Testing a U value

Take three temperatures:

All temperatures to be measured on centigrade scale.
Make sure the room is in a stable heat state as possible that is the heating has been consistent for several hours and outside temperature is not fluctuating.

  1. Outside temperature 
  2. Inside Room temperature
  3. Surface temperature


Equipment required; Contact or Inferred thermal thermometer. Weather station or thermometer that will measure air temperature.

  1. Calculate the difference in room temperature and outside temperature and then draw a vertical line on the chart to the corresponding temperature.
  2. Calculate the difference in room temperature and the wall or window surface temperature and draw a horizontal line where it passes the curved line. (Note: for decimals interpolate between curves). Then read off the left hand side of the chart the estimated ‘U’ value.

Estimate U Value

Download pdf version of this chart

Note; This method of determining a ‘U’ value under site conditions is approximate and dependant on the accuracy of the measurements and quality of the measuring equipment but never the less can give a good indication how a material is performing.
Best results are achieved at night where conditions are more stable and there is a greater difference between inside and outside temperatures.