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R-value of concrete plus slate?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by gyrfalcon, Apr 7, 2013.

  1. gyrfalcon

    gyrfalcon Minister of Fire

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    I've bought (but not yet installed) a Hearthstone Homestead, which requires a hearth with an R-value of 2.5. My house has a rather handsome DIY slate hearth for a freestanding stove (iow, no fireplace) the previous owners had built. My Tribute only needs ember protection, so what the hearth was constructed of is only now an issue. (The owners are deceased, so can't ask them.)

    It's a raised hearth about 4" high and opening up one end to have a look revealed about a half inch of concrete on top of 1-inch plywood supported by a grid of 2x2's.

    Obviously, that's not good enough by itself, but I'm wondering what 1/2 inch of slate on top of a 1/2 of concrete adds up to in R value. Anybody know?

    I really don't want to tear up the existing hearth (which is a good deal larger than what's required for the Homestad) and start from scratch, so I need to figure out what I'd need to put on top of it to get to that 2.5.

    What would you do?

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  2. webby3650

    webby3650 Master of Fire

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    R-.75? I think cement board is only R-1 per 1". Unless you have solid masonry, 2 layers of Micore is the only realistic material that will achieve the value that you need over wood. Maybe you could trade the Homestead for a new Heritage, they only require ember protection now.
  3. gyrfalcon

    gyrfalcon Minister of Fire

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    Thanks. That's about what I was afraid of.
    I looked long and hard at the Heritage, but there's just no way to make it work with the very tight clearances I can get in this room, which are hardly a half-inch to spare with the Tribute or Homestead. Why they haven't redone the Homestead design I can't imagine. They probably will next year or something, after it's too late for me.

    I'm totally not clear, either, on the key differences between cement board, micore and Durorock. Got a quick list, by any chance?

    Also, the manual talks about using 1/8-inch of "horizontal air" as an insulating layer, but you can't put a 350-pound stove on a 42-inch wide board of anything over air without supports without it sagging right down through that 18-inch. You'd have to have something to support the top layer ever few inches, I would think. But what do you use to support the top layer that doesn't conduct more heat than the air, and nullify the insulating value of that air where the supports meet the bottom layer? I totally don't get this.
  4. KaptJaq

    KaptJaq Minister of Fire

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    You can view a list of R values here: http://www.chimneysweeponline.com/horvalue.htm

    This table lists the R values for various materials for specific thicknesses. R values are additive, just add the values for each material. An R value of 2.5 is pretty high. You probably need to find some Micor 300 or mineral fiber board to get to 2.5 without too much thickness.

    Most recent discussions I've seen are against stagnant airspace as an insulator. Current thinking is that it should be open at both ends to let the hot air flow out and be replaced with cool air. Your manufacturer would have the final say. If you can use airspace use a non-combustible sheet (Durock?) on the bottom, strips of non-combustible to create the airspace, then another sheet on top. Then you have to top it off with something strong (solid sheet of blue stone?) to support and distribute the weight of the stove. All the time adding the R values to make sure they exceed the stove's requirement. I would probably use some micor and not the airspace.

    KaptJaq
  5. Jon1270

    Jon1270 Minister of Fire

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    Durock is a brand of cement board -- hard and dense, but low R-value. Micore is light and soft, essentially a robust, fireproof ceiling tile. It is high R-value, but fragile. You'll need both. Plan on tearing off the existing stuff down to the plywood, then put down a couple layers of Micore, then cement board (screw through cement board and Micore, into the plywood). Then lay whatever tile or stone you want on top of the cement board.
  6. xman23

    xman23 Minister of Fire

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    I"m curious did you know what the hearth requirements before you bought the stove? I don't think I did when I bought mine. I see so many people here that deal with this issue after they have the stove. I wonder if they would have picked another stove If the sales people more. If some of the manufactures can build stoves that require only a fireproof floor, they all can. I believe my Jotul had two leg sizes. The 6 inch leg I have needed only a fireproof base. What about a bottom stove heat shield? I used a rear heat shield to reduce the rear clearance. Also my single wall pipe has a rear heat shield.
  7. Jon1270

    Jon1270 Minister of Fire

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    This is a good thought; I know some models offer an optional bottom heat shield, so the OP should definitely ask about that.
  8. gyrfalcon

    gyrfalcon Minister of Fire

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    Ah. Thanks. The fog lifts...
    Problem with tearing the existing hearth apart and rebuilding it is that it's way beyond my skills, not to mention tools, so I'd have to hire somebody to do it. Between the cost of the materials (including replacing the lovely wood trim around the exterior) and the labor, I'm getting right up to or maybe over the cost of just saying to heck with it and buying the pre-fab hearthpad for $450 or so and laying on top of what's already there.
  9. gyrfalcon

    gyrfalcon Minister of Fire

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    I did, and it concerned me, but as above, this is the only stove in the universe, as far as I've been able to tell, that fits the clearances --- with double wall flue and rear heat shield --I can give it. And I was hoping (in vain) the existing hearth, which is quite handsome and looks substantial, would turn out to be more substantially built than it is. I have no idea what sort of woodstove the previous owners had, but given where I am in the middle of farm country, odds are it was a pre-epa cast stove, so I guess I'm just lucky they didn't burn the house down. They didn't build all this and the backing on the wall just to roast marshmallows over a toy stove once in a while.

