Q: Can you coat existing windows.

A: No. The low-E coating gets added to the glass at very high temperatures in the factory. You can buy films to apply to the glass from the inside, but these are not low-E coatings (i.e., they do not selectively reflect either UV or IR light) but they darken the whole window. They are good for blocking intense sun but they do not reflect the IR light that keeps you warm in winter.