I've noticed that the
cold-rays from my windows are less harsh when the roller blinds are down, so I waited for the really cold weather to come in to test it. Armed with an IR thermometer I did some measuring.
The temperature of the room was 18°C, while it was -19°C outside, so that's a 37 degree (°C) differential across the double pane window! I measured the surface of the window's glass to be at 11.8°C with the blind rolled up (open). When down, the blind's surface temperature was 14.5°C, while the glass underneath was only 8.8°C. I went outside to measure the temps of the two windows, but there was no difference between them (around -16°C).
I'll try some bubblewrap with aluminum foil on one side tomorrow and see the difference that makes, but there are slim chances that the rest of my housemates will want to look at a window ornament like that