Dad has asked several times about the windows in the Palin home. We have double pane here, but the cold air still gets through. Is there special glass used for such a large expanse of windows?
You could try an additional glass on the outside.
That size of glass is at least double paned and filled with an inert gas. And its not uncommon to install triple pane.
I have a double pane e-rated window thats supposed to be pretty efficient and yet I can still feel the cold through it so I would think for that size of a window they may be triple.
All my house has double paned windows, the very high quality ones are drawn down to a vacuum and they add a dry inert gas so as to prevent fogging, and yes they are expensive.