After a short holiday break (and because it was too cold to work!), we installed the windows in February.  These are Anderson brand double-pane argon-filled casement windows.  Update 2022.  Insulation factor. These windows are fantastic.  It can be -10F outside and putting your hand on the glass inside the house, you can’t even feel the cold. I have never put curtains over them because I love the pretty snow forest outside and the home stays nice and warm.