What is the Source of the Approximate "R Value" Chart?
Journal of Civil Engineering Research
p-ISSN: 2163-2316    e-ISSN: 2163-2340
2014;  4(2A): 42-50
doi:10.5923/c.jce.201401.09

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  To determine the "R value" of a wall, insulated window or ceiling
  requires three simple temperature measurements:
  
  
  The "air" or ambient room temperature.
  The inside surface temperature of an exterior wall.
  The outside surface temperature of the same exterior wall.
  
  
  The difference of the interior and exterior wall temperatures is
  calculated and this is labeled as "Temperature Difference, Interior to
  Exterior Wall". Now the difference between the "air" or ambient and
  the interior wall temperature is taken and this is labeled as
  "Temperature Difference, Air to Interior Wall". The interior to
  exterior difference is taken and the corresponding point is selected
  on the bottom of the graph given below. The air to interior difference
  is selected on the vertical axis of graph. Curve is chosen from these
  two points.