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

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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