Need help with an overbag to combat damp down bag

[Travis Leanna]

I know there's a lot on the forums on overbags, but I need help understanding what materials (insulation and shell) are best for an overbag, and whether or not they really help keep the moisture out of the main down sleeping bag. Do they really help push the dew point out of the down? Or are VBL's more effective in keeping the moisture away from the down? Any help is appreciated!

[N. F.]

Are you sleeping in a tent?

[Michael Febbo]

It depends on where the moisture is coming from- internal can be combated with VBL and/or an overbag, external will be helped by an overbag but a good shell fabric is a better option.

Overbags as made by MEC and Rab are heavy, but the synthetic layer will help keep frozen moisture (internal) from your down bag if conditions are right for it. Unless I am mistaken, it has to be cold enough for the dew point to be near the overbag, not so warm that it is still in the down… could be wrong about that.
An overbag has worked for me in Adirondack winters. However, for trips less than a week I see little utility if you already have a warm enough bag.

If you are in a very humid area with temperatures between 30-40, it is hard not to have everyting get damp and not dry out. Synthetic bags have their place.

[Roger Caffin]

If you are talking about a single layer of fabric, like a bivy bag, then they can actually make the moisture worse by confining it.
If by overbag you mean a second insulation layer, then yes, they will help keep the inner bag dry. The overbag may get damp of course.
VBLs will stop the moisture from getting into the bag in the first place.

Cheers

[Travis Leanna]

Yes, I'd be in a tent. Sorry everyone, I almost forgot about this post!! I have a Marmot Pinnacle 15* down bag. Regardless of conditions, it seems to be slightly damp from my sweat/evaporative moisture with each use. I know its not coming from the outside (as in from tent walls or rain) so I'm thinking a VBL is something to consider

Roger,
I've experimented with the big plastic garbage bags as VBL's, and am not too keen on how damp I and my clothes get. However, in some instances, that is better than the bag itself getting damp. How do "real" VBL's out on the market compare to the trash bag test?

[Franco Darioli]

Sounds to me that you are sweating because your bag is too warm. Maybe you get into the bag too cold so feel the need for a warmer bag than necessary once you warm up (?)
That 15f rating may not be that accurate but how often do you sleep down to those sort of temps ?
What mat are you using ?
Franco

[Roger Caffin]

Hi Travis

> I've experimented with the big plastic garbage bags as VBL's, and am not too keen on how damp I and my clothes get.
Yep, as Franco said you are sleeping too hot and sweating. Proper use of a VBL requires that you be just a shade cool all night, for the now-obvious reason.

Cheers

[Travis Leanna]

I got the bag in November, and have used it all winter. I've used it in temps ranging from upper 30's to single digits. I kind of was thinking that I was sleeping too hot, especially in the upper range of those temps, but its so warm and cozy!!!

Franco, I'm using a Big Agnes Insulated Air Core mat (not super UL, but really comfortable)

Roger, sleeping slightly cool with a VBL is something I was not aware of before. That will help with trying to figure out my system.

I'm looking into a new bag for the warmer months now, something in the 40 degree range. I'm also considering a quilt.

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