This question was asked at an energy efficiency workshop hosted by Sustainability Ireland, that I attended before Xmas. The advice given was that if you're going to use the chimney balloon, then it is advisable to put a cap on the top of the chimney as well, to prevent water entering. If you use the balloon on its own, the water will still enter the chimney void and build up around the surface of the balloon - from there it could start seeping into the brick or blockwork of the chimney and create much bigger problems.