Insulation Ratings: R Factor, K Factor & C Factor

The R Factor, K Factor and C Factor of Insulation

InsulationInsulation terms can be quite confusing to anyone outside the industry. If you’ve ever bought insulation for your house, you know that insulation with a high R factor is better. But what, exactly, does that mean? Did you know that the R Factor depends on other factors?

When it comes to buying more specific insulation products, like removable insulation jackets for hot pipes, understanding the particulars of the three measures of insulation is key. In order to understand the well-known R factor it’s important to understand the factors upon which it relies, the K factor and C factor.

If you are seeking out the formulas to calculate these factors, check out our R, C & K Factor Formula Conversion Table that lists all the formulas discussed in this article. For more information, read on!


I Want
K Factor C Factor R Factor
I Have K Factor C=K-factor/in. of thickness R= in. of thickness/K-factor
C Factor K=C-factor in. of thickness R=1/C-factor
R Factor K=in. of thickness / R-factor C-1/R-factor
None of
the Above
K=BTU-in / hr – ft² – °F C=BTU/(hr · ft · °F) R=h · ft² · °F/BTU

The K Factor of Insulation

What is the K Factor of Insulation?

The K factor of insulation represents the material’s thermal conductivity or ability to conduct heat. Usually, insulation materials have a K Factor of less than one.  The lower the K factor, the better the insulation. The textbook definition of the K factor is “The time rate of steady heat flow through a unit area of homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.”  That’s a mouthful.

Simplified, the K factor is the measure of heat that passes through one square foot of material that is one inch thick in an hour.

How Do I Calculate the K Factor of Insulation?

If R factor is unknown, the formula to calculate the K factor of insulation is:

K factor = BTU-in / hr – ft– °F
British Thermal Unit-Inch Per Square Foot Per Hour Per Fahrenheit Degree

If R factor is know, this easier formula can be used to calculate the K factor:

K factor = inches of thickness / R Factor

How is the K Factor of Insulation Reported?

K factors are reported at one or many mean temperatures. The mean temperature is the average of the sum of the hottest and coldest surface temperatures which the insulation material is exposed to.

Put more simply, the testing apparatus that determines the K factor of an insulation material places a sample of the material between two plates, hot & cold, and the average of the surface temperatures of those two plates equals the mean temperature. Here is an example of an insulation material’s K factor report:

k factor report

via Nomaco Insulation

Notice that as the mean temperature rises, so does the K factor. It’s important to observe the K factor & mean temperature when comparing insulation.


The C Factor of Insulation

What is the C Factor of Insulation?

The C factor stands for Thermal Conductance Factor. The C factor, like the K factor, is a rate of heat transfer through a material. The lower the C factor, the better the insulating properties of the material. It is the quantity of heat that passes through a foot of insulation material.

The C factor is dependent upon the thickness of the insulation. The thicker the insulation is, the lower the C factor will be and thus the better the material will be at insulating. This is one of the main differences between the K factor and C factor, because generally the thickness of an insulation material will not affect its K factor.

How Do I Calculate the C Factor of Insulation?

If the K factor is unknown, the formula to calculate the C factor of insulation is:

 Btus/hour per square foot per degree F of temperature difference

If the K factor is known, this easier formula can be used:

C factor = K factor / inches of thickness

The R Factor

What is the R Factor of Insulation?

pink fiberglass

image by Jack Amick

The R factor pulls together all of the information of the other factors and makes it easy to judge the effectiveness of insulating material. The R factor of insulation can be found most easily of the insulation factors discussed, and it is the most popular indicator of a material’s insulator properties. Generally it is listed on an insulation material’s label. The R factor stands for thermal resistance. The higher the R factor, the better the insulation.

The textbook definition for R Factor is: the quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow through a unit area. Aren’t textbooks supposed to be helpful?

To simplify, the R factor is a variable value that measures the ability of a material to block heat rather than radiate it. The variable is the C factor, which is dependent upon the thickness of the material. It is the opposition to the flow of heat energy.

How Do I Calculate the R Factor of Insulation?

