Bivy “Inside” Sleeping Bag/Quilt as a Partial Vapor Barrier for Condensation?

The Rab is a real VBL while the MLD breathes (so not a VBL). Other than some mylar emergency bivies, most bivies are designed to breathe some (so are not VBLs).

If you want to try using a VBL, all you need is a plastic bag. A mattress bag, christmas tree disposal bag, two large contractor or leaf bags taped together, or you can even buy large plastic “survival sacks” made for the job. Western Mountaineering also makes a VBL liner, similar to the Rab.

Opinions on VBLs vary widely, even among arctic explorers. Alex Hibbert loves VBL-everything, all the time, when he is adventuring in subzero (F) temperatures. Aaron Linsdau carries a VBL but only uses it when necessary because he doesn’t like them. Both of those guys have skied across Greenland and explored Antarctica. Both have YouTube videos about their VBL preferences.

Before spending a lot of money on a silnylon VBL, try a plastic one to see whether you even like the idea at all.

As for “better”: A VBL works differently than an overbag. A VBL is foolproof in that it will always keep condensation (from you) out of your insulation. An overbag may trap condensation from the atmosphere). Many people find overbags more comfortable. Tradeoffs.

By using a bivy you might decrease condensation development within your quilt or bag through its semi-vapor barrier function although its effective performance depends on multiple factors. At low temperatures within the range of 25˚F to 0˚F certain users discover bivies useful yet their unvented design might still create moisture accumulation. Synthetic over-quilts function more reliably than classic alternatives for condensation control while preventing excessive moisture accumulation. Tests reveal that the Rab vapor barrier liner stands as an effective solution to moisture protection when your main goal is body moisture control.

“Before spending a lot of money on a silnylon VBL, try a plastic one to see whether you even like the idea at all.”

A convenient and cheap plastic to evaluate is a plastic garbage bag.  Cut holes for your arms and head.  I’ve done that before.  My evaluation was that it was comfortable enough but didn’t seem to add much warmth or anything else.  I seldom go below 20F which is maybe why it didn’t work too well for me.

That will just be a vbl for your torso, but maybe that would be enough to evaluate.

Thank you all for the insight.  I’ll probably will forgo trying my MLD bivy (or plastic bags) inside my quilt to see if it reduces condensation.  The more I’ve researched the more unpleasant vapor barriers (partial or full) sound regarding sleep comfort in the temps I overnight in.  I do not plan on any artic expeditions in extreme cold over multiple days so I think I will stick to my 50F synthetic over-quilt system and is probably worth the extra ounces to continue bringing it along. I just retuned from a two nighter in Yosemite at 15F and my over-quilt system worked fine.  Just was curious what others thought.  Thanks again.

I suspect that an over quilt is really only useful in those same extended arctic conditions