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Old 10-01-2008, 06:59 AM
Tim F's Avatar
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Default Question about insulation

Hi all,

I'm looking at a castable insulation to go under my hearth and wondering if anyone can tell me if my maths is in the ballpark. The insulation that I am looking at has a K value of 0.18 at 500C, and I understand that R = 1/K, so the R value per inch = 1/0.18 = 5.5.

The FB plans call for 4 inches of vermiculite concrete mixture. Google seems to be telling me that loose vermiculite has an R value of around 2.1, and I assume it would be slightly lower with the cement mixed in - call it 2? So 4 inches of vermcrete has an R of around 8.

So is my thinking right that 2 inches of this castable insulation will be more effective than 4 inches of vermcrete?

I'm very open to the idea that I don't know what I'm talking about here
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Old 10-02-2008, 08:56 AM
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Default Re: Question about insulation

I'm no expert on this, but I'll jump in. In general, members of this board (myself included) have had good luck with 2 inches of insulation board instead of the 4-inch vermiculite concrete layer. I think James said that 2 inches of the board that FornoBravo sells is a slightly better insulator than 4" of vermi-concrete. The more insulation the better, of course.

Several months ago, I compared the thermal conductivity values for several different types of boards that people were using, and they ranged from about 0.69 to 1.3 (in BTU in/hr deg.F sq ft) at 540 deg.C (1200 F). The products included Insblok-19, SuperIsol, FB Board, Marinite, and Kaowool board (Note: Kaowool loses strength when wet).

As a concrete example of conductivity, I was told that with a constant heat of 1100 degrees F on one side of Insblok-19 (with a thermal conductivity of 0.84 at 1000 degrees F), the other side will be about 200 degrees F.

I hope that helps!
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Last edited by dbhansen; 10-02-2008 at 02:42 PM.
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Old 10-03-2008, 03:27 AM
Tim F's Avatar
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Default Re: Question about insulation

Sure does!
So the thermal conductivity value you are talking about looks like it's in english units? If so, english units = 1.73K. That puts the castable I'm looking at (Moral Coolcast) 0.27 EU and the board (V-Lite 100 AL) 0.20. That seems like it's a lot lower than the range you mentioned - have I found a really good product, or just made a mistake in my maths somewhere?
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Old 10-03-2008, 07:43 AM
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Default Re: Question about insulation

Yes, English units. Sorry, I don't know the conversion, but here is a website that converts thermal conductivity numbers.

I think most people building wood-fired ovens (at least in the U.S.) stay away from the castable insulating materials and use vermiculite or perlite + concrete instead, probably due to cost and availability. Some have said that castables are "fussy" to work with, and it can be difficult to get the mixture right. Using vermiculite is a "tried and true" method for these ovens, and usually inexpensive.

When looking at insulating board, be sure to check on the other properties besides conductivity, such as density, compressive strength, resistance to water damage, and shrinkage.
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