Help me! - understand cold weather condensation

**Rapt** · 10-31-2007, 07:45

Great article Sk8tr dad

Originally Posted by kohburn

for the sake of maximum heat a vapro barrier works well, but it lacks in the comfort area if you end up wet, so really the ideal is a vapor retarder that slows the transfer of vapor to the outside air to both retain as much heat as possible but also allow enough diffusion of moisture to avoid condensation inside the bag, inside the insulation, or on the outside of the bag.

so it sounds to me like your underquilt is simply providing the right balance of breathability with insulation.

This is a strong argument for something like goretex as a lining ... Which is what I did with my overbag... I did this work through on my own about 10 years ago and came up with this concept. Then made three overbags this way. As faras I know they are all still in use. I find mine exceedingly comfortable when used as an overbag, or when used on its own as a sleeping bag. Even when its hot and humid out.

**Rapt** · 10-31-2007, 07:58

Originally Posted by warbonnetguy

good point, you would need a full coverage vb to keep that from happening.

i'm still trying to figure out why i've never had condensation issues with my sil uq, even when the top of my down bag is wet and the bottom of my tarp, and in a wide range of temps (40's-20's), the uq has always stayed dry. it eludes me. maybe it has something to do with the slight inefficiency of an uq alowing some breathability. the hammock itself doesn't really collect condensation either. there is obviously a very complex set of variables involved with condensation, which can drasticly effect comfort and warmth, and breathability isn't necessarily the end all solution, (hence the wet down sleeping bag, which has happened more than once., usually in colder conditions).

My thoughts here are as follows... The inside surface of your UQ is warm, warm enough that no moisture can condense on it, further it inevitably has some air space between you and it, unlike a pad that you lie directly on, so it doesn't have the direct "pooling" issues with liquid sweat coming from your body. That combined with a highly breathable upper layer means that all the moisture in your "warm" chamber ends up traveling upwards through your upper quilt/bag.

Let me repeat, the reason your NON-breathable UQ works is because the air next to it is kept warm enough to avoid any condensation.

Just like the vapour barrier in house construction works. It keeps moisture out of the insulation in the walls and unless you have seriously high humidity in the house and poor breathability/ventilation or poor insulation there is no issue with moisture consdensing on the walls.

However in your hammock scenario if you went to a Sil shelled upper bag too I think you'd be less comfortable....

**sk8rs_dad** · 10-31-2007, 15:05

Originally Posted by kohburn

thats possible with a vapor retarder not with a true vapor barrier as by definition the vapor can not get beyond it.

Sorry my use of the word "barrier" raised an alarm. Clearly the material needs to allow vapour to pass through.

**warbonnetguy** · 10-31-2007, 19:41

you're the man Rapt, that's sort of the direction my thought was going too

since the uq is actually 2 vb's with insulation between them, the insulation keeps the vb against my hammock plenty warm (as warm as my back is) so moisture can't actually condense against it b/c it's not cold (if it was only sil on the outside of the uq, this is what would happen). this is similar to what happens with ccf too since it is an insulated vb as well, but with ccf, it is pressed tight against you, so it eventually gets 100% humidity and saturates the pad. so it might not really be condensing due to cold, but rather increasing humidity till the air (or lack therof) can hold no more. with the uq, there is more of a dead air space between my skin and the insulated sil. this means more moisture holding capacity between me and the vb before the air gets saturated and soaks everything, this combined with a seal that is not quite air tight (like ccf against skin) allows air to escape fast enough to get rid of excess humidity/moisture.

so you say using a gore tex over bag reduced condensation on your sleeping bag? it created a less cold dead air space between the sleeping bag and outside that slowly breathed out the moisture?

seems like it would condense on the inside of the gore tex just like it condenses against the shell of the sleeping bag.

**Rapt** · 11-01-2007, 06:37

Yes that's what I'm saying. The reason it didn't have the condensation issue is because I kept the goretex warm.... I made my own overbag and went against tradition and used the gore on the INSIDE of my overbag. That way the dew point is moved outside the gore layer, but because the gore slows the transfer process it limits the amount of moisture present to something less than can be evaporated from the outer bag in normal humidity.

For me this overbag works great by itself down to about 10C (50F) and when used as an overbag with my down bag is great from there down to -35C (-31F). That's as cold as I've used it down to. Even at that temperature the moisture was still passing through the outer bag before condensing or freezing. There was a lot of frost on the outside of the bag that just brushed off, but not any significant weight gain or reduction in insulation value of the overbag.

I've made a few of these bags for friends and family and as far as I know everyone has been very happy with them.

**kohburn** · 11-01-2007, 08:03

Originally Posted by sk8rs_dad

Sorry my use of the word "barrier" raised an alarm. Clearly the material needs to allow vapour to pass through.

no alarm, just trying to clarify because the wording can get confusing

**kohburn** · 11-01-2007, 08:06

Originally Posted by Rapt

when used as an overbag with my down bag is great from there down to -35C (-31F). That's as cold as I've used it down to.

i don't think i've ever experienced -31F except through wind chill. can't imagine it.

**Rapt** · 11-01-2007, 09:55

Camping New Year's Eve in Algonquin Park, Ontario, Canada. No wind chill thankfully, it was perfectly still. I wore fleece long underwear, smartwool socks, and a polarfleece hood. I slept in the dual bag system I described above with a 1" thick thermarest, and 1/2" ccf pad.

Toasty warm the whole night.

**warbonnetguy** · 11-01-2007, 20:08

ok, so it wasn't just a gore tex bivy, there was insulation outside of the gt layer, this makes alot more sense now.

**Rapt** · 11-02-2007, 07:17

Its something I came up with about 15 years ago, and I never understood why the commercial guys didn't go that way when they discovered that there were problems with moisture condensing on Goretex when it was the cold surface barrier.