I am replacing the door from my family room to my 3-season sunroom. Currently, an exterior grade patio slider is in place. I am looking to add french doors instead, so when using the room, I can open it up wider.

My question is around interior vs. exterior grade doors. Exterior french doors are a lot more expensive than interior grade. Do they provide that much more insulation? The doors do not need to protect against wind/water, etc. obviously. However the sunroom is unheated (has a heater, but we keep it off most of the time except when using the room for parties, etc). We are in the Baltimore, Md. area so we do have cold winters. 

Would interior single pane doors, with perhaps a cloth doorstopper underneath keep the main part of our house warm enough or do the double pane/weatherstripped doors really provide that much more protection?