Don't know if this is something that would be useful to share as we're heading back into some warm weather again in the North East. I have an old house 1930's buid so not great insulation. Thinking about passive sollitions that I could afford I got some silver backed .5" foam board at the hardware store. The sheets are 48"x96", and half inch thick are around $20/per. You can cut it across to a width that sits on the window sill. My windoes are around 29" wide so I think I got about 3 - 30"x48" pannels. I dressed the edges in duct tape and when I put them in I can keep the house 15-20deg cooler.
https://n2qfd.tumblr.com/post/657093209059704832/living-in-a-house-built-in-the-1930s-in-the
4 months ago
https://n2qfd.tumblr.com/post/657093209059704832/living-in-a-house-built-in-the-1930s-in-the