This is the material I used behind my radiators to reflect the heat back into the house and insulate the walls. Basically, it's Styrofoam with a sheet of foil attached to one side. It came in a good sized bundle so there was lots left over after the radiators were all done.
At the time I didn't have any plans for the extra, then I thought of a possibility! My old basement windows are drafty and until I can fit replacements into the budget I thought I was stuck with that. With a little creative use of duct tape I enclosed the entire window opening adding a striking touch of classic sci-fi to my basement decor. :)
I am having a little trouble getting the regular duct tape to hold to the blocks so I'm going to pick up some Gorilla tape tomorrow and fix that. An added benefit is this blocks the grow lights from shining out and possibly annoying the neighbors.