We use a very robust and durable waterproof plastic that undergoes a very effective sealing process for maximum waterproofness and impermeability. However, we prefer to say that they are water-resistant. In fact, it is best to cover the bags with a tarp if they are to remain outside for a long time. They are also made of UV-resistant plastic, which protects the bags and bedding from sun damage for 12 months. As a preventive measure, we also recommend covering the bags with a heavy cloth during extended periods of outside storage (more than three months).