I assume that you are talking about waterproofing the outside of the bag to keep rain water from getting to the down. The problem is that waterproofing a bag will also make it vapor (sweat) proof and you will get condensation on the inside of the waterproof fabric. This condensation will wet the down and the down will loose its insulation properties. You could also waterproof both the inner and outer fabric. The down would stay dry but you wouldn't.