Structure hardening is a key strategy to help mitigate building destruction during wildland-urban interface (WUI) fires. While hardening all exterior components of a structure is important, windows have specifically been identified as a vulnerable part of a building. The purpose of this study is to characterize the heat transfer through single- and double-pane windows constructed of plain and tempered glass. Double-pane windows with and without low-emissivity coatings and with either air or argon-filled gaps are included in this study. Small-scale experiments were performed on 23 cm x 23~cm windows exposed to a radiant panel producing centerpoint heat fluxes of 10, 20, 30, 40, and 50 kW/m2 to the exposed side of the glass. Each experimental condition was tested in triplicate. Total and radiative heat flux was measured 5.1 cm behind the unexposed side of the glass at the center of the window. Additionally, total heat flux was measured in the bottom corner of the window to characterize the difference in uniformity of heat transfer across the plane of the window. Surface temperatures on the exposed and unexposed side of the glass were measured in various locations using type K inconel-sheathed thermocouples. Tests lasted for either 20 minutes, until glass failure, or until frame failure. Times to glass crack and failure were recorded. Results showed that double-pane windows reduce heat transfer through a window compared to single-pane windows (13-43% and 39-60% of incident measured, respectively); additionally, the application of a low-emissivity coating is effective (heat fluxes measured were 5-17% of incident). Plain vs. tempered glass and air vs. argon-filled pane gaps do not yield statistically different results in heat flux measured behind the window. Temperatures were not uniform across the plane of the glass on both the exposed and unexposed sides. Finally, tempered glass had better survivability than plain glass (22/23 and 0/16 survived at incident heat fluxes up to 30 kW/m2, respectively), and double-pane argon-filled windows consistently survived longer than double-pane air-filled windows.