Took this technique I learned in prison to make it bearable during the heatwave:
- Wet the window with a sponge or rag.
- Put aluminium foil up against it. Shiny side out. Cut beforehand on angled windows like that or use the roll on vertical surfaces.
- To cover the overlap, just wet the foil already on there, but be careful not to pull it off at the edge. An overlapping strip will stick to the other ones.
- Use masking tape to cover any overlapping edges that come away, as well as the edges that have a hard time connecting to the windowsill.
This keeps out an incredible amount of heat; you can feel how hot the foil itself gets during the day, while keeping the inside nice and cool.
I use this product on all the windows in the summer and winter. It insulates a bit and also reflects back.
The one thing better than inside is to have something on the outside, but that’s not possible for me, because it gets really windy here.