Most people have heard of RValues, which are used for rating common building materials such as fiberglass insulation and glass. However, many texts which cover stoves and fireplaces use KValues instead of RValues. Although the two are somewhat related, there are differences.
RValue: The higher the RValue, the better the insulating properties of the subject materials. RValues are most often used to express the thermal resistance (ability to stop heat flow) of a building wall, ceiling or floor. Because of this, most RValues are calculated at normal temperatures of approx. 75 F. RValues are easy to add together so calculating the total RValue of a wall is simply done by adding the values for the sheetrock, insulation, sheathing and siding.
Kvalue is a measure of heat conductivity of a particular material. Specifically, it is the measure of the amount of heat, in BTUs per hour, that will be transmitted through one square foot of material that is one inch thick to cause a temperature change of one degree Fahrenheit from one side of the material to the other. The lower the Kvalue for a material, the better it insulates. If the Kvalue of the material is known, the Rvalue per inch can be determined by dividing 1 by the Kvalue (Rvalue per inch = 1/K value). The LOWER a KValue, the better its performance as an insulator.
R or K values have nothing to do with whether a material is flame proof, flame resistant or combustible. Styrofoam, cork, wood and polyester are just some examples of materials which are good insulators but will burn or smoke dangerously when exposed to excess heat.
Technical  For those who desire to calculate their own K or R values, please use the following formulas:
1. R value can be calculated by dividing the thickness by the K value.
For US calculations, we use inches as the unit of measurement.
“In the inchpound units, thermal resistance is measured in degrees F times square feet of area times hours of time per Btus of heat flow.”
Rvalue = thickness / Kvalue
2. K value is the inverse of the RValue. If one is known, the other can be calculated.
“units of Btuinch/hour per square foot per degree F”
Thickness/k value = R value
or:
Divide the inches of thickness by R.
k= inches of thickness / R
KValue Example: A wood stove may call for a floor which has a K factor of 1 or less. A product such as Micore 300 Board from USG has a KValue of approx .43 per inch. Therefore a 1/2” thickness of this board would have a KValue of .86, which meets the requirement of our example stove.
RValue Example: A stove or fireplace may call for an floor with an RValue of 1.5. The same board above is rated as having an RValue of 2.33 for a one inch thickness. Therefore, 3/4” of the Micore 300 Board would meet the specifications for this stove.
Summary: R and K values are related, but K is the value commonly used for specifying materials for use with stoves and fireplaces. Be sure that your choice of insulating material for high temperature applications is noncombustible.
With K values, the lower value is a better insulator. With R Values, the highest number is better.
For low profile hearths, it is best to use manufactured materials such as Micore and Cement Board (Durock, Wonderboard, etc.) as these will allow hearth thicknesses of from 1/4” to 2” with most stoves and fireplaces. Most other common building materials will require at least 3” of thickness and usually much more.
See the information and additional links at our companion article below:
https://www.hearth.com/talk/threads/woodstovehearthconstruction.147784/
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Example of Hearth Calculations  this is for a Hearth requirement of approx R=1.15 (figures taken from Ceramic Tile manufacturers trade association)
The assembly that we will evaluate is a layer of Micore 230 and a layer of ½” UtilACrete. The first step is to convert the k values of the materials in question into R so that we may add them up and determine if they will provide the necessary insulation value required by the manufacturer.
Micore 230 has a k value12 of .43 so –
1 divided by k = 2.32 times the thickness .375 (3/8”) = 0.87
UtilACrete (cement tile backer board) has a k value of 1.6 so – 1 divided by k = .625 times the thickness .5 (1/2”) = 0.3125
Add the values together 0.87 plus 0.3125 = 1.1825 This R value is an acceptable assembly.
What if we decide to use only one material? In this example, only UtilACrete cement board?
We could use the published RValue of UtilACrete13 which is .31 in the ½” material and add them up to the value of the minimum required which is R=1.16
1.16 divided by .31 = 3.74 This assembly would require 3.74 layers of ½” UtilACrete to reach the necessary Rvalue required. Obviously, you would have to round up to the next layer, which would mean that you would have two inches of UtilACrete.
WOOD  K VALUES  WHAT DOES IT ALL MEAN?
Post in 'Articles' started by webfish, Oct 17, 2015.
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