The disadvantage with bubble wrap is that you won’t be able to open the window. I had three windows I wanted to insulate but also open. They are aluminium windows, so at first I thought I’d just stick my plastic (not bubble wrap, but I’ll get to that later) to the frame of the window, but that would’ve only made a 5 mm gap which wouldn’t be enough. So what I did was buy some foam from Para Rubber and cut it into 1 cm wide strips. My foam was 5mm. I made a double layer stuck onto the glass at the bottom of the window, and then put a single layer around the rest of the frame all stuck on with double-sided sellotape. Then I put my plastic on top and trimmed off the excess.
Now I have an insulated window that doesn’t look too geeky, works brilliantly (you just have to touch the surface to tell it’s working: it feels warm, not cold), and I can open the window.