@whreq ouch! I'm aware of the risk, but understood it mainly happens when the window is unevenly heated, and it's pretty uniform in my case. Or perhaps I've just been lucky.

Good extra reason to experiment with putting the foil outside!

@whitequark cool. Did you tape them on the inside or to the outside?

we use a reflective sheet inside which helps a lot but gets the glass pretty hot, I'm curious if putting space blankets on the outside would make a meaningful difference for those really warm periods.