We had our back windows and door replaced today, with timber framed double glazing. Our old sliding door had become quite difficult to open and close, and last year I discovered a one inch by about 1cm gap in the top of one of the frames, which would let in hot or cold air depending on the season, and spiders I'm sure. I plugged it up with cotton wool then, but things should be far more efficient now. They are the windows/door for our lounge and dining area, which can get quite hot in summer, especially in the mid afternoon. I'm sure a lot of winter heating went literally out the window too. I'm having trouble loading a before photo, but I'll put it up soon if I can.