eVent dry bags – can they dry a damp down sleeping bag?

Interesting….

I haven't found most of these type of bags to be truly waterproof, though for keeping things dry inside a backpack works pretty well. What advantage do these have over cuben fiber?

It wasn't my particular question but I'll answer yours!

So I was converted to the idea of a piece of eVent in a drysac because it allows you the compromise of a fairly well squished small sleeping bag which is kept dry but it can still shape itself to the hollows in the base of your pack because you buy one just large enough to get your sleeping bag with a little bit of stuffing, roll it and clip shut but there's still air inside. You then place it in the bottom of your pack and then add all your other gear ontop. The eVent lets the air out, as much as is needed to respond to the weight of kit above and to close your backpack. This will produce the MINIMUM compression you need, plus not a round hard object in your pack. Different approach to say a large drysac inside your backpack with the sleeping bag at the bottom and the risk of water running in to wet your sleeping bag if you open your drysac during a hike.

Video showing eVent's air-permeability
https://www.youtube.com/watch?v=eKG6Wx3OkwA

I bought one of the Podsac types last year and it worked a treat.
Here is my 0C rated sleeping bag in its mesh bag to show the degree of loft
http://i334.photobucket.com/albums/m421/NigelHealy/camping/DSC00435.jpg
here is in in the Podsac drybag, the slipper to scale shows I've made say 10 seconds of effort to squish it in.
http://i334.photobucket.com/albums/m421/NigelHealy/camping/DSC00436.jpg

Then I placed this first in my pack and everything else ontop. I'd say is squished down to about 30% of the size above.

My question is everytime you squish and unsquish you're exchanging air with the outside world plus you can place it where it will get sunshine, whilst still resisting water getting in from splashes and possibly get a bit more drying of any damp inside. Is that a real expectation or not? I've heard two contradictory views on this. Anyone?

Hmmm.

The process at work here is called diffusion. It is driven by a concentration gradient. ie, if there is a higher concentration of water vapor inside, then it will try to diffuse thru the pores to the outside. This is because nature seeks the "randomness" or "disorder" , which is defined as entropy.

Now, in effect the concentration of water vapor in air IS determined by the temperature as long as it is in equilibrium with a liquid water phase. So in a way, what they were telling you is sort-of true. When you are wearing an event garment, and you get hot and are making perspiration , the air inside the jacket has a higher concentration of water vapor (determined by temperature and the vapor pressure of water) so it diffuses thru the porous membrane to outside. However, that doesnt tell the whole story, because the air outside is not saturated unless its basically raining. The relative humidity outside could be very low in dry climates, and the relative humidity inside the jacket high, and both at the same temperature, and moisture transport would still occur.

Back to drying a wet sleeping bag. The air inside the bag may be saturated in equilibrium with the bag at whatever temp it is at, ie 100% relative humidity. The air outside bag is likely lower humidity and unsaturated, so yes moisture diffusion might occur at some low rate.

But as the day warms up and temperture rises, the sleeping bag inside the drysack wont warm as fast, and it is possible that it ends up having a lower moisture concentration that the outside air. In that case, none will occur.

But a bigger problem is just plain heat input and molecular diffusion of the water through the sleeping bag itself. Squished up in a drybag, you have virtually no convection,(wind or mechanical agitation), just pure diffusion to rely on and that is very, very slow thru a thick dense medium. It wouldnt matter much if you put sleeping bag in a net bag with no barrier, it simply isnt going to dry unless you can spread it out and heat it up, etc.

I'll add the anecdotal evidence to this thread – I used the S2S dry bags on both the CDT and PCT, and never had my sleeping bag dry inside it… any time they were damp going in, they were damp coming back out.

Martin, an eVent drysac has opportunity for mechanical agitation, if you squeeze it pushes air out, if you release the loft of the bag will suck air back in, in a generally moving hiking situation there will be some air exchange going in, some agitation, it is also on argument to go for a larger drysac so it will loft up larger and still squish smaller, for max air exchange. In the bottom of your backpack the opportunity to loft will be little, in the top of you back the opportunity for squishing will be small so in general no matter what the exchange is going to be small.

Ryley – great an anecdote I take seriously, basically this idea DOES hold water (i.e. eVent dry bag will not really dry a bag out).

I'm buying another one of these eVent type bags for packing reasons.

Are you kidding me! The responses above are polite/technical ways of saying this is ridiculous. Put a damp towel in your eVent drysac under any type of optimal conditions for any amount of time and it will be damp when you pull it out. Seriously, put a damp towel in your eVent drysac and try to make it dry. Put it in the dryer, oven, sun with zero humidity etc. Whatever you are comfortable with, short of melting your eVent bag ….. the towel will be damp. Your damp down bag in normal circumstances will be far worse. If you have a damp baq, air, sun and open exposure are the ways to get it dry.

