Thermal radiation is a function of temperature.
https://en.wikipedia.org/wiki/Stefan...3Boltzmann_law
You probably did experience some heating from the radiation reflecting back at you. Those mylar "blankets" they give marathon runners after the race do work...to an extent. Once the mylar touches the skin, you can feel that it isn't helping you keep that warm because thermal conductivity & thermal convection sucks the heat away quicker. For your window, your absolute best value would be to insulate it with insulation to dramatically decrease both conduction and convection. I believe these foam boards even have the Mylar film on them:
https://www.lowes.com/pd/Expanded-Po...875-ft/3365568