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Rigid foam insulation in Hearth pad construction

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Jakub Dudek, Nov 14, 2012.

  1. Jakub Dudek

    Jakub Dudek New Member

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    Hi All,

    My first post here. I've been researching construction of a hearthpad for a hearthstone (homestead) stove i am about to acquire. As you may know depending on the setup, the hearth pad can require quite a high R value (6.6). I've been reading online quite a bit in the past few days and it's become clear that it is difficult to attain such values without a hearth pad that is rather thick.

    Then, browsing home depot, i came across rigid insulating foam, such as supre TUFF foam insulator and or owens coarning foamular. (home depot links embedded). These materials have R values upwards of 5 for an inch of thickness.

    In all my reading over the past few days, I've not seen any one mention these materials for constructing hearth pads. Now I suppose there may be a good reason for that :) so I apologize if this is a silly question, but is there a reason why this stuff should not be used as one of the layers in the hearth pad? And why would that be?

    Thanks very much for your wisdom!

    -Jakub

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  2. velvetfoot

    velvetfoot Minister of Fire

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    It melts.
  3. Augie

    Augie Feeling the Heat

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  4. begreen

    begreen Mooderator Staff Member

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    An air space filled with Roxul batt insulation is a possible solution here. It is a mineral wool and completely non-combustible. I believe Lowes carries it, not sure about HD.
  5. Seasoned Oak

    Seasoned Oak Minister of Fire

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    I guess the earth pad reuirements are in case something(embers) fall out of your stove. It sure cant be cuz the stove is hot on the bottom, mine is always very cold.
  6. Sprinter

    Sprinter Minister of Fire

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  7. begreen

    begreen Mooderator Staff Member

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  8. ozzy73

    ozzy73 Member

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    Jakub, i would defenetly stay away from the rigid stuff. We insulated our basement with this + R14 batts on the walls.
    The product is great but it is higly flamable and toxic.

    Roxul, Micore / Cement board is the way to go here.
  9. bag of hammers

    bag of hammers Minister of Fire

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    +1 on Roxul. HD in Canada carries it, seen it in a couple other building supply stores. They have a "safe and sound" version - essentially the same as their regular "comfort batt" insulation but in a more dense 3" batt. They also make a semi-rigid board (firestop) that comes in a 1" x 16" x 48" sheet. Not sure how easy it would be to get hold of just a couple pieces of that, but that would probably be great for a hearth if you didn't want to raise it too much.
  10. Jakub Dudek

    Jakub Dudek New Member

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    Thanks everyone for the answers. Rigid insulation is clearly not the way to go.

    The homestead i will have has the 4 inch legs, however it itself will be sitting on a proper fireplace hearth. That brings me to page 14 of the user manual as far as setup is concerned. All I need is the extension in front to be 2.5. My hearth is flush with the floor, so to not make it look funny, I wanted to make a 2.5 hearth to cover the entire surface... The stove would then be on a hearth pad which would be on the fireplace hearth.

    My challenges are that I only have 2 inches of play. Raise anymore than that and the stove exhaust won't fit under the fireplace, which is exactly 24" high. So I have to get to 2.5 in ~1.75 inches.

    Roxul sounds great, but 3.5 inches is too thick. If i could cut it across into an 1 inch piece, the r value would still be ok, but I'm wondering how it would not sag under the wight of cement board and almost 400 pounds stove.

    I see air gaps are good too (r-0.9 for 1/8 of an inch), though i have no idea how to introduce air gaps between sheets of cement safely. Any ideas there?

    Finally, micore seems very difficult to find, though i found one place online that has micore 300.

    there's always the store solution, the shop say they have a 6.6 hearth pad that's 1.25 inch thick (how that's possible i don't know). but that's 500$. I'd rather not go that route if at all possible.

    Ideas welcome and thanks!

    -Jakub
  11. bag of hammers

    bag of hammers Minister of Fire

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    Hi Jakub. I think a previous post (begreen)mentioned steel studs with roxul - basically framing the hearth with metal to support the hearth top layer,with insulation set between metal studs. Furring strips might also work? Just spit-balling there, as I've used metal furring strips as spacers for heat shields on the walls.
  12. Wolfy

    Wolfy New Member

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    Check out my installation;)
  13. begreen

    begreen Mooderator Staff Member

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    If you are just building the hearth extension at R= 2.5 then Micore or Fiberfrax will do the trick. Search around, it's also used as acoustic sound deadening in ceilings, walls and cubicle dividers.
  14. rijim

    rijim Member

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    Not sure what you have for a stove; if you haven't already done so, contact them to see if they offer a bottom heat shield. Bottom shields can reduce the requirements for the hearth.
  15. velvetfoot

    velvetfoot Minister of Fire

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    A local building supply store ordered the Micore for me.
  16. Wolfy

    Wolfy New Member

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    The tec. Data says not to exceed 85 f / 70% h. ? Not being a pain,but won't the hearth get hotter then that ?
  17. begreen

    begreen Mooderator Staff Member

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    That limitation is unrelated to hearth insulation concerns. It's about avoiding high humidity. The example given might read clearer if it said 70%RH@85F. I'm wondering if this is possibly related to mold concerns?

    "Exposure to excessive or continuous moisture should be avoided. Installation and occupancy conditions not to exceed 85 °F/70% RH."

    Give them a call if you need product info: 800 (874-4968)
  18. Dakotas Dad

    Dakotas Dad Minister of Fire

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    It looks like you could go with steel studs, turned flat, then rockwool batt in between/inside. (split the batt) It gets an r value around 3 per inch.. you would be 1.5 inches thick.Use wonderboard or durarock under your tile or whatever to get up to the finished floor height you need..

    Are you figuring to get under your lintel with a standard transition (oval to round adapter), or the close clearance adapter?

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