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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. Highbeam

    Highbeam Minister of Fire

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    I am building the pad for my Hearthstone and it requires a 1.2 Rvalue. I want the pad as low as possible since it is in a small room and I want max chimney height. Subfloor is wood and reinforced but the stove is heavy and many of the hearth building materials are also heavy. The pad will be 6 feet wide by 4 feet deep and tiled. I like extra big hearths vs. premanufactured ones.

    Micore you say? Great idea...

    So I have contacted the stove shops, the manufacturers of hearth pads, the building supply outfits, and even the listed distributors for micore in my state. Unless I want to buy a pallet of it then there is no micore available in the state. Most folks, even stove shops, think I am crazy. "Micore? What's that?" I have read through several threads and finding the stuff seems to be tough everywhere but impossible here.

    As I go down the list of accepted materials I find gypsum or plaster board. I assume that sheetrock with its combustible paper does not qualify as gypsum board in this application. So is there a more common name for the gypsum board? The R value is only 0.45 but that is twice as high as durock at 0.20. So it is not out of line.

    6 layers of durock will get heavy, tall, and just bulky to deal with but it might be my only choice.

    Does anyone have any ideas or even better would be a puget sound source for micore?

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  2. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Go to an office furniture bankruptcy sale. Nobody ever believes me but the primary use of Micore is the substrate for cubicle walls. Most of the time they have to pay people to haul off office cubicles. It deadens sounds and meets the fire codes.
  3. jqgs214

    jqgs214 Minister of Fire

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    many "cement boards" are gypsm
  4. jtp10181

    jtp10181 Minister of Fire

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    There are some fireside stores in California, they could definetly get you some "HX4" board which I belive is very similar to micore 300. We get it in bundles of 5, but they should be using it for thier installs and have it on hand already. I think the sheets are somewhere near 20x60.

    http://www.fireside.com/

    They might be willing to ship them out to you.
  5. Highbeam

    Highbeam Minister of Fire

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    I emailed a few times now with the USG folks that make micore and they are not surprised that I am unable to find it instate. Out of state or even mail order is not on the path of least resistance so I am to the point now of looking at alternatives. The charts also list "horizontal still air" as being dang effective. So I assume the metal stud hearths detailed on this site took advantage of the air space for the R-value. Does this air space need be be ventilated such as a rear heat shield does? Or can it be sealed up? I am trying to avoid a huge stack of durock for fear that my floors aren't up to the task. This pad will be 4x6' and the beams underneath don't benefit much from distributing the weight.

    Can I seal up my hearth pad "air gap"?

    Oh and are the metal studs available in sizes other than 2x4? are they structurally sound laying on the ground so that they are only 1.5" tall? Thanks for your help on this folks
  6. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    It seems one could use ceramic fiber blanket insulation between the studs to gain a bunch of R-value also. It is available at kiln shops, on eBay and such.
  7. babalu87

    babalu87 New Member

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    Too bad some of you werent closer
    I have plenty of Micore left over, I guess I take for granted how easy it is for me to get it locally.
    When I picked up my sheet there was a broken corner on it so the warehouse worker gave me another piece with a sample cut out of one corner. Ended up with pretty much two full sheets and have enough to do two more hearth pads.
  8. begreen

    begreen Mooderator Staff Member

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  9. webbie

    webbie Seasoned Moderator Staff Member

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    Bab, what type of supply house did you find yours in?

    I have found that drywall supply houses often have strange types of hi-temp insulation, although I am not certain about Micore.

    Does McMaster-Carr sell it - they are mail-order.....

    http://www.mcmaster.com/

    Search for insulation or millboard........

    But you can probably find something similar local.

    Does HearthStone sell a bottom heat shield? (additional to any that might exist on it already)
  10. Highbeam

    Highbeam Minister of Fire

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    Don's installation inspired me to all but settle on an air gap with metal studs as my main R-value getter. Per the hearthstone manual- horizontal air space gives 0.92 per 1/8". The key thing that I need to be reassured of is that the air space can be static and does not need to be ventilated in any way. Just as in Don's install, I want to tile in the skirt of the elevated hearth.

    So (3.5"/0.125) * 0.92 = 25.76 R value for the air gap. Plus 0.2 for a single sheet of durock. Heck just round this up to R value of 26.

    Go big or go home.
  11. Highbeam

    Highbeam Minister of Fire

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    No additional heat shield available for the bottom of the heritage. There is a cast iron ashpan there already. I bought the rear heat shield to get my clearance to combustibles out back down to 7".
  12. webbie

    webbie Seasoned Moderator Staff Member

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    With that much excess R factor, I would feel safe without ventilation - but I might use two sheets of the durarock just for making the hearth firm. One layer of it spanned over studs would be subject to movement (and cracking of the tile, etc.).

