I'm building a breadbox that will hold multiple loaves of bread. I was wondering if it would be a good idea to put a gasket around the lid to reduce air infiltration but I don't know what is more important. So is the key to keep out light or air or both?
Mold grows best in warm, moist conditions. It's a fungus, not a plant; light has nothing to do with it. Fresh air doesn't really make it grow either, but circulating air does help reduce condensation, depriving mold of moisture. So you really don't want to seal a breadbox - that'll just help create a nice moist environment for the mold to grow in.
Breadboxes are really just about keeping other pests away from your bread, while still being able to keep it at room temperature. (It goes stale faster in the fridge.) If you're worried about mold growth, you probably just want to freeze some of the bread.