Adding an extra layer of insulation to double glazed windows

[alankearn]

As heating is going to be a lot more expensive In three of the rooms in our house that are not used a lot I am thinking of adding another layer of insulation using made to measure pieces of two inch thick seat/bed type of foam on to the double glaze glass
Two of the the windows are approximateley two by one an half metres
The windows dont look out to anthing special so would'nt not look strange from the ouside
Does anyone think this would be a an effective

 

[Alec_t]

It would cut down heat transmission into the room, but that heat has already penetrated the glass so would be trapped between foam and glass. I don't know if that would cause any problem with the glazing units?
If the windows were clearly visible by outsiders, then painting the foam black would make it less noticeable.