How did Professor Sherman make his balloon as light as possible in The Twenty-One Balloons?

In The Twenty-One Balloons, Professor Sherman made his balloon as light as possible by being very careful about how he designed and constructed his basket.

Since he wanted to stay up in the air for as close to a year as possible, he knew he needed to construct a much larger balloon with a much larger basket than usual. The basket had to be large enough that he could live in it; therefore, he followed French balloonist Nadar's design and constructed a basket house for himself.

To make the basket house as light as possible, one thing he did was think of a new means of ballast other than using sandbags for ballast. Normally, people throw sandbags overboard when they want the balloon to climb higher. Since sandbags would be too heavy to keep on board, Professor Sherman developed the idea of using extra food and pails of garbage for ballast. The second thing he did is furnish his basket house as lightly as possible. He did so by designing a mattress out of his balloon material that could be filled with gas, just like his balloon. He also made chairs and a table out of "balsa wood and bamboo" (p. 44). Plus, he only brought paperback books on board. Finally, he knew he needed to think of a lighter weight solution should he crash into the ocean. Instead of bringing a lifeboat with him, he had waterproof suits designed out of his balloon material and brought a "cork lifesaver" (p. 44). He also brought only lightweight men's clothing with him.