The thermal barrier in a sealed unit, is created by still dry air which is a poor conductor of heat, being trapped between the two panes of glass.
The spacer bar is the bridge between the glass. The spacer bar is also full of desiccants that will attract moisture. These desiccants make the air dry.
In the hot weather, the sealed unit will expand under pressure.
Then as the temperature drops the sealed unit will contract, again putting more pressure on the seal. Over time the seal on the unit will weaken, then ultimately the seal will open.
Usually the seal on the bottom of the unit opens, as this is where the damp sits.
When this happens, the dry air will be forced out of the unit under pressure.
Then as the temperature drops and the unit cools down, moist air will be sucked into the unit through the open seal.
As the temperature stabilises, the seal will close trapping the moist air inside the now re-sealed unit. As this process which is known as “solar pumping” repeats itself, more moist air will become trapped within the unit.
The moist air will be absorbed by the desiccants in the spacer bar. But as more moist air is sucked into the unit, the desiccants will reach a point of saturation.
When the desiccants have reached their saturation point, the heavier moist air will sit in the bottom of the unit and when heated by the sun, the heated vapour will rise to the top of the unit, then as the temperature drops, the vapour will condense and run down the coldest of the two sheets of glass.
There are always water particles in the air, but you can’t see these particles in the warmer dryer weather. However, in the winter months as it gets colder, the water particles get larger until they become visible, we call this mist or fog.