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?
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.
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.