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Hearth redesign questions for new insert

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by noce, Jan 21, 2013.

  1. noce

    noce New Member

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    I'll be posting some info & questions on a plan for a hearth extension--based on LOTS of good intel i see in this forum, but i have a quick question that i hope some of you who have worked with Micore might have an answer to:

    --Could 1/2" Micore compress to 3/8" under 1/4" durock + thinset + tile?

    (the inside edge of the micore would butt up the current hearth material raising 3/8" above the hardwood floor the micore would lie on. I would like the Micore to compress down enough for the same piece of durock to extend from current hearth over the Micore...)

    thx!

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  2. begreen

    begreen Mooderator Staff Member

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    No, I don't think so. Though I would be concerned that 1/4"cement board will flex too much on the soft micore. That could lead to cracked grout.
  3. noce

    noce New Member

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    does your flex worry lessen much if it were 1/2" board?

    i was considering a few small and well-placed shims to screw the board down tight with, but that would screw up the entire R-value (requirement = 1.0).
  4. begreen

    begreen Mooderator Staff Member

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    Yes, 1/2" is what's recommended. It should be screwed down through the micore at regular intervals. I think every 8" is what is recommended.
    noce likes this.
  5. noce

    noce New Member

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    Anyone familiar with Fibrefrax (rigidity + strength to underlay hearth)?

    i have seen some references to both in the same sentence enough on the forum to *almost* assume so. but i am hoping not...

    we are considering to extend the current hearth by layering new on top of it, but the surrounding wood floor is about 3/8" lower. the extension needs R>=1.0. i was hoping to just lay a sheet of durock across current hearth and extend it the required 10" we need, and have enough support from compressed Micore or Fibrefrax under the extended portion.

    (response to my separate post suggests that Micore will be too squishy to support the durock firmly.) and 1/2" of squished (to 3/8"? separate thread on that question suggest NOT)
  6. Jon1270

    Jon1270 Minister of Fire

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    No, you're not going to compress Micore this way; it's not that soft. If it were soft enough to do that then you wouldn't be able to tile over it. Compressing it would wreck the r-value anyhow so there's no point.

    I would remove the hardwood and build up from the subfloor. Even better, remove the original hearth material and have both micore and cbu cover the whole area.
  7. TheBaron

    TheBaron New Member

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    The hearth extension we are building right now is similar in concept, but a bit thicker (bed of thinset, 2 sheets of micor, and 2 sheets of cement board, screwed at regular intervals as bg mentioned)

    I'll attempt to attach some pictures.

    The micor didn't compress at all in this instance.

    Attached Files:

    noce likes this.
  8. begreen

    begreen Mooderator Staff Member

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    It's coming out beautifully Baron.
  9. begreen

    begreen Mooderator Staff Member

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    I merged the posts. Don't depend on compressing the insulation. If you did that it would lose its insulation value.

    Would it be possible to remove the flooring down to the sub-floor in the area of the hearth extension? That could get you an additional 1/2 or more depending on the flooring.
  10. noce

    noce New Member

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    baron-that pic on the left helped alot here on the home front where the boss is wondering, "you're going to tear up our living room floor to do what?"

    BeG--thx for merging. cutting the wood is NOT ruled out, but when trying something straightforward with the kitchen floor a couple decades ago, the layer-removal opened up a "rabbit's hole", and i am not the handyman to make it work on the fly... may come back to that if i can't get a level finished surface across current hearth to extension area that can be crack-worry free. ...

    Baron - did you have significant difference in height of the prior hearth above the wood flooring before you began?
    Baron - did your research suggest anything particular about the tiles you chose for under the box back in the fireplace? simple ceramics? (why not soapstone?)

    following your lead, maybe:
    OPTION A
    layer # 1 - thinset on the wood only to get nearly level with the hearth
    layer # 2 - 1/2" micore over thinset+ original hearth (+ back into fireplace?)
    layer # 3 - 1/2" durock
    layer #4 - approx 1/4" thinset
    layer #5 - approx 1/4" tile => raises hearth 1.5" (i can spare up to 2-1/8" max without screwing up clearance to combustible top facing.)
    **fireplace - 1/2" stones /tiles of some sort subbing for the last two layers

    OPTION B
    maybe a 1" slab of stone for layers #3-5?

    thx 2 any comments!
  11. begreen

    begreen Mooderator Staff Member

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    Can you inspect the subfloor from underneath the hearth to see what shape it's in? If it's good, that is going to give you the most clean look. If not, I would put the 1/4" thick durock under the micore instead of thinset.

    What insert is going in here?
  12. noce

    noce New Member

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    BeG - good question about looking from below... i have these acoustical-looking tiles hung as basement ceiling, and i have never taken one out (or replaced it!) probably a little "project" by itself for me to pull and replace. so, no, i have not seen subfloor from below.

    i am attaching jpegs here (finally) showing what i am trying to do with an Osburn 2200i.

    ideally, the solution addresses the roughly 3/8" difference in height b/w the wood and hearth surface.

    also looking for interesting options to level/underlay the insert box in the fireplace, once the hearth (and extension) are raised.

    i hope the pertinent specs i keyed in are legible in pics..else i can retype.

    Slide1.JPG Slide2.JPG
  13. begreen

    begreen Mooderator Staff Member

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    It looks like this is not just a heart extension, but a hearth remake. Is the intent to remove the existing tile on the hearth?