    Strange, but I have seen UL approved stoveboards that are thin sheet metal over one layer of fuzz (like veggie or sound board). I have also seen UL approved mats which use gypsum and then a layer of ceramic tile on top.

    In terms of rules of thumb, keep this in mind. The high requirement for that stove probably related to the ash pan castings that hang down. When they test the stove with firebrands (dry sticks stapled crosswise), you can bet that there is no layer of ash on the grates and also that the ashpan is probably empty. That means the intense heat radiates downward without anything stopping it.

    In the real world, things work differently. You will be using cordword, there will be ash on the grates and chances are that the ashpan will usually have some ash in it. I would be very surprised if that hearth hit over 200 degrees on top - whereas in testing it probably hit 500 degrees plus (maybe more).

    I did once start a fire with an improper hearth - way back in WV at the same time we burnt the shed down with the sauna stove. We needed some heat and the house had a fireplace....but the hardware store tin stove would not fit into the opening to vent. So we took the legs off of it, and sat the bottom of the oval sheet metal stove directly on a stoveboard which was on the small hearth and projected onto to hardwood floor that was flush with it.

    Well, a few days of firing the stove and we started smelling something burning.......you guessed it, we ripped the stove out and the floor was charring! Of course, this was ZERO clearance with the sheet metal of the stove sitting on an el cheap stoveboard........yes, an airspace works wonders.
  13. Highbeam

    Highbeam Minister of Fire

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    It's amazing that we keep using or caring about micore when an air space is so much more efficient (both in thickness and cost) at giving us R-value. The hearthstone folks did confirm with me that the air space does NOT need to be ventilated and that supports are allowed within the air space whether metal studs or any other noncombustible material that will support the weight. This would indicate that if a guy were to cut 2" wide strips of 1/2" thick durock, lay them down on the subfloor in rows on 12 or 16" centers and then laid a full sheet of durock on top of that then an air space has been created with an R value of 3.68.

    I still think I'll use the studs for an elevated hearth since I can use the 3.5" air space to route my outside air plumbing.
  14. elkimmeg

    elkimmeg Guest

    correction Highbeam airspace is a bout .97 per inch not .92 per 1/8" Ever hear of high hats 1.5" instead of a full 3.5" on a 2/4 metal stud you coild keep a lower profile and still get a decent
    R -value Pm me if you need a more detailed description
  15. wg_bent

    wg_bent Minister of Fire

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    Well, that's how mine is designed. 3 layers of Durock for a hearth surface over metal studs with about a 3" ventilated air space. Covered with tile. Actually in front of the stove at the end of the hearth I can feel the thickness of the durock and tile, and the tile gets quite warm, the underside of the hearth is always cool. personally I think that's a good strategy.
  16. Gooserider

    Gooserider Mod Emeritus

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    Wierd coincidence - You are the second person today asking about Micore, and I just posted a link to one of the US distributors with a state by state dealer list over in the other thread earlier this morning.

    Gooserider
  17. webbie

    webbie Seasoned Moderator Staff Member

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    How about a short micore entry in the wiki - goose, can you do it......and put that link?
  18. mayhem

    mayhem Minister of Fire

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    Nearest Micore dealer/distributor to me when I built my pad a couple months ago was about an hour's drive. No local building supply stores had any idea what kind of product to sell me so I finally just went shopping at HD...the durarock brand cement board is rated at r=.13 per 1/4 inch thickness. I wound up using 6 layers under my stove, then tiled over it.
  19. colsmith

    colsmith New Member

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    I am surprised that the stove stores near you don't have Micore. Ask what they use to build hearths when they install stoves, I can't believe they use all those thick layers of Durock all the time. Micore was recommended to us by one of the stove estimate dudes who came to our house. We couldn't find it anyplace, so we asked at the store where we were buying the stove. They don't technically have it for sale, but they use it when THEY build hearths. They agreed to sell us one 4 x 8 ft. sheet of Micore 300 for about $48. It was easier than putting down all the sheets of durock we would have needed, so we went for it. With all that air space it seems like you have less support under the (always heavy) stove, I suppose that is why people aren't all keen on it Mind you I was a computer engineer not a structural engineer, so what do I know.

    Maybe you asked the wrong person at the stove stores, can you speak to one of their installers and ask again? The regular salesguy didn't know anything about Micore at our local fireplace store, but the installer was quite knowledgeable (it is his family's business.)
  20. Highbeam

    Highbeam Minister of Fire

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    I asked the three local stove shops and after them telling me they had never heard of micore, I would tell them why I wanted it, and then they would tell me that they use premanufactured pads. None of them actually make the pads, maybe it's a dying art, not as profitable as the pads. The installer that two of the three stove shops use also had never heard of it.

