Thermophysical Properties Research Laboratory, Inc.
Phone : 765-463-1581
How To Calculate an R-value
We typically do not report R factor values in the reports that are issued. To calculate a R value from a
thermal conductivity value you would need to do the following.
Lets say you have a thin layer (.010 inch) of insulated paint that has a thermal conductivity value of .006 W/cm*K.
In order to calculate a R insulation value you will need to convert to BTU units so you would multiple the
0.006 W/cm*K value by 693.81 to convert it to BTU inch /(hr ft2 F) which gives you 4.1628 BTU inch /( hr ft2 F).
You would then take the reciprocal of the result because the thermal resistance is the reciprocal of thermal
conductance. The equation would be 1.0 / 4.1628 BTU inch /(hr ft2 F) to get a value of .240 which you would then
multiply by the thickness of material being used, in this case 0.010 inches, to get an R insulation value of .0024
thermal resistance. A ONE INCH thick layer of this material would have a R insulation value of 0.24 thermal
resistance. A TEN INCH thick layer of this material would have a R insulation value of 2.40 thermal resistance.
This is very different from the 15.0+ R insulation values common for a foam block or insulation batting.
The REFLECTION of the heat is not used to calculate a R value as defined by the United States Department of
Commerce. The reflection of heat has nothing to do with the real insulation (R) value. An example of reflection,
if you paint a house in the hot desert with a thin layer of white paint it will reflect the heat from the direct
sun by reflecting the suns energy but that thin layer of paint will not insulate the house from the cold at night.
If you expect to paint your building with a thin coat of insulating paint and cut your heating bill in the winter
you will be very surprised if your bill remains roughly the same. In the summer it will reflect some of the direct
suns rays away from the building making it easier to cool but other white paints will also do this.
Copyright 2011 TPRL, Inc.
3080 Kent Avenue
West Lafayette, IN 47906
Last updated 1/5/2012