I have a patio door that gets direct sunlight around midday, but because of evergreen trees and a huge hill it's maybe only an hour a day.

I want to replace the current patio door because the current one is 40 years old and my office desk is now right next to the glass. I sit 24" from it. Plus I open it 10 times a day for dogs to go out. There is no way the current one will do anything but freeze me out all winter.

But in the replacement I have a choice of clear glass or Low E. The Low E is $100 more, which sounds like a bargain. The door has an r value of 2.0 and with Low E it has 2.8. I don't know what all that means but it sounds like it is 1/3 more insulating.

but is that only for sunlight? and does Low E really insulate better? Basically, is the LOW E coating for real or for just a worthless add-on?