    Hearthstone's owner's manual which is online for the heritage clearly calls out 0.92 per 1/8" of horizontal still air. 0.97 per inch also works for me but either Hearthstone has a misprint in a very important part of the manual or they used a different source for their R-calue charts. It wouldn't matter for my install but the inspector would have a hard time looking at my manufacturer's chart and calling BS.

    The air gap can be filled with as much necessary support as is needed. Hearthstone authorized steel studs or other material as needed for support. I figure that I will be careful to provide a steel stud directly beneath the final location of the stove legs.

    I of course had no problems finding the micore distributor list but that doesn't do squat for me when the distributors in my state only sell pallets and are not willing to refer me to retailers. An actual list of retail outlets would be far superior.
  21. Gooserider

    Gooserider Mod Emeritus

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    Will try to write something up, but no guarantees - there's this big party coming up that's getting a lot of my attention right now... :coolgrin:

    I don't know what the quality is on those distributor links - it is just one that I found somewhere and bookmarked, figuring I might need it some day, but I haven't actually tried to see if they are useful yet...

    As to why Micore instead of an air space? I think it depends on the design objectives. Assuming Elk is right about the .97 / INCH value for air space I think Micore still is the champ for the highest R-value per thickness.

    If you are trying to build a hearth that is as close as possible to flush with the existing floor, and you have a stove that requires a high R-value hearth, then I think Micore is the only option that will work as part of the sandwich, Working on the assumption that the floor is typical construction of plank subfloor, 3/4" particle board or ply floor, with carpeting over it, you have about 1-1.25" to work with. Per the Chimney Sweep's
    R-Value Cheat sheet 1/2" of Micore, 1/2" of Durorock or Wonderboard, and 1/4" of ceramic tile will just fit and give a 1.3+ R-value... (You could get more by taking out the subfloor to get down to the bare joists, but Elk says that's a bad idea, not sure why...) I don't see any other options that would get it thinner, or keep the same thickness.

    If you are going for a raised hearth, there is no real need for the Micore, as it doesn't offer any major advantage over the air-space or stack of Durorock approach...

    Gooserider
  22. elkimmeg

    elkimmeg Guest

    Goose one can got to bare joist but they have to replace the load bearing some how I wouls suggest 2/8 2/10 0r 2/12 solid blocking frush with the top of floor joisr between the floor joist spacing. Ususlly this should give you 1.5" to work with for a flush configuration of the floor.

    Of the products out there cement boards I would opt to work with Hardi Backer board every time . It is much easier to work with, easier to cut.
    '
    I think a 3'/5' at Home Depot cost about $11 for the 1/2" thickness.

    Just checked the website R 0.26 per 1/2"
  23. webbie

    webbie Seasoned Moderator Staff Member

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    That would be 4 layers for most hearths...really not too bad. As I mentioned before (Elk, correct me if wrong), this stuff is really not designed for spanning distances on floors with weight so at minimum doubling it up (even if R is reached with air space) might be prudent.

    Elk mentioned the lower profile metal channels in an earlier post - I think these are what I used for the ceiling in my jam room - very low profile and then even have a small lip where you could screw or nail to floor. They have a wide bottom which screws will cut right into (no drilling).

    Available at sheet rock supply specialty places. As I said before, when I visited the local place I was amazed at all the various building materials and insulation that I was previously not familiar with.


    Pic of the channel enclosed - pay no attention to the thing which hangs vertically - that is for ceiling hanging. I assume Elk is talking about using this channel upside down (those little flanges) toward the floor.

    This stuff is dirt cheap, BTW.

    Attached Files:

  24. begreen

    begreen Mooderator Staff Member

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    I've only used Durock brand board because it is 100% cement with a fiberglass backing. Hardibacker board is fire resistant, but contains 10% cellulose which is I assumed is considered combustible? Elk am I in error? The cellulose content is the same reason our tile setter says he never uses HB board, especially in a wet area.
  25. jtp10181

    jtp10181 Minister of Fire

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    About the wet areas....

    "Hardibacker® is a unique, cement based, water resistant substrate for tiling that can be
    used on walls, floors and countertops. Particulary suitable for use around baths, showers and
    in kitchens, it will keep your tiles just where you want them. Stuck hard to the surface. Tried
    and tested in millions of homes worldwide, Hardibacker® tile backerboard has a 10 year
    guarantee, and is easy to cut and install."


    I read someplace, I thought on the hardibacker webpage that it was an approved non-combustible material. But now of course I can't find it... well I found something now.

    "Can I use Hardibacker® Backerboard for fireplaces or barbecues?
    No. While Hardibacker® Backerboard is an interior non-combustible product, it is often used with barbecues. It is not a recommended or a warranted application."


    http://www.jameshardieeu.com/pages.php?pages=faq
    http://www.jameshardieeu.com/pages.php?pages=products&subpage=hbacker

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