Cold weather sleeping bag with membrane?

Anyone have much experience with cold weather sleeping bags that have "waterproof breathable" membranes?

Bought a Marmot CWM a while back and didn't notice till it came that it has a membrane on it. The plan is to only use it in a tent. At super cold temperatures the only moisture issues are from you're body and seems to me like waterproofing the bag would only make this worse as well as making it more difficult to dry the bag in the sun.

Seems like just about every -40 bag out there includes a membrane though so maybe I'm missing something? . Are they assuming everyone uses a VBL at those temps?

In the past I've never found I need a VBL as long as I get the opportunity to put the bag in the sun every 3-4 days or so.

I have a Marmot Col (-20) with the membrane and I love the thing. Never had an issue with moisture in or out of the bag, though admittedly I sleep very cold.

A lot of it has to do with condensation inside the tent. I've been camping in the winter and literally woken up with icicles hanging from the roof in the morning from the moist enhaled air. When the sun hits the tent and heats it up, that all melts, dripping onto your bag. That's where having the WPB barrier helps keep your bag dry.

from "professor popsicle" a leading expert in hypothermia and cold weather/immersion injuries, he trains local SAR teams

umanitoba.ca/faculties/kinr…

note that the material is a decade old, WPB fabrics have improved considerably since then ....

also a synthetic overbag/quilt will help prevent BOTH internal and external moisture

more on the popsicle ....

outsideonline.com/1821211/m…

;)

ive used the mountain hardwear wraith (-20F) quite a bit. one big benefit i saw was that the exterior of my bag would be covered in wet frost from condensation in the tent when i woke up in the morning and i could simply brush it off of my bag. if there were no membrane, the frost that was melting from my body heat would have been able to soak into my bag.

i never have any issues with condensation inside my bag. one thing i do is fully unzip the bag and flip it inside out then strap it to the outside of my tent if its not snowing. if youre at high altitude this will kill some of the funk from a long trip and allow the down to dry (although, again i never noticed any moisture issues within the bag). a lot of the really cold expedition bags have black interiors for this reason.

I also have a Marmot Col (-20 F) with a membrane. Used it in and out of tents, never had a problem. I certainly wouldn't be concerned if I was looking for a cold weather bag and my options all had membranes. If you're concerned about drying it in the sun, just flip it inside out. I have used a VBL and sometimes not. More often not.
Don't think about it too much. Be happy you don't need to worry too much if you accidentally dump the water you've just melted onto your sleeping bag.

Thanks. I kind of picture the inside of a bivy sack when I think of waterproof bags which obviously based on responses doesn't seem to be the case