We propose a method to determine the effect of Ag2O on heat mirror production processes. A heat mirror is a low emissivity ( low-E ) film with high visible transmittance, high infrared reflectance and vanishing infrared transmittance. We carried out a computer simulation to understand the effect of Ag2O on the spectrum of three-layered low-E films. We then prepared three-layered low-E samples with ITO buffer layers, which were deposited on the Ag layer at 0.6kW, 1kW, and 2kW sputtering power. The simulation showed that the peak visible transmittance decreased, the FWHM of the transmittance widened, and the infrared reflectance was lowered as the amount of Ag2O was increased. In experiments, the three-layered sample with the buffer sputtered at 0.6kW exhibited a lower peak transmittance, broadened FWHM, and decreased infrared reflectance, suggesting that higher levels of Ag2O were introduced. We also prepared five-layered low-E samples, which are of typical heat mirror structures, with buffer layers similar to the three layered samples. The transmittance and reflectance spectrum of the five-layered sample with the buffer sputtered at 0.6kW also suggested a higher Ag2O content than other samples. Therefore, studying the spectrum of the three-layered low-E film, along with the simulation, can be an economical way to optimize manufacturing processes in the production of five-layered low-E films.