    Inside of the fireplace if the adjusters on the stove can't raise it enough, you can put non-combustible support back there. It could be metal studs on their side, hat metal, 1.5" brick, firebrick, etc. Then put a sheet of metal on top to make sliding in and out easy.
  14. noce

    noce New Member

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    the hope is to simply layer material on top of the current heart (as the baron appeared to), and avoid tear up or out.

    i like the sheet metal slide, if access is going to be an issue for maintenance etc. adding it to the list for installer discussion. - thx.

    for brick underlay in the fireplace itself... i don't know how hot the insert box gets on the outside and whether significant benefit will accrue from warming up something like soapstone bricks underneath it, but i am looking around for what others have done--even if the raisers will lift the box a full 1-1/2".
  15. TheBaron

    TheBaron New Member

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    Good evening,

    Took me a while to find the old pictures (prior to putting the hearth on this past week). Attached is a pic of the old flush hearth (same tile as the new hearth, what can I say... I liked it). The new insert is now in, and we have done a few small burns getting hotter with the second one. Waiting on the trim kit (they 'forgot' it) and to finish the border tile.

    So for our install, the first step was spreading out thinset over the area where the board was going to go down (to level out any low spots), then two layers of Micor (with thinset and screws between each layer), then Durock (again with thinset and screws). Top layer we used regular mortar and grout for the tiles.

    Tiles we went with were natural slate, which do absorb some of the heat and will transfer and hopefully store some of the energy created by the fire. Also, it looks good with the stone surround of the fireplace.

    Hope this helps, I can probably dig up more photos if you have a particular question.

    Attached Files:

  16. Jon1270

    Jon1270 Minister of Fire

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    FWIW, when I rebuilt my hearth this past fall I asked a lot of advice over on the John Bridge tile forums, and one of the moderators there put me in touch with a USG (manufacturer of Micore) representative who warned that Micore is vulnerable to moisture, and that even the water in an application of thinset could cause problems. I realize that installing Micore this way is fairly common, but as far as USG is concerned it's a totally non-approved, off-the-grid practice. The rep I corresponded with characterized Micore as basically glorified ceiling tile, and a sort of legacy product that they don't sell much of or put significant resources into anymore. Unfortunately he wasn't specific about what water does to the product, but it's probably a good idea to minimize the amount of thinset you use in direct contact with it.

    With my insert, relatively little heat is radiated from the bottom of the firebox. At the moment my stovetop temp is about 560F, but the bottom of the air jacket is only ~130F.

    The levelers on the back of my Osburn consist of 1/2"-13 screws, with ordinary hex nuts welded to the sides of the insert near the back corners. The provided screws have small, square heads (small because there's no clearance for a typical bolt with a larger hex-head). I replaced those screws with hex cap screws (Allen head), so that I could use a ratchet with a long extension and a universal joint fitting to adjust the levelers after the insert was slid back into the fireplace; mine was a tight fit too, and there was absolutely no room for wrenches back there. Anyhow, 1/2"-13 hex cap screws are a little hard to find, but you can get them in a variety of lengths if you need a larger range of adjustment.

    Another thought -- make the hearth deeper than the minimum. I held mine right at 18", but would add another 6" if I had it to do over. The stove throws a lot of radiant heat through the glass when it's really cranking, and the surface temp of the first few strips of hardwood beyond the hearth can hit 145F. That's not hot enough to damage the finish or cause me any safety worries, but the MC of those few strips of flooring has been driven down enough to make them shrink, opening visible gaps between the strips; tile would be better there. It would also be nice to have a bit more space there for tools and such.
  17. begreen

    begreen Mooderator Staff Member

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    Good post Jon, thanks. Moisture from the thinset being sucked into the Micore was my concern. I see no need for this step.
  18. Jon1270

    Jon1270 Minister of Fire

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    In most tile installations (i.e. non-Micore) the thinset is a way to forgive irregularities in the subfloor or subsequent layers, ensuring there are no voids. I skipped the thinset around the Micore, but I was perfectionist about making sure all the mating surfaces were very clean and flat. I drilled all my screw holes, then picked up the layers and vacuumed out the drilling dust before screwing anything down. I tightened all the screws to the same torque, more or less, as if I was replacing a head gasket. I've been walking on the tile and burning in the stove since October and it still looks good so far.
  19. TheBaron

    TheBaron New Member

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    Interesting information about the moisture. I had the hearth work (the 4 layers) done by the fireplace contractor, and I'm tackling the tile on my own. They used the first layer more as a leveling step and it was a fairly dry mix, I didn't notice any sign of damage or noticeable moisture infiltration into the micore (at the edges it seemed ok). I suppose one wouldn't want to put weight onto the sandwich of materials until things had totally cured/dried out.

  20. noce

    noce New Member

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    good intel--thx.

    do you suppose the problem is eliminated if use thinset in contact with micore ONLY on the hardwood to level BELOW the micore, presumably letting it dry very well? i suppose there is latent moisture even in dry thinset, but, again, ok?

    then i'd put durock directly on top of that micore, with thinset on top of the durock, then tile...

    my unit will fit in according to this pic below--TIGHT too. (solid line is current fireplace wall)
    Slide3.JPG


    Yowza! another good piece to know. i need about 10" MORE for the minimum extension in front of the bay windows of the 2200i. 16" more may be tough in this room (the hearth is already 17-1/2"), but i will squeak the extra inches that i can.
  21. noce

    noce New Member

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    sry 4 multi-post--BARON--anothr useful photo. i appreciate this.
  22. Jon1270

    Jon1270 Minister of Fire

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    I wouldn't (didn't) worry about latent moisture in cementatious products under the Micore. I'd be more concerned about movement in the hardwood underneath it.
  23. Jon1270

    Jon1270 Minister of Fire

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    Also, look for opportunities to insulate around the insert. I didn't, and mine is losing a lot of heat unnecessarily. I will be pulling it all apart and reinstalling when the weather improves.

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