Balsa typically can weigh between 4 and 14 pounds per square foot. For our use, it can be categoried into 3 types.

"A" Grain - Lightest in weight. Light color, not much grain pattern. Easy to bend without splitting. Typically used in gliders, wing sheeting, and other applications where it must be formed around a shape. Often called "Contest" balsa, and is specifically available from bulk suppliers like BUSA, Lone Star, and Superior. You will occasionally find some in hobby shops.

"B" Grain - Medium weight and flexibility. A good all around wood for general building use. More pronounced grain pattern. Good for fuselage sides.

"C" Grain - The heaviest and strongest grade. Doesn't bend much without splitting, so use it for flat applications. Very hard with a pronounced grain pattern, often grayish in color. Good for bulkheads or wherever much strength is needed.

For your wing, match the thickness with what's already on there.