covering up windows

I have a back porch that was added on to the house. The walls are insulated but the roof isn't and the windows are old single pane and there's 5 of them that don't open. Its really cold in the winter and really hot in the summer. I was wondering if I covered them up with foam insulation board, except for one (so I can see into the back yard) if that would make a big difference, or would I have to cover them all? This is only temporary as it will be a while before I can get the back porch leveled out and get better windows in.