wrote in message

I have noticed that the rooms with skylights are MUCH cooler than other
rooms, making me conclude that heat is being lost through the
skylights.

It could also be that those rooms just don't get as much heat anyway. While
your conclusion may or may not be correct, the reasoning is wrong.

If the skylights don't have some sort of double pane or storm panel, add
one. Watch for excessive heat buildup though, when the sun shines