Generally speaking, the thickness of the door itself is not the most important factor to consider when comparing heating costs. It is wiser, instead, to consider the weather-stripping seals, how they work and their long term durability. The folding doors have fewer seams requiring weather-stripping than do the overhead doors, and therefore there is less surface area to seal.
The weather-stripping on folding doors are also more weather-tight. The hinges cause the weather-stripping on the door’s side edges to squeeze against the jamb, while the guiding track and the locking mechanism cause the weather-stripping on the door’s upper edge to squeeze against the header face. The weather-stripping on the bottom edge of the door squeezes against the floor because the hinges lower the entire door structure as it is closed.
The weather-stripping on the folding doors lasts for years. When the folding doors are opened, the male-female design of the weather-stripping between the panels causes an immediate detachment from each other and the weather-stripping on the door edges also immediately separates from the door opening surfaces, which means they are not abraded by each other. Further, when the door is opened, the hinges lift the door off the floor, thus saving wear on the door-sweep weather-stripping at the bottom of the door.