Sleeping Bag Q: WPB or not?

Pertex outer, every time!

Never ever put anything on the outer layer which might stop moisture from escaping. If it can't escape, guess where it stays?

Cheers

Would the down just absorb moisture before it reaches the outer fabric?

Definitely not WPB. The only reason for WPB is to keep rain splatter and condensation drip from getting into your bag from the outside. At 0 to -15, I can guarantee you won't have either. But moisture from your body will most definitely move through the bag and you don't want it stuck in there.

ETA: I meant to comment on this:

"Would the down just absorb moisture before it reaches the outer fabric?"

You say that like it's a good thing. Why would you want a WPB in the first place? Any bag that'll keep you comfortable at 15 below is not the one you want when there's liquid water to worry about. If WPB has any effect at all it will be a bad one.

I think that I would avoid any special WPB sleeping bag shells.

When I went to Aconcagua, the coldest that it got was about -5*F, but there was some wind and snow. My team of three was in a strong North Face expedition tent, so we were well-protected from the weather outside.

One morning, we woke up and it had been the coldest during the night. The entire inside of the inner tent was white from frozen water vapor. That meant that lots of water vapor had successfully escaped from our sleeping bags. That's the way you want it.

Incidentally, we kind of worked the rest days differently. From base camp, we would carry a half load up to the next higher camp, drop it, and then return to base camp to sleep. Then we would carry the second half load up to the next higher camp and sleep there. We continued that up to 19,500' for the high camp. So, we never exactly had any rest days. We just broke up the days so that we never had any maximum load days at high elevation.

There were others there who had flimsy tents, sort of light-backpacker style, and those seemed to work terribly in the high wind.

–B.G.–

If you really need WPB sleeping without a shelter then why not get a WPB bivy? That way you can use it without the bivy on calm nights so it can dry out.

"Would the down just absorb moisture before it reaches the outer fabric?"

The outer fabric would prevent the moisture from evaporating and escaping, especially in sub freezing temperatures with a thick winter bag.

In addition to all of the previous comments add: The WPB cover will also also slow the drying of the bag when you are in the field.

"The WPB cover will also also down the drying of the bag when you are in the field."

Please explain.

–B.G.–

I think he means it will slow the drying time.

"I think he means it will slow the drying time."

That is why the Sun was invented, to dry sleeping bags.

–B.G.–

Up here in rainy bc the sun god is a false god for several months of the year

The rain god is the only true god and he demands the sacrifice of high powered down

;)

That is why the Sun was invented, to dry sleeping bags.

Not so helpful when you are out in the field for 10 days of rain or whiteout.

Thanks all, looks like Pertex it is.
I like those illustrations.

Just out of curiosity does anyone know why so many cold weather sleeping bags use WPB outers? Some manufacturers make all below 0F rated bags with WPB outers.

"Just out of curiosity does anyone know why so many cold weather sleeping bags use WPB outers?"

They probably feel that they can charge more money for the product whether it does any good or not.

–B.G.–