Read the operating instructions for the hand-held infrared thermometer and set it to measure temperatures in degrees Fahrenheit. Measure the temperature of an interior wall or a piece of furniture away from sunlight and HVAC vents. Write the temperature on a piece of paper and label it “ambient temperature."
Find an exterior wall that is out of direct sunlight and easily accessible from the inside of your home. Go outside and measure the temperature of that wall. Note the result. Label the exterior wall reading, “outside exterior wall temperature.”
Take another temperature reading back inside your home, approximately at the same place on the other side of the wall where you took the outside temperature. Note the reading on the paper and label it “inside exterior wall temperature.”
Compute the difference between the “outside exterior wall temperature” and the “inside exterior wall temperature” by subtracting the smaller from the larger. Write down the result. Label the temperature difference, “Temperature Difference, Interior to Exterior Wall."
Determine the temperature difference between the “inside exterior wall temperature” and the “ambient temperature.” Write the temperature difference down and label it, "Temperature Difference, Air to Interior Wall."
Use a chart such as the one at "professionalequipment.com/images/r-value-chart.gif" to determine the approximate R-value of your exterior walls. Find where your “Temperature Difference, Interior to Exterior Wall” and "Temperature Difference, Air to Interior Wall" figures intersect on the chart. The closest slope on the chart will give you the R-value of your home's insulation.