Condensation on a partially covered window

Factual Questions
1

I have two identical windows. On the inside of one of them I placed a piece of 1/2" rigid foam insulation over the opening. It’s similar to how you might shrink wrap a window, but the rigid foam is just friction fit to the inside of the window trim. It’s not a perfect seal, there are tiny gaps, but it blocks light and cuts down on convection. I could use temporary caulk to seal it if I wanted…

My question is, why does the covered up window develop huge levels of condensation compared to the uncovered window, which just has a little? I’m just curious as to what physically is going on to create more condensation.
To me, it seems logical that the covered window would have greatly reduced convective airflow, so LESS condensation, but in fact the opposite is true.

Is it because covering/insulating the window means that the temperature difference is greater… So what air does travel over the glass condenses more water out?

Is it creating some kind of stack effect where warmer air cools, falls, and exits out any gaps on the bottom, in effect not actually reducing convection much? Maybe acting in combination with the above?

2

There is probably plenty of moisture just in the airspace between the foam and the glass, also as it condenses out it is going to draw more moisture in through the gaps.

Usually the solution to condensation on windows is to increase airflow. In a cold climate removing screens in the winter and leaving the furnace fan on help alleviate window condensation.

This is a real problem in nice homes with wood sills; lots of damage from moisture if it is not dealt with. Here in Calgary windows are just double pane but really should be triple pane to help deal with condensation.