

    This'll result in an R-Value well in excess of the 0.4 I need for installing the Progress Hybrid (with ash lip). As long as the seams aren't a problem.

    Anyone see a problem with my design? I might still replace the brick with metal studs.
  7. begreen

    begreen Mooderator Staff Member

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    I don't like the airgap. Cement board does not have a lot of freespan strength. This is a very heavy stove. My concern is a leg punching through the weak backerboard.

    I would recommend skipping the air space. It weakens the structure considerably. Instead directly screw the sheets to the plywood using their special screws every 8". If you want additional insulation, switch to Durock Next Gen which has a higher R value of .78 per inch.
  8. siddfynch

    siddfynch New Member

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    Thanks, begreen. My plan was to underlay the actual stove footprint with solid brick (no airgap). This would be roughly 24" by 30". It's the rest of the hearthpad that would have the "joists." Does that eliminate your concern?

    One reason to go with with the brick was to raise the elevation of the pad. The decision to use "joists" once out from under the stove location was to save money, and overall weight on the floor. But, I posed the question to get feedback exactly like this, so let me know if the idea still stinks even with solid brick directly under the stove.
  9. Joful

    Joful Minister of Fire

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    How you gonna get the stove into place without putting its entire weight, and likely that of two men, on some unsupported area? Also, what if you switch stoves someday?

    Why not consider building up with something lightweight, like Micore? Killer R-value.
  10. fishingbum574

    fishingbum574 New Member

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    Greetings,
    New to the Forum but been reading many and gaining significant amount of insight and tips to the whole woodburning in the newer stoves. Was a woodburner back in the 80's with a Buck insert and have been out it for the last 15-20 years or so. Recently got back into it with the install of a Jotul Castine F400.

    Just a suggestion with the Hardi-Board...Take a look at your local lumber yard, not Lowes/Home Depot. They usually have a Wonderboard selection including 4'x8' sheets. That could help reduce the number or eliminate the need to seem your application at all.
    Jack
  11. Waulie

    Waulie Minister of Fire

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    One .5" sheet of NextGen is all you need for R-value for the Progress. If you want a raised hearth, why not just make one out of 2X4's and plywood with NextGen and tile overtop? It will be way more solid, lighter, easier to build, and still meet your hearth requirements. If you want a higher R-value in case of switching stoves some day, you could just add another layer of NextGen, or two, or however many you want.
    PapaDave likes this.
  12. begreen

    begreen Mooderator Staff Member

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    I see no necessary gain by this design, but it introduces possible weight issues. There's no need to make such a heavy hearth. Just use 2x4 studs, plywood, and cement board to raise it.

    +2
  13. siddfynch

    siddfynch New Member

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    Ok gang, thanks for all the assistance. Based on the comments above, I made a couple significant changes:

    1) I went out and bought a sheet of Next Gen (did not know this was made; it's not listed in the R-value listings posted in the Info section);
    2) Bought a 4x8 sheet, which I didn't know was made - no seams!
    3) Scrapped the "brick joist" idea, and went with 2x4 framing, laid sideways.
    4) Since I already had cut a layer of Hardi Backer, I left this in as a bonus layer under the Next Gen. Can't hurt, right?

    So what I now have is 4" of construction material for the raised hearth (surface tiles not added yet), with 2 layers of cement board that should provide an R-Value of 1.02 (.78 for the 1/2 " Next Gen, .24 for the 1/2 " of Hardi). From the bottom:

    1) 3/4" CDX plywood, with a thin layer of cork rolled on to pad the pad.
    2) 1 1/2" of 2x4 lumber, laid sideways; joists are 12" on center, then blocked.
    3) another layer of 3/4 " CDX.
    4) A layer of 1/2" Hardi backer (2 pieces butted together).
    5) A layer of 1/2" Next Gen (full sheet, not butting). This is screwed down thru the prior layer, into the wood. I assume there are no heat transfer problems with this.

    Next steps: add a center piece of tile where the stove will sit, install stove, then finish hearth surface once I can select more tile that matches the stove.

    Hopefully this helps someone else designing a pad. Thanks for the help, and feel fee to weigh in further.
  14. jeff_t

    jeff_t Minister of Fire

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    Sounds better. Next Gen is .78 per inch, though, so .39 for a 1/2" sheet.

    I tiled the entire hearth before moving the stove into place. I laid down some plywood scraps to protect it.
  15. PapaDave

    PapaDave Minister of Fire

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    Not sure why you put the cork in there. Also, as Jeff said, r value of NG is .39/layer.
    Tile the whole thing before putting the stove on there. How big are the tiles anyway?
    You could eliminate the first layer of CDX.
    Have fun. If you have bad knees, or even if they're just getting bad, consider raising the hearth some more. No more kneeling to load, etc.....
  16. mudr

    mudr Member

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    Hi guys,
    Hijacking the thread here. I read somewhere on here there was a debate regarding Durock's "next gen" products usage in a hearth. Was that resolved? Also, if I do micore, next gen, then tile (1.03+.39+.04ish= 1.46ish), would someone give me crap for not getting the 1.5? This would all be on top of 3/4 osb on top of 2X8s.

    Off topic even more. First post, long time lurker. Thanks for all the info you have already given me.
  17. jeff_t

    jeff_t Minister of Fire

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    Round up? ;lol

    I like plywood vs OSB. It's only a sheet or two, spend the extra.

    And welcome.
  18. siddfynch

    siddfynch New Member

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    PapaDave, the cork is to protect the wood floor...in case the hearth ever needs to come out.
  19. begreen

    begreen Mooderator Staff Member

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    The bottom layer of CDX is doing nothing but adding weight. Put the cork on the bottom faces of the 2x4's. You have R= .63 by current design. (.39+.24)
  20. PapaDave

    PapaDave Minister of Fire

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    I still think the cork isn't needed, but if it makes you feel better, I guess it won't hurt.
    I'm with begreen.....
    Cork, 2x4's, cdx, whatever depth you need of Next-gen for your r-value, then tile it.
    Isn't the PH somewhere north of 700 lbs.? The hearth isn't going to move unless you fall against it while break dancing....in which case you've got other issues (not including the break dancing).==c
  21. dorkweed

    dorkweed Guest



    When I built my raised hearth; I built a 2x4 frame w/12" centers, covered with 1/2" plywood, then 2 layers of 1/2" Micore, then 1 layer of 1/2 Durock next Gen and then porcelain tile. My hearth exceeds the R value required for my 13NC.
  22. siddfynch

    siddfynch New Member

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    Thanks PapaDave. Even my awesome breakdancing couldn't move it, but earthquake tremors sure could. We had a 5.8 the other day that might've done it. I went ahead and screwed the pad down, even though, like you said, it would take a lot to move the thing. Hopefully we never find out.
  23. PapaDave

    PapaDave Minister of Fire

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    Looking forward to the finished product.
    The PH too.

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