Marmot MemBrain on Sleeping Bags

I don't have one of these bags, so take this with a grain of salt, but I would never get a down bag with a shell that doesn't breathe well. I have a couple Marmot jackets with the MemBrain and it does not breathe nearly as well as the standard Helium shell fabric. Not even close–it's almost like a garbage bag in comparison. Fine for a rain jacket, but not a sleeping bag.

Sleeping bags shouldn't get wet from the outside. If they are going to come in contact with lots of snow or rain or condensation, get a bivy or a better shelter. The way bags get wet and lose loft is from drying wet clothes using body heat or just from skin and lungs releasing moisture while asleep. So, I'm more worried about moisture escaping from the bag than from it coming into the bag. That's why a waterproof shell on a down bag makes no sense to me. Maybe in some hardcore mountaineering situations there might be some exceptions, but I'm talking about normal backpacking, including lots or rain and snow. A good DWR every now and then, like Revivex, will "touch up" the water resistance on the normal shell and will be more than adequate. In this case the cheaper bag is probably better.

Putting a membrane fabric on a sleeping bag is a big No-No. It just keeps water vapour inside – as liquid water.
Another bad marketing idea.

cheers