Originally Posted by gmcttr
Down proof does not mean waterproof at all.
To clarify... Some down proof fabric is waterproof. Some is not. The two terms to do not interchange in any meaningful way.
IMO the best way to determine if fabric is downproof is buy from vendors who know what they carry. Mill ends and surpluses are great. But if you are going to invest in quality down, don't do the fabric on the cheap. You'll most likely be sorry.