A: Hi Jeff. The Naturepedic® Wooly 3" Organic Wool Mattress Topper. Shell: 300-thread count cotton fabric. Fill: Organic Wool Batting. Wool naturally regulates temperature and wicks moisture, keeping you comfortable in both warm and cool weather. There’s a reason why sheep's wool has been a staple across the generations. It’s highly functional with unique properties that synthetic variants have been unable to reproduce. Its unique structure can absorb up to 30% of its own weight in moisture without feeling wet. It draws moisture away from the body, keeping you dry and comfortable, which is especially beneficial for bedding. It's also highly breathable as it allows air to circulate, preventing overheating and ensuring comfort across all seasons. While the topper is not waterproof, should you use a water proof covering, you will not experience these qualities. Also, it is best to allow your topper to breathe. We recommend to cover your topper with a cotton cover or with your fitted sheet. Thank you for your inquiry.