If the foil is visible from outside it signals behaviors that are widely disapproved of. To that end, it would be wise to put something between the foil and the glass, perhaps colored paper or arbitrary fragments of unwanted posters.

Light may also come in around doors. In this case, a folded flap of duct tape with foil inside may be attached to the edge of the door on the swingward side and on the frame on the contra-swingward side. That may eliminate all light.

Particularly thin (cheap) foil may get have small tears that let through points of light. A piece of duct tape will patch those.

Blacking out windows is less beneficial than adapting to a conventional day/night cycle when possible. Natural light improves quality of life.