There is a Chart labeled Approximate "R Value" of Walls, Windows, Ceiling & Floor in one of the responses to the question How can I measure the r-value of a wall? What is the source of this table?

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  • 1
    Web search will find many versions of this, most commonly from companies which sell insulation-related products. Ultimately, it comes from standardized measurement procedures.
    – keshlam
    May 5, 2015 at 0:38
  • Thanks keshlam. Do you know what organization developed these standardized measurement procedures and/or where I could find those measurement procedures?
    – Benjamin
    May 5, 2015 at 18:51
  • There's an ASTM standards document covering this but I don't think they originated the measurement. Not sure who did. It's related fairly directly to the k- value and u-value measurements since they're all describing thermal conductivity but scaling it differently.
    – keshlam
    May 5, 2015 at 19:46

2 Answers 2


Journal of Civil Engineering Research
p-ISSN: 2163-2316 e-ISSN: 2163-2340
2014; 4(2A): 42-50


To determine the "R value" of a wall, insulated window or ceiling requires three simple temperature measurements:

  1. The "air" or ambient room temperature.
  2. The inside surface temperature of an exterior wall.
  3. 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.

Figure 14


R-value is actually a unit of measure for thermal resistance, just as meters are a unit of measure for length.

The physical property of an insulator's thermal resistance is quantified by the ratio of the temperature difference and the heat transfer per unit area per unit time (aka, heat flux).

So your units would be

  • Kelvin x Meters^2 / Watts for SI units
  • Hours x Feet^2 x °Fahrenheit / British Thermal Units for freedom units

Note there's a conversion factor of 5.678263 when going from Imperial to SI measurments.

  • 1
    This is true, but unlike length, heat flux is not so straightforward to measure. It requires an elaborate temperature-controlled environment on both sides of the insulation assembly to be tested. It's not like your Average Joe can whip out a measuring tape. Also, thermal resistance is not actually constant across all temperatures... fiberglass insulation is about 20% more effective at 32 ºF than at 100 ºF. And some types of rigid foam insulation get worse at colder temperatures, not better. So the testing procedure is important.
    – Hank
    Sep 9, 2015 at 22:59
  • Wow, this is good. I learned something today.
    – Lee Sam
    Mar 17, 2017 at 4:51

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