Is tin foil the cheapest for darkening a room?
Answers:
even cheaper would be to go to a local grocery store and ask for cardboard boxes on delivery day....they are free, and many places will save them for you if you ask. Great resource for moving boxes as well.
Other answers:
YES
YES
i dont know what you are trying to acheive but if you are doing a grow room white painted walls work better than tinfoil
It worked for me!
I had curtains on the inside, though, so it wouldn't look SO gross. I lived where it didn't matter much, for the back window was barely seen from the road.
cardboard