Appowner is correct.
balsa is like any other wood, the conditions of it's storage will be the determining factor for the condition of it's usefulness and lifespan. it is very a porous wood, it absorbs moisture like a sponge. like any other wood,.... if stored for long periods,...storing in a cool dry place out of direct sunlight and sealing the ends of a board with shellac or fingernail polish will do a lot to keep it dry and light. be sure it is laid flat and/or if stacked straight up, leaning as little as possible.