    It's quite maddening that this stove has twice the R requirement of the entire rest of the Hearthstone line when it's otherwise such a perfect size for so many people.
    raybonz likes this.
  10. gyrfalcon

    gyrfalcon Minister of Fire

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    I didn't think to ask, but if the excellent and experienced sales guy at the dealer knew of one, he would have said so, given the amount of time we spent together fussing about the hearth requirements and the Hearthstone brochure doesn't mention one. But a very fast Google turns up numerous mentions of a Homestead bottom heat shield, so I will pursue that further. It sure would make all of this a lot easier...
  11. raybonz

    raybonz Minister of Fire

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    I built my hearth and have never done anything like it before.. Really not too hard to do and more physical than anything. Durock cuts with a utility just like drywall and the layers get screwed and thinset to each other and to the floor. I bought the tile from Best Tile and they will cut the tile for free if needed but I borrowed a coworker's elcheapo saw and it worked fine. If you do more than one layer stagger the seams. Good luck!

    Ray
  12. gyrfalcon

    gyrfalcon Minister of Fire

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    Thanks for that link. Unfortunately, the best stuff isn't even for sale anywhere retail, as far as I can figure out. I think I'm going to give in and get the (&*%^^% prefab hearth pad.
  13. gyrfalcon

    gyrfalcon Minister of Fire

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    OK, after googling and reading, it appears that the optional bottom heat shield is only for the hearth-mounted Homestead, which reduces the floor R-value from a whopping 6.6 to the same 2.5 my freestanding model requires. I'll call the dealer tomorrow, but I'm thinking the freestanding model comes with the bottom heat shield already. :mad:
  14. begreen

    begreen Mooderator Staff Member

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    The Homestead comes either with the short 4" leg hearth model or the freestanding 6" leg model. The 6" leg model needs R=2.5 and the 4" leg model needs the whopping R=6.6.
  15. gyrfalcon

    gyrfalcon Minister of Fire

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    I take your word for it that that's the only difference, though the Hearthstone materials I have make no mention of 6-inch legs making a difference in the hearth model. It just says 6.6, period. I was asked when I bought the stove for a freestanding install whether I wanted 4 or 6-inch legs, and again, no mention of it making a difference in the hearth requirements though we'd been chewing over that for a good while by that time.

    There also is apparently a bottom heat shield available as an option for the hearth-mounted stove, which the Hearthstone materials also don't mention but which various posters here and elsewhere say reduces the the R-value needed to 2.5, unless I totally misunderstood or misremembered, which is of course possible.

    I suppose Hearthstone could make all of this even more obscure to the potential customer if they tried really hard...
  16. begreen

    begreen Mooderator Staff Member

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    Last paragraph on page 9 and first on page 10 of the Homestead manual cover the two configurations.
  17. rideau

    rideau Minister of Fire

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    So here's a thought.

    If the hearth model, with 4 inch legs, can have its clearance reduced by that massive factor by having a heat shield installed, and the only difference between the hearth and freestanding model is the leg length, and the freestanding model can have its clearance reduced by using 1/8 inch of horizontal air as an insulator, why the heck can't you mount the bottom heat shield that is used on the 4 inch leg model on the free standing model to get your 1/8 inch of horizontal air and reduce your clearance? Bottom heat shields definitely are effective in reducing heat radiated to the hearth pad.

    And if for some reason you cannot use their heat shield (I cannot imagine why you couldn't), then why couldn't you have one made for the stove that gives you the anywhere up to 2 inches of horizontal air?

