Not enough R value. Sweep's Library - R Value of Hearth Materials Concrete has an R value of 0.095 / 1" Durock Next Gen is 0.78 / 1" (0.39 / 1/2" by the table). If the stove doesn't require a high R value in the hearth, that would definitely be quick and easy solution. Mine required I think R 2 (D2!) 21" of concrete would have sunk my battleship.
I'm with you GS, except for all that concrete.... Air is the best insulator, so unless you plan to Come right off the floor with stacks of durock and such, if you framed the structure up at least on 2x4's you'd have 3 1/2" of air between the underside of this monolith and the subfloor of the home... Even if you needed to allow air to pass thru the hearth framing... Or block up (tastefully) the feet/ pedestal of the stove....
I was looking at that site last nite, hoverwheel, I noticed there was no listing of insulating refractory. A 55# bag of castable Kastolite or Mizzou, both of which are available rated up to 3000F might be quick, affordable addition to the hearth.... $40-65 a bag.....
Good idea Eric. How durable is that stuff though? And, what's the rating on it. I have no experience with it.
Looking for r values... Typically it's rated by temps. As for durability, it would need covered with durock before tiling....
I'm no maths expert, and I know from reading back on some of your posts Kimberly, you've got it going on in that department, so if you go with this and plug the applicable numbers from here (thermal conductivity) might you arrive at the R value? Paul bunion, you're excellent with math- can you chime in on this?
Look at the units section on the wiki, 1 of the metric 'R's is equal to R5.67. The chart for the material lists it as being a little over 1 Metric R. So it's about R5.5. BUT, I don't see how thick it is for that value other than the above numbers for weight are for a cubic meter. I don't think you want a hearth that weighs a metric poop ton.
Some of the light weight insulative cements they have now days is upwards of a R-2 per inch. At that rate a pad of 3.5 inches would be more than adequate.
I never paid much attention in math class and avoided any advanced mathematics and I have absolutely no familiarity with the metric system but Im pretty sure that a metric poop ton is a whole lot heavier than a regular metric ton or even a standard ton that would probably create a sinkhole.
One cubic meter of water weighs 1000kg, or one metric ton. It stands to reason that a metric poop ton would be one cubic meter of poop. Now, I could not find a reference on the weight of poop, but since it sinks, I have to believe it's a little heavier than water. I'll guesstimate one metric poop ton is around 1200kg.
Totally dependent on recent diet. After 7-11 pizza I can guarantee that metric poop ton is a little more.
If a person was to pour a 1.5" pad of lt wt insulative cemento get the r value needed. Let it cure and then pour over it with standard cement. Because the lt wt cement is not as resilient as standard and have a 2" standard cement casing over it and around it to provide strength and support. I am no cement engineer but a hillbilly technician.
Just got to talkin to the new guy at work on the way out. He worked as a cement chemist on his last job. He said no need for the cap like I was thinking. He said properly mixed and poured insulative cement can handle at least 500 psi and fully cured and done right would not be afraid to double it.
Found Roxul Rockboard 80 with this spec: Thermal resistance (R value/1 inch at 75 ºF: 4.1 h ft2 ºF/Btu to ASTM C518 I am not familiar with R ratings; most of time I just see a single numerical value. I can get six pieces of 2 in boards 24X48 inches which will be plenty for a hearth.
I know we got to play by the rules, but that just sounds ridiculous. I can't imagine any stove that would literally require 21" of concrete to be safe. In fact, if you poured a pad say 4" thick or so, I still couldn't imagine this being insufficient for any normal stove. Is 0.095 accurate? Heck when I moved in here the POs had an old no-name smoke dragon sitting on thinner than standard brick. I noticed too that the mortar was cracked in many places (and completely missing in some) and there was no mortar bed underneath, just wood floor. It was some time before I remedied this, yet the only problem I had is when a hot ember shot out of the stove and went clear onto the carpet.
I happen to agree with you and while I make every reasonable attempt to comply with the rules that make actual sense... If you go with concrete, it may not meet code (so you may have trouble with the inspector or the ins co) but I'm pretty confident your house will not burn down because you shortchanged the hearth by using 4" slab instead of 21"
Alright hypothetical situation. Around here none of this stuff is inspected. But lets say an inspection was required. You did actually pour a concrete hearth pad. But you made it 'only' a foot and a half thick. 18". The inspector is going to see a stove sitting on 18" thick of concrete and probably say holly shick! Do you think he would actually verify the thickness and look it up to see if it meets code? Or just check it off and say your nuts. lol. No one would ever do that so this is silly, but just curious. Sorry for the tangent!
