small trick: put it on the outside.

Most modern window glass heats up as it filters out UV and a few other light bands. My previous flat had windows that would go as high as 95C!

All that heat WILL radiate inwards and through the foil.

if you out the foil outside, you prevent that heat buildup and radiation.

You can also buy cheapo reflective film you can install on your windows permanently, for the same effect.