    Another option, it would seem to me, if you need a horizontal air space that is larger than the area that would be covered by a heat shield on the stove, would be to construct a slate platform on top of your hearth, with simple slate or other stone pillars supporting it at required distances. Just like you'd build a tower out of blocks, Maybe put 1 to 2 inch stone or slate supports every six inches or whatever distance is required for support around the edges of the platform, and in staggered positions underneath throughout the platform? To get your required air space without tearing stuff up? It could look nice, just a two layer slate installation with a slight airspace. I might make the airspace high enough to easily run a flat cleaning tool under it...maybe 1 1/2 inches high?
  18. gyrfalcon

    gyrfalcon Minister of Fire

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    Well, that's an interesting idea I will think about. Thank you. (how come this place is the only place that has practical advice about this stuff??!) I have to check with the dealer tomorrow, but I'm pretty sure the model I'm getting does come with a heat shield already on it. At least I assume Hearthstone isn't so stupid as to market and sell the damn thing with such a high R-value requirement and make no mention of the availability of a heat shield to reduce it to something more reasonable. The chart of all their stoves in their brochure is just striking from largest to smallest stoves... 1.0, ember protection, 1.0, 1.2, 6.6, 2.5, 0.6, 1, 1 ,the middle two being the two Homesteads. Sheesh. If they can make a stove as big as the Mansfield that only requires ember protection, what the &*#$?
  19. rideau

    rideau Minister of Fire

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    Looks to me as if the Heritage is 3/8 inch wider than the Homestead but has 1 inch lower clearance to combustibles, for essentially the same or slightly smaller total clearance to combustibles in width? It does require a wider hearth, but also a less deep one, and only requires ember protection.,

    Maybe I'm missing something.
  20. Joful

    Joful Minister of Fire

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    Nonsense! You have the entire Hearth.com community here to guide you, and the required tools are not expensive. I have watched many here build raised hearths with success, even when it was clear many of them had zero prior experience. Figure out what you want it to look like, describe what you want here, and we'll guide you thru the process. You can do this.
    raybonz likes this.
  21. gyrfalcon

    gyrfalcon Minister of Fire

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    Rideau, my email notification sent me a much longer post of yours beginning with the words "What exactly are the clearances you have?" which isn't here on the thread for some reason.

    In any case, the problem is in the front, from the glass to the edge of the hearth. (It's not in a corner, btw, but against the front wall of the house, maybe 10 or 12 feet from the center of a long front room.) The rear and side clearances are fine. But the hearth is 43 inches deep and the Heritage 6 inches deeper than the Tribute or Homestead, meaning the front edge of the hearth would be about 8 inches shy of the hearth requirement for the stove. The hearth is raised about 4 inches, so extending it wouldn't be trivial, and the room itself is long but not very wide and that extra eight inches looms large. In fact, the hearth isn't even quite deep enough for the Tribute, but we fudged a couple of inches on the install..

    The flue goes straight up about 6 feet, then makes a 90-degree turn to go through the top of the wall to the outside, where it connects directly to the bottom of the exterior flue (stainless, insulated, class A, etc.)

    There's enough clearance at the back with the heat shield so that the stove could be moved back 8 inches with no problem, if it weren't for the flue. Is it possible to put that precise a bend into the interior flue to accommodate that? Edit: Adding that the draft is superb verging on too much, so an extra bend shouldn't cause any draft problems.

    And given the crappy construction of the hearth, I'd *still* have to put a hearth pad on top of it, though it would only need an R-value of 1 or even a little less. (The Heritage does require more than ember protection.)

    So add the extra cost of the Heritage over the Homestead (not huge, but maybe $300 or $400), plus redoing the flue plus the hearth pad, and I'm back to thinking it makes more sense to just get the Homestead and that $450 super-duper hearth pad and have it extend a couple of inches past the existing hearth in the front and be done with it.

    The room itself is quite wonderful, but the front is broken by three windows and a door, each narrow end has a window, and the inside wall is broken by two wide doorways on either side, a very wide interior unglassed "window" into the kitchen and then the staircase smack in the middle. The flue is literally in the only place I could have put it, squeezed in between the masonry chimney that vents the boiler and a window. The previous owners just vented their stove through the same chimney as the boiler.
  22. begreen

    begreen Mooderator Staff Member

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    Would it be possible to put a less expensive hearth extension pad at floor level and go with the Heritage? As always, a picture here is worth a thousand words. There may be other options we are not visualizing.
  23. gyrfalcon

    gyrfalcon Minister of Fire

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    OIC, you mean end up with a two-level hearth? I never even thought of that possibility. There's very elegant 2-inch-wide wood trim around the hearth, and I assume that would have to come off and be replaced by... what? Metal flashing of some sort?
    (I do know pix are most helpful, but the chip in the camera needs to be either reformatted, which I'd have to learn how to do, or a new one purchased, which I'm reluctant to spend very sparse cash on, and I need new batteries before I can even do that! In any case, the nearest source is 20 miles away, so not one of those run out to the corner store and back in 5 minutes deals...)
  24. begreen

    begreen Mooderator Staff Member

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    There may be practical options but it is hard to say what will be best without seeing what you are seeing. Put some batteries in the camera, go to the options menu or use the trashcan icon and delete some of the pics on the chip to make room to take some more pics.
  25. gyrfalcon

    gyrfalcon Minister of Fire

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    Done that already, but it's not full, it actually needs reformatting because it's eating pix.

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