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How thick should a concrete hearth pad be?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Coreywh, Jan 24, 2013.

  1. Coreywh

    Coreywh New Member

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    I want to make a concrete hearth pad to go directly on a wood floor. I am trying to figure out how thick it needs to be. I have an older jotul 118 and it calls for 3/8 inch asbestos millboard. I am also thinking about adding vermiculite to cut down on weight. But, I would like to know how thick it should be with just concrete and wire mesh for reinforcement.
    Thanks

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  2. pen

    pen There are some who call me...mod. Staff Member

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    If I remember correctly 3/8 mill is supposed to be something like 0.45 for an r-value. Concrete is listed for 0.08 r-value per inch. That means, you're looking at 5 5/8 inch of concrete to get that r-value.

    At the end of the day, there are much better ways to make a compliant hearth pad. Also, there are ones for a fair price at places like tractor supply or Lowes (if they are still in stock).

    pen
  3. Coreywh

    Coreywh New Member

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    What about adding vermiculite. I heard that if you add enough (10:1) you can make the concrete 20 times less conductive. My hearth pad needs to be 40" X 60". I am having a hard time finding that size. My girlfriend hates the way that 2 put together looks.
  4. pen

    pen There are some who call me...mod. Staff Member

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    How about just a couple of layers of durock? 1/2 inch is .39 r-value each (you'd need 2 layers of that) or 5/8 (if you can find it easily) is .49 for an r-value itself, then tile over it.

    BTW, it needs to be durock for those values by name. Things like permabase are not appropriate due to combustible foam beads inside them.

    I have no experience with adding vermiculite or perlite to concrete, sorry. If you are bound and determined to use concrete, perhaps this site could help http://www.johnbridge.com/vbulletin/index.php ? Their expertise is with tile, so I'd figure they'd have concrete knowledge as well perhaps.

    pen
  5. Coreywh

    Coreywh New Member

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    I called durrock and they told be that their product was only good up to 200 degrees. My stove has 8 inch legs. is that suitable.
  6. pen

    pen There are some who call me...mod. Staff Member

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  7. pen

    pen There are some who call me...mod. Staff Member

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    Here's another bit of info from durock publishing

    http://www.usg.com/rc/system-catalogs/durock-cement-board-system-guide-en-SA932.pdf

    It's also listed as passing the E136 test for noncombustibility. If you find the requirements for that, it's wayyyyy above 200.

    I just can't figure how all their most current data sheets disagree with their customer service guys.

    pen
  8. Coreywh

    Coreywh New Member

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    Thanks- I will use durock.
    thanks again-
    Corey
  9. mtcox

    mtcox Member

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    I have a 3.5 inch concrete hearth with a layer of Micore and Durarock. I used the concrete as a heat sink. Once it heats up, it keeps warm for 6-8 hours after the fire dies out.
  10. pen

    pen There are some who call me...mod. Staff Member

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    Called durock and and received an answer that made sense.

    Basically, they told me that the Next Gen durock won't burn, period. So it is non-combustible. However, they don't recommend it for applications that exceed 175 degrees because that exceeds the curing temperature of the next-gen product. As such, while it will never burn, it could potentially crumble/disintegrate over time.

    I just used a straight piece of durock as a heat shield at a buddies cabin next to a double barrel wood stove. The sides of which will glow red from occasion and I'm certain the durock will see over 175. I'm curious to see how it will hold up over time.

    With this info, I'd personally would still be willing to use the stuff for a hearth pad, but may consider using something else for a wall shield in a home, especially if tile were to be placed over it.

    pen
  11. jankdc

    jankdc New Member

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    Do you consider hardibacker better for the hearth pad?
  12. Jon1270

    Jon1270 Minister of Fire

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    Hmm, the chart of materials in my Osburn manual lists Durock as being R-0.52 per inch.
  13. ddahlgren

    ddahlgren Feeling the Heat

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    I used 2 1/2 inch layers or durock and tiled the top layer with some left over kitchen til and trimmed it with some left over veneer bricks around the edges so the whole cost for me was about 100 for the 4 x 8 sheet of durock. For behind the stove i put up some metal 2 x 3 gong vertical and covered that with some 16 x 48 inch sheets of .090 brushed aluminum with the last 6 inches of the ends bent to 30 degrees. It is very light easily meets code and cost the same as a 4 x 8 of durock. Most metal supply houses have a 'drop zone' that has cut pieces left over from another job and i bought 6 of them for around 40 and the studs were just a few bucks each at Home Depot. Had a local metal fab shop bend them while I waited for 40 bucks in the beer money kitty. The thing took less than an hour to put up and if you like the look of brushed aluminum as I do it looks real sharp. best thing is instead of soaking heat up it reflects it back into the room.. .
  14. pen

    pen There are some who call me...mod. Staff Member

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  15. Jon1270

    Jon1270 Minister of Fire

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    Your link is busted, but I found the document. Looks like Osburn got it wrong.
  16. ddahlgren

    ddahlgren Feeling the Heat

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    I doubled checked mine and it is actually 2 layers of 5/8 rather than 2 layers of 1/2 as I had stated previously
  17. begreen

    begreen Mooderator Staff Member

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    .52 is the R value for an inch thickness of the original Durock cement board which is no longer available. The same R value applies to 1" thickness of Wonderboard. A 1/2" sheet is R=.26. Durock NextGen is R=.39/ half-inch and R=.49 for five-eighths thickness.
  18. ddahlgren

    ddahlgren Feeling the Heat

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    When did the Next Gen come out?
  19. begreen

    begreen Mooderator Staff Member

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    A few years ago I think.
  20. ddahlgren

    ddahlgren Feeling the Heat

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    Thanks that means the piece I bought 6 months ago from a very busy lumber yard no doubt has the Next Gen.. In all honesty the stove i have only called for a sheet of .026 metal under the stove. I am assuming because of the fire brick in the bottom of the firebox then a layer of steel the an 1 1/4 inch gap for combustion air and the air for the convective enclosure around the stove then on a 12 inch pedestal off the floor coupled with a tiny 1.3 cu ft fire box. The manual made no metion if on a combustible or not combustible floor. If they are correct i think I may be wildly oversized in pad protection but better than waking up to the smoke alarm...
  21. begreen

    begreen Mooderator Staff Member

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    It probably was NextGen. Old inventory of the original Durock seems to have disappeared last year. It would have been lighter than 100% cement board and clearly marked NextGen on each sheet. There is no problem with exceeding specs. Personally I like the idea a lot. Our hearth is oversized and insulated for our stove too.

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