Assuming you are talking about setting your plant pots inside this box rather than planting directly in it and that you don't allow heat to build up in it then "the-more-reflective-the-better" and foil will reflect the most.

If you will do a bit of research on on 'hydroponic grow boxes' you'll find this is quite common. Plus over on the Growing from Seed forum here there are many discussions about using foil coated sheets like those cheap foil emergency blankets to enclose shelves for seed starting to increase the lighting.

Dave

Thanks John, Dave Took a peek at some of the other forums... I think I'll just start everything a little later then I did last year so the weather will be warm enough to put the seedlings outside during the day and under my pitiful tiny light at night. I have sprouting down but the growing up to transplant is a challenge.

Simplest possible job: tape foil to the outside of an aquarium tank (there I go again...). Keeps cats out too.

Rhoder - I see you are in zone 9b-10. You should get good germination just leaving your starting trays in the sun.

John A

If you find foil to be difficult to maintain (wrinkles, tears, etc...,) then just go with plain old, flat white paint. Some people do like foil and mylar, but white paint is the easiest to maintain and apply. Make sure it's flat white, not glossy. You might think glossy is shiny, and therefore better, but its not. Most of the high efficiency indoor grow rooms are just painted flat white.

Actually flat white paint will reflect more light than aluminum foil or Mylar.