Here's one for the Forum scientists

[thechimneysweep]

A site visitor just turned me on to a potential hearth material called Rescor (view at (broken link removed) ).

My questions: would the values listed under "Thermal Conductivity (BTU-in. /Hr. Ft2 ºF)" be the R-values per inch of the variations of this material listed? And if so, what temperature would be applicable? It seems to me a hearth will never get hotter than 500f, so I would guess that would be the line to use.

 

[madison]

I think that you are interpreting it correctly, here is a link that seems to concur, I found it by searching "NFPA hearth conductivity" : (broken link removed to http://www.brimfieldma.org/Pages/BrimfieldMA_Building/Floor) Protection.pdf

perusing the website of the mfg ((broken link removed to http://www.cotr.com/vo/cotr/)) and found some interesting products. One was a product for stovepipe sealing called 2300°F Resbond™ 907GF Adhesive & Sealant. Also there were other adhesives for their products that I found interesting.

Regarding conductivity, I'm fairly certain that the conductivity of a material would change with the wave lenght of the radiation. So the conductivity will increase with shorter wave lengths. Or as shown in the table, as the temperature increases (wavelength decreases) conductivity increases.

Conversion web site: http://www.unitconversion.org/unit_converter/thermal-conductivity.html

I would guess more importantly that the hearth would have to resist the accumulative effects of heat over time, and not just the highest potential temperature. Though both qualities would be desirable.

The product literature does seem to imply that it would work, though the manufactured size of the material may be a limiting factor ...18" x 24"

Thanks for the info. I hope i did not confuse the masses.

 

[fossil]

Thermal conductivity and thermal conductance are measures of a material's ability to conduct heat. Thermal resistivity and thermal resistance are measures of a material's ability to resist the transfer of heat. Of course, they are related...in fact they are reciprocal to one another provided you've very carefully made sure the units you're dealing with are the same. When we're talking about hearth installations, we're usually concerned with a material's insulative properties...which means we're looking for low conductance/high resistance to heat flow. The terms conductance and conductivity are related, but not the same thing. Likewise for thermal resistivity and thermal resistance. Depends on whether the area of the surface is included, or just the thickness of the material. There are also differences in the use of the terminology between Europe and the US. But, in any case, no...anything labeled "Thermal Conductivity" is not the same thing we refer to as R-value...although you might be able to get there from here. I'd direct the question to the material's manufacturer and see what they have to say. Rick

 

[don_faithful]

I have trouble with conductance, too. I think mathematically, resistance R values can be added to get the total R factor. K is the symbol normally used for thermal conductivity while C is for thermal conductance. C = K / T where T is the thickness. In other words, thermal conductivity K depends in part on the thickness of material T multiplied by the termal conductance C. Or K = C x T. Not to confuse the issue, R = 1 / C. So you can have a bunch of R values: R1 = 1 / C1, R2 = 1 / C2, R3 = 1 / C3 and so on. So Rt = R1 + R2 + R3. The manufacturer might in the instructions specify a certain R value for the floor pad or hearth extension for instance. Anyways, I am not an authority on this subject. I got these details from a really old fireplace manual containing examples of the calculations.

 

[don_faithful]

Just incase I completed screwed up my explanation, I'll pass along the example in the book. The idea here is that we need the R value. If we don't have the R value, we can get it from the C value. If we don't have the C value, we can get it from the K and T values.

So for a layer of marble 3/4" thick, C = 11 (conductivity) / 0.75 (thickness) = 14.66
So R = 1 / 14.66 (this being C) = 0.068 the R factor

For a layer for Micore 300 3/8" thick, C = 0.458 (conductivity) / 0.375 (thickness) = 1.22
So R = 1 / 1.22 (this being C) = 0.82 the R factor

Conveniently we can add R factors whereas we could not add conductivity, I think. So, Rt = 0.068 + 0.82 = 0.89

I don't believe K is constant over the heat spectrum. The amount of heat energy that is conducted depends on the temperature differential. The calculations above are based on K values at 75 degrees mean. Rescor 360 according to your PDF has a K of only 0.45 at 500 degrees. I'm not sure how to adjust for the different temperature basis. That is something I would like to really like to know.

 

[begreen]

Rescor seems like it might make a better firebrick replacement material on the interior of the stove ala the experiments precaud has been doing. For a hearth, I'd be concerned it might be too brittle unless used as a substrate? Maybe not? Any idea what the product costs?

 

[thechimneysweep]

So for a layer of marble 3/4” thick, C = 11 (conductivity) / 0.75 (thickness) = 14.66
So R = 1 / 14.66 (this being C) = 0.068 the R factor

For a layer for Micore 300 3/8” thick, C = 0.458 (conductivity) / 0.375 (thickness) = 1.22
So R = 1 / 1.22 (this being C) = 0.82 the R factor

Trying to follow the same math shown above:
The chart for Rescor 360 Standard Grade shows thermal conductivity (K) at 500f of 0.45 / inch.
So C = 0.45 / 1, or .45
So would R = 1 / .45, or 2.22?

 

[labrador]

Now I know why I hated math