Some possibilities for windows. The windows in my place are wafer thin, and I suspect one of the main ways weather is getting in and out.

• Magnetic double glazing. A way to add removable double glazing to windows
• Bubble wrap is a good insulator! Spray water and it sticks to the window!

Now, I've just done an experiment with bubble wrap. I had 2 rooms basically facing the same way that got morning sun from about 7am to noon, and measured temperature differences between them.

It does help with reducing temperature movement. That's on the way up as well as on the way down. From 10am to 5pm it's colder in the insulated room as it doesn't heat up as fast. Then from 6pm to 10am the insulated room is warmer, as it doesn't cool down as fast. The difference wasn't huge; only about 1C max.

This was without any active heating/cooling. I suspect insulating would be a much better advantage if you were pumping warm/cool air into the room and maintaining it.

External temperature ranges over a 3 day period were from a 17.8C max to a 5C min. Wider/faster temperature fluctuations would also see a benefit to insulation. The biggest difference (advantage bubble wrap!) was on the coldest day when external temperatures were always below the internal temperatures. It stayed warmer than the non insulated room.

I'll give it a go in summer to see what will happen as well....