In situ measurement of U-values

The measurement of the U-value of a built structure presents great difficulties, since the thermal mass of most structures and the inevitable diurnal temperature variations prevent steady state conditions ever being realised. A conventional U-value meter takes a simultaneous measure of heat flow and temperature difference across the wall, and computes the U-value as their quotient. This can result in gross errors, large enough to render the technique virtually useless. Reliable measurement becomes possible by recording heat flow and temperature difference over a period of some days, and calculating the U-value as the quotient of the means. Using this technique it is possible to obtain satisfactory measurements of insulation even in the presence of sunshine, or with intermittent heating regimes. The paper gives examples of the technique in practice, and practical advice on achieving a satisfactory measurement.