Which reflective material is better for a grow box?

Mylar, Sisalation, White Plastic Sheeting, Aluminum Foil or Foylon?

Mylar gets around 92% reflectability. Has to be perfectly smooth though.
Flat white paint gets around 86% reflectability and is cheap and easy to refresh.

panda film (white plastic sheeting) is at about 90% and super easy to work with.

roll of mylar

Andrew_McHerb said:

Mylar, Sisalation, White Plastic Sheeting, Aluminum Foil or Foylon?

Click to expand...

Aluminum foil burns your plant. That is not good to use!

Nothing bad is going to happen if you use aluminum foil., but it's a pain in the ass to work with. It's a sign of lack of preparation, which tends to correlate with failure. That's why people think aluminum foil CAUSES failure.

If you use aluminum foil, nobody will help you with your problem. They will all blame it on the aluminum foil until you remove it.

I voted for aluminum foil because it's the easiest to line a cardboard box with!

Flat white paint.

Also depends on the climate you live in. I live in a relatively damp climate and had high humidity problems flowering during winter using plastic sheeting.
Then I started using just flat white paint and RH is better

Like DeeTee said, flat white paint.