Glass is a weak insulator compared with a foam-filled wall. Commercial glass doors therefore use two or three panes with a sealed gap, often filled with argon, and a low-emissivity (low-E) coating that reflects heat. This reduces both heat gain into the cabinet and the chance of condensation on the outer face.
In very humid conditions, insulation alone may not be enough. Freezer doors and some chiller doors add a transparent conductive coating on one pane that is warmed by a small electric current. Keeping the outer surface above the dew point stops fogging, so customers can see the product and do not have to open several doors to find what they want.
Heated glass uses electricity, so better units control it by humidity or adjust its output. Condensation on a heated door that used to stay clear can point to a failed heater connection or a broken pane seal. A fogged gap between panes usually means the seal has failed and the glass unit needs replacing.