Exactly as he saidneither side, you want to go for a trip to walmart and get emergency blankets or mylar wrapping paper or windshield reflectors. there mad cheap, and not hard to find
and the reason for this is that tin/aluminum foil reflects more heat than it does light, and while it is reflecting light, it's imperfect surface has a tendency to create hotspots that are damaging to your plants.
when you get your emergency blanket, it is sometimes difficult to determine which side let's light through, and which one reflects it; the key is to make note of the final fold in the blanket, the inside fold has the reflective surface.
p.s. i do not at all suggest using tin foil. your better off avoiding the risks of foil.
he means white paint, as in get some, and use iti dont have a white closet
yeah i set it up already with the dull side out because thats what i heard before and since i wont have much heat for the room since the outside of the room is ran by a a/c im trying to get warmth in the closetI have had no problems ever with the dull side of foil. Just be as neat as you can - no wrinkles!
Mylar is ideal, however...