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.
The downside is obviously you get zero natural light. And your neighbors might think you’re weird, but they’re roasting while you’re not so fuck em.
You could probably tape the foil to a piece of cardboard cut to size and that would make it easy to remove once the sun isn’t shining straight at your window and put it back before morning. I don’t know if the cardboard would affect it, however.
I’d use a piece of insulation foam board, cut to fit the window. Lighter than something like plywood, way more insulating, and can be easily squished into place once it is cut to size. Just hole the board up to the window, mark where your cuts need to be made, line one side with shiny foil, tape it along the back, and you’re done. Now you have an insulating reflective panel that can easily be popped in during the daytime heat, then pulled out at night.