There are a few formulas to calculate the R factor of insulation, depending on if your K factor and C factor are known. If they are unknown, you can use this formula:

degrees F times square feet of area times hours of time per Btus of heat flow

If your K factor and C factor are known, you can use these formulas which may be easier to use:

 R-factor = 1 / C-factor
 R-factor = thickness in inches / K-factor

Keep in mind that these factors are specific to the materials being measured. For instance, if you take two pieces of batting that are rated at R 11 and put them together, you won’t get R 22 coverage. Understanding the ins and outs of the factors that help describe how effective insulation material is will go a long way to helping make the buying process easier.


About Thermaxx Jackets

Thermaxx Jackets was founded over 25 years ago with a single purpose: to save energy when traditional stay-in-place insulation is not practical. Combining expertise in heat loss, wireless monitoring, insulation design, and several other disciplines, we’ve become the #1 provider of removable insulation jackets and covers.
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13 Responses to Insulation Ratings: R Factor, K Factor & C Factor

  1. syed mohabat ali says:

    i need to know the k factor of our cooling tunnel ,we have ambient of 26 deg c and we need temp inside tunnel 6 deg c,what should be the k factor and what should be the dencity of pu foam ?

  2. Will jones says:

    Hi Syed,

    It seems you are asking about a tunnel rather than a pipe or valve. Is that correct?

  3. Heather says:

    “Insulation Ratings: Calculating R Factor, K Factor
    & C Factor” ended up being genuinely compelling and
    insightful! Within the present day society that’s tricky to deliver.
    Thx, Earlene

  4. Luis M. Rivera says:


    I bought a freezer with 100% rigid Urethane insulation. It was poured-in-place with density of 2.2 Lbs/cu ft, K = 0.125, U=0.025, R=40. This insulation got wet with 1 gallon of water in a 8ftx4ft panel. Since Urethane is a hygroscopic material, does this material with these specifications absorb large amount or small amount of water?

    • Todd Cook says:

      Cured polyurethane foam absorbs very little water, and the foam structure should be closed cell, so that water cannot infiltrate through the insulation either. I would expect the water to make very little difference in the performance of the insulation.

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  6. C G Baker says:

    Have 2″ fiberglass wrap around duct. ….
    1″ sound liner in other ducts …..
    Question , what is the “K” factor difference between these two insulations

    • Will Jones says:

      The units for “K” are Btu-in/hr-ft²-°F (How many BTU’s will pass through a 1” thick piece, 12”x 12” square in one hour per degree of temperature differential. In order to compare “K” factors we must look at each different type of material at the same temperature. Most data I’ve seen uses 75F for the temperature constant.
      Duct wrap: 0.3 Btu-in/hr-ft²-°F
      Gyproc Sound bloc: 0.35 Btu-in/hr-ft²-°F
      John Mansfield Equipment Spinglass: 0.23 Btu-in/hr-ft²-°F
      Owens Corning 700 series sound board : 0.24 Btu-in/hr-ft²-°F
      The lower the “K” factor the better the thermal insulating properties of the material.

  7. A Sawyer says:

    Hello. Have a question for you.

    Our specifications call for insulating a pipe on a domestic hot water system with 1″ of insulation with a conductivity range of .22 to .28 btu-inch/hour/square foot. They do not state what type of material is required for the 1″ of insulation nor do they state the required R-value. Is there a way I can determine an R-value of insulation that will comply with the conductivity range?

    • Will Jones says:

      What they are asking for is fiberglass pipe insulation which has a conductivity range as shown in your post. The lower range (.22) would be for a needled fiberglass with no binders, the upper range would be for regular pipe insulation that has binders to hold the fibers and mold the fibers into a round shape. The binders aid in conductivity so the rating is higher. With K factors or C factors the lower the better. If you want to know the R value simply divide the thickness (in this case 1″) by the K factor (.25 average) which gives R-4. R values is not a good indicator of high temperature and high performance insulation and should not be used. The engineer did a good job with the specification by using a K or C factor.

      • A Sawyer says:

        Many thanks and good explanation. Was hoping there was a way to reduce the overall thickness of the insulation as it complicates installation in the tight cavities we are having to maintain.

        Thanks again.

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  9. information is very useful. Which will help to take correct decision of material selection.
    I am thankful to the author..

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