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How to stack layers of cement board (e.g. durock)?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by morphic, Feb 14, 2013.

  1. morphic

    morphic New Member

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    First time poster, but I've been reading the forum like mad for several days ;)

    I'm planning on getting the Englander/Summer's Heat 13-NC. The installation manual says that it requires an R value of '2'. I calculate that, using cement board alone, I would need stack eight 1/2" sheets to achieve that R rating. Assuming I need to put thinset between each layer and I need to wait 24-hours for the thinset to cure before putting on the next layer, this will take up to 8 calendar days!!!

    Is there anyway to build it faster? Can I skip letting the thinset cure between layers? - or can I skip the thinset between all but the top/bottom layers?

    PS: as a side question, do people like the 13-NC around here?

    Thanks!

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  2. mellow

    mellow Resident Stove Connoisseur

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  3. bag of hammers

    bag of hammers Minister of Fire

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    depending on how elevated you want / need your hearth to be, maybe you have other / better options IMHO. For example, if it were me, I would think about maybe building up a couple inches using metal furring strips or studs, set on narrow centers (for lots of support), with roxul or similar fill in all the spaces - a sort of framed / insulated hearth pad. Just thinking out loud (I only needed ember protection so I have durock over plywood for a base, with tiles on top of that). If I recall, there are also other insulating materials that have been discussed here (Micore?).
  4. ddahlgren

    ddahlgren Feeling the Heat

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    Sounds like they are tranferring their lack of engineering to your door step..
  5. Highbeam

    Highbeam Minister of Fire

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    Yeah, it's pretty lame to have to build this much R-value into your hearth. I had to get a 1.2 for my heritage and it sucked. Ended up using an "air space" made by building a 3.5" thick box out of steel studs and skinning that with 2 layers of durock and tile.

    As to your question. Since you are using thinset AND screws to hold the layers together, I would not wait for the thinset to cure between layers. The screws are maintaining your gap whether you like it or not. Of course, stagger the seams, use the durock tape and the durock screws. Is your floor stout enough to support all of that weight?
  6. ddahlgren

    ddahlgren Feeling the Heat

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    It is more than pretty lame it is lack of concern for use for the task involved.
  7. raybonz

    raybonz Minister of Fire

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    If memory serves me correct Durock NexGen has an R-value of .39 per 1/2" so 5 sheets is pretty close but to be extra safe you need 6 sheets.. If you decide to build with the Durock make sure you thinset between all layers and the subfloor and stagger your seams if you will have seams.. The hearth requirements of the NC-13 are ridiculous Englander should address this issue.. I wouldn't buy a stove that requires such a high value as there are many stoves that are ember protection only with no R value requirements or a very low R value that's easily achieved..

    Good Luck,
    Ray
  8. TheBaron

    TheBaron New Member

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    Mine required R2.9 I believe. We used Micore (2 layers), then two layers of Durock.

    Screwed and thinset between each layer. Drier mix than usual as the Micore absorbs some water.

    Some folks on here recommend against putting thinset against Micore however the contractor installing mine did not share that opinion. Likely good advice if you have a very flat surface you are mating to.
  9. begreen

    begreen Mooderator Staff Member

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    Use no thinset for the micore layer, just screw through the Durock to hold it captive. If you use Micore 300 it will give you R=1.03 for a 1/2" thick sheet, but if you can get Micore 160 it goes up to R=1.27. Add a couple layers of Durock Next Gen on top of 1/2" Micore 160 and you have a sandwich value of R=2.05.
  10. Highbeam

    Highbeam Minister of Fire

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    I was unable to source micore in my area. If you can find it then that will make life mch easier.
  11. Jon1270

    Jon1270 Minister of Fire

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  12. morphic

    morphic New Member

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    Thanks everyone for the quick replies!

    At first I was discouraged to get micore because I couldn't find it in my area, but I see I can order it online - four 2'x4' 1/2 sheets of 300 (not 160) for $95.50 from Sears for $100.29 ($95 + $0.01 S/H + $4.78 tax), which is about $10 cheaper than anywhere else. Here's the link to it: Drolet Micore-300 Mineral Fiber Board 48" X 24", 4-Pack. Stacking two layers, I'd have 48"x48" @ R=2.06. With that, I don't need any more insulation, but would still need a CBU, so one layer of 1/2" durock. I'd need two sheets to cover 48"x48", which would be about $20, so all said, about $120 for R2.32 in 1.5". Using 8 layers of Durock, I would've needed a total of 10 sheets, which would be about $101 (inc. tax) for R2.08 in 4" - so the micore solution is $19 more expensive but 2.5" less thick! Then there's the weight. The all-durock solution would be 324 pounds, where the micore solution would be just 74 pounds (249 pounds less!). Using micore is totally worth it :)

    Thanks again!
  13. morphic

    morphic New Member

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    So what's the prevailing forum wisdom? Is thinset needed between any of the following layers in my stack?
    • micore-durock (tiling surface)
    • micore-micore (insulation layer)
    • plywood-micore (base)
    Of coarse I'll use thinset on top of the durock for the tiles, it's the other layers I'm unsure about...
  14. KaptJaq

    KaptJaq Minister of Fire

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    I would not put thinset on the micor layers. USG data sheets show that micor absorbs the water from the thinset and that affects its properties. I personally would not put thinset anywhere except between the tiles and the cement board. Use screws to connect the other layers.

    KaptJaq
  15. Jon1270

    Jon1270 Minister of Fire

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    Thinset between substrate layers is there to fill in irregularities, not for adhesion. In most tile work, which doesn't use Micore, thinset forgives a lot of sins. But Micore is, according to the manufacturer, not tolerant of moisture. When I built my hearth, I made up for the absence of thinset by being perfectionist about the flatness of all the mating surfaces, and by torquing all the fasteners very consistently using a rechargeable drill's torque adjustment. It's only been a few months, but the tile job looks great so far.
  16. tekguy

    tekguy Feeling the Heat

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    i ended up getting it for free... its used in partition walls for office furniture like at any large company, I lucked out and got partition walls that where going to be trashed and pulled Micore 300 sheets out of the middle
  17. EatenByLimestone

    EatenByLimestone Minister of Fire

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    One hearth I built used micore. I stacked 2 layers and screwed them down using plaster washers. Hardie board went down next and was screwed down. Then came a layer of thinset and the tile.

    Another one was for a cabin. I just stacked 3 layers of Durock next gen. They aren't even screwed down, but the wife has not told me how she wants it to be finished. I'll make it pretty after the boss gives me her instructions.

    Matt
    raybonz likes this.

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