lzaharis wrote:If I were you I would just block off the baseboard heat
under the windows. as that is where a lot of your heat is going.
Setting aside drafts and convection heat moves to cold, that's a law of physics, it's never the other way around. For example if you hold an ice cube in your hand it doesn't get cold because the ice itself is making it cold. It gets cold because the heat in your hand is being conducted into the ice... melting it. You're losing heat.
It's the same thing with your window, assuming its sealed the cold from a window is not coming into the house. As the warm air comes up against it the heat is conducted through the window and radiated to the outside and now it's cold air. Because hot air rises other hot air in the room is going to push that down so now you have column of cold air coming off the window. This is going to happen with or without a radiator under the window.
By placing a radiator under the window you can counteract this column of cold air preventing cold spots in the room. As the column of cold air coming off the window meets the column of warm air from the radiator it merges and is pushed out into the room. That's why you put a radiator under the window. Without the radiator under the window the cold will continue down the wall and spread out over the floor producing a cold spot by the window.