The purpose of this study was to determine the precision of an instrument (window thermal transmission instrument) that measures thermal transmission through a window by determining the precision of the instrument under specified test conditions and under different temperature levels. Seven conventional window treatments were tested at three temperature levels, and the variation in thermal-transmission coefficient was measured. Analysis of variance and priority contrasts was used to determine the effect of window treatment and temperature on the precision of the instrument. The analysis indicated that temperature level affected the precision of the instrument. Precision was better at the highest temperature [333 K (140 F)] than at either of the other two temperatures [321 K (120 F), 327 K (130 F)]. Though no differences in instrument precision resulted when groups of window treatments (roller shades nr draperies) or various types of roller shades were tested, differences were found when various types of draperies were tested.