Why would you ever put a damp item inside a vapor barrier?

FYI, if you try to dry a towel in the eVent bag in a dryer, put a similarly damp towel in the dryer outside the bag and report the results on backpackinglight.com. This would be a great example of the limited transfer of vapor through eVent, or other similar fabric.

"My question is everytime you squish and unsquish you're exchanging air with the outside world plus you can place it where it will get sunshine, whilst still resisting water getting in from splashes and possibly get a bit more drying of any damp inside. Is that a real expectation or not? I've heard two contradictory views on this. Anyone?"

Nigel, I'm not quite sure what you are asking in the above. I have what I believe to be a similar question. When I roll up my Silnylon tent and force air out in the final steps of rolling, am I damaging the vapor barrier in the Silnylon? Is any potential damage significant?

Hi Nigel,

Maybe putting a heat source in the bag such as a small bottle with hot water or a heat pad might provide enough heat to push the moisture out.

Cheers,

Stephen

well I bought one of these last year for my sleeping bag and used it for the packing reasons (smallest size, shaped around other gear in pack), never even crossed my mind then the eVent would have some drying capability til I was looking to a larger one for larger sleeping bag, and was comparing reviews of the two manufacturers and then came across this

http://www.ultralightoutdoorgear.co.uk/jmt2011gearreviews.html#podairstreamlite

"If there was ever any doubt about the breathable eVent end of the sack it was dispelled immediately – often on the morning the foot of my sleeping bag was damp, having come into contact with the tent, needing to get on the trail I packed it damp – however because it was hot the bag was invariably bone dry when I took it out of the compression sack next – the eVent panel had allowed the bag to dry out – while it was compressed in my pack – awesome."

So there is something I can't figure out.

However to me its settled, I'll not assume the above reviewers experience will be repeated.

Now then, for the sake of science I will do an experiment. I happen to own a larger 20L type eVent based SeatoSummit I use for backpack traveling. I'll put a damp (not sodden, lets be realistic) item inside and place in the California sunshine for a few hours and report back in next few days. Interesting to see what happens….

oh – another reason to go for eVent dry bags is if you're flying.

I had a Thermarest liteseat I used to act as a stiffener in a daysack for travelling, I had it not pumped hard, at altitude the lower pressure DELAMINATED the Thermarest. Fortunately I swapped it at REI.

But it DID make me think, what if I'd made a tightly squeezed drybag and at altitude it stretched weakened/ruined a drybag? It would require for damage for the bag to be fairly stuffed with kit so the bag was close to its max capacity so it simply can't expand into the excess space, AND for there to be plenty of air inside.

Compressing a wet down bag sounds like a bad idea in itself. Getting a down bag dry with some usable loft is hard enough with a dryer at home, let alone laid out in the sun when on the trail. Even if the temperature and humidity differential were perfect, your trip would be long over before down would dry. I think the end result would be a goose flavored brick.

"compressing" well you don't really with this type of drybag, its over-sized so its only compressed by gravity from other kit above it. Its only compressed as much as is required without you thinking. That's the whole point.

The surprising idea from the review above is you have some hope of drying it packed.

1. dont get yr down bag damp

2. if you do dry it out in the sun and wind

3. if you dont have either … know all the tips and tricks on how to dry it otherwise

4. if you still screw up, go synth … alot of very experienced people here use synth for good reason

5. just because its on the internet doesnt mean its true … id trust the manufacturer over some internet "reviewer"

for a credible source on how to dry wet down go here … http://www.andy-kirkpatrick.com/articles/view/wet_down_how_to_cope

as a side note, my EB down vest got the bottom half damp on a climb a few days ago, i didnt seal my nalgene tightly enough in the pack … so i simply put my 19$ old navy synth puffy over it and used body heat and the moisture transfer to the synth to dry it out on belays …. by noon it was dry … knowing what to do and having the "right" systems is what matters … not fancy gear

The review that reported a dried out bag after compression most likely just spread the moisture throughout the bag, in my opinion. It was wet at the foot end, stored, then taken out again "dry," at least visibly, because the moisture was more evenly distributed throughout the bag. This would help it dry faster during use, but is the same result as using any compression or tight stuffing method.

Using a heat source to dry out a bag while stored in the Event sack was mentioned. Sounds like it could work in theory, but would be a bit of hassle. Also, would it really "breathe" out that well if it was stuck in the bottom of a pack with another layer of (usually) waterproof fabric against the Event?

Finally, I have noticed that when I have used compression-style sacks, I can get just as good of compression using a standard roll-top stuff bag as any with valves or similar (like the Event type). You just burp out the excess air from a slightly oversized compression sack and get a flattish, mold-able shape.