Answer and Explanation:
When Thoreau says his house wasn't ready for winter and was just "a defense against the rain," he means the house he has built for himself is still unfinished. It does have walls and a roof, therefore being able to protect him against the rain. However, it does not yet have a chimney or plastering. The walls are made of boards, so he basically doesn't have much defense against the cold. Nevertheless, Thoreau begins to live in it anyway in the month of July, 1845.
