I found this on finding R-factors:
How to Determine R Value
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.
To find the inside "air" or ambient temperature, measure half way up on an interior wall or a piece of furniture such as a couch or table at about 3 feet above the fllor. Be sure to make your measurement away from any outside influences such as windows
, vents or heating from direct sunlight. Record this temperature on a piece of paper.
For the wall temperature measurements you will need to pick a place along an exterior wall that you can access the same point on either side of the wall. Ideally, neither side of the wall should be in direct sunlight. Make the exterior measurement and record the temperature. Now make the interior measurement directly opposite the point the exterior reading was taken and record the temperature.
Take the difference (subtract the smaller number from the larger number) of the interior and exterior wall temperatures and label this "Temperature Difference, Interior to Exterior Wall". Now go back and take the difference between the "air" or ambient and the interior wall temperature and label this "Temperature Difference, Air to Interior Wall".
Now, take the interior to exterior difference and find the corresponding point on the bottom of the graph on the last page of this booklet. Move up from that point to the line that is closest to the value you found for the air to interior difference. Look for the slope that comes the closest to this point on the graph. This is the approximate R value of the wall.
Example: Assume a difference of 40 degrees for the exterior wall value and a 3 degreee difference for ambient to interior wall. The slope that comes the closest to this point is R=10.