Here's an interesting article on the down bags.

http://www.backpacking.net/gear-reviews/sleeping-bag-review/sleeping-bag-review.html#water


WATER RESISTANCE

Although none of these bags are intended to be weather proof, they all are afforded a generous application of Durable Water Repellent (DWR) treatment to protect the down insulation from getting wet and becoming dysfunctional. The following tests were performed to observe how each bag performed, in relation to the other bags. The goal was to (1) examine how each bag's shell material responded to persistently applied water and (2) examine how each bag responded to water persistently applied to its always vulnerable seams.

We first created a little crater in at least two places on each bag and filled with eight to ten ounces of water. We let the water remain for at least 1/2 hour. After removing the water, we visually checked for wetout of the shell material, dampness/wetness of the down and dampness/wetness on the inside of the bag. The second test was similar except the puddles of water were positioned directly over the seams.

All bags passed the first test with relatively similar results. All bags failed the second test, with relatively similar results.

* Nunatak - Great DWR -- BUT water seeps thru seams.
* Marmot - excellent DWR, very water repellent, water beads up & rolls off -- BUT water seeps thru seams.
* Big Agnes - excellent DWR, very water repellent, water beads up & rolls off -- BUT water seeps thru seams.
* Western - very good DWR, water resistant, water does not soak thru but does dampen shell -- water seeps thru seams
* Moonstone - very good DWR, water resistant, water does not soak thru but does dampen shell -- water seeps thru seams
* Mountainsmith - very good DWR, water resistant, water does not soak thru but does dampen shell -- water seeps thru seams
* Exped - good coat of DWR, water resistant, water does not soak thru but does dampen shell -- water seeps thru seams

TEST #1 RANKINGS (shell material):

1. Nunatak Alpinist (like a duck's back)
2. Marmot bags (nicely beads up)
2. Big Agnes bags (nicely beads up)
3. Moonstone Lucid 800 (shell slightly damp)
3. Western Mountaineering bags (shell slightly damp)
3. Mountainsmith Bags (shell slightly damp)
4. Exped Hummingbird (shell damp)

TEST #2 RANKINGS (seams):

1. Moonstone Lucid 800 (took longer to soak thru seams, but end result the same - very wet inside bag)
2. Nunatak Alpinist (very wet inside of bag)
2. Marmot bags (very wet inside of bag)
2. Big Agnes bags (very wet inside of bag)
2. Western Mountaineering bags (very wet inside of bag)
2. Exped Hummingbird (very wet inside of bag)

Incidentally, I also included the Western Mountaineering Apache Super Microfiber in this water test. As you know, the microfiber material is windproof and highly water resistant. Well, just so you know, the seams on these highly water resistant bags (be it microfiber, dryloft, or equivalents) will allow water seepage into the bag the same as the bags which rely solely on a DWR coating. The only advantage to the microfiber or dryloft type fabrics is that they will afford longer-term durability and offer water resistance after their DWR coating wears off. Don't presume, then, that a very expensive dryloft bag will provide the functionality of a waterproof bivy sack. You might indeed, get very wet and become very disappointed.