An insulated glass unit is two or more panes of glass held apart by a spacer and sealed at the perimeter. Glass itself conducts heat readily; the insulating work is done by the captured gas layer between the panes, which suppresses convection and slows conduction across the assembly.
The spacer is the quiet protagonist. Early units used aluminum, which is highly conductive and creates a cold band around the perimeter of the glass — the place condensation appears first. Warm-edge spacers, made from structural foam or low-conductivity composites, substantially reduce that edge effect.
The perimeter seal is where units die. It is a sacrificial component under constant thermal cycling: the cavity expands on hot afternoons, contracts on cold nights, and the sealant flexes with it thousands of times a year. Eventually it fatigues, the desiccant in the spacer saturates, and moisture begins condensing inside the cavity where nobody can reach it.
This is why fogging is a glass problem rather than a window problem. The sash, the frame, the hardware, and the finish are typically unaffected. Replacing the sealed unit restores the original performance without touching anything else in the opening.