I'm going to start work on a down sleeping bag for a friend soon, he just won't convert to quilts. He always uses a bivy so I was thinking that it would be a good idea to use a Waterproof/Breathable fabric on the outside to guard against condensation affecting the down. Has anyone done this and are there any problems I should be aware of. As long as it is breathable I can't see a big problem with it.
I know Nunatak recommend Epic which is highly water resistant for quilts used in bivys and I was thinking going for a breathable coated nylon might not be a problem either.

WP/B fabrics are not 100% breathable. If he already uses a wp/b bivy then the moisture that gets trapped by the bivy will now be trapped by the outer layer of your sleeping bag minus exhalation vapor released into the bivy.

This will cause your down to get wet even worse than just the bivy I would imagine. With a normal set up and a bivy, at least the DWR coating of the bag provides some protection from condensation.

The temperatures used for the bag also play a role. I know Epic treated fabrics were having issues where the condensation would freeze in cold weather and effectively block the transfer of moisture, leading to more condensation buildup. It seems that this occurs in a sleeping bag as well whenever the freezing point of the temperature is somewhere in the insulation.

I think your best bet is to stick with a traditional bag, or if intended for cold weather use consider making it a non-breathable material like silnylon or cuben for the vapor barrier effect to minimize condensation. The latter of course limits you to layering extra insulation clothing on top of the bag instead of inside it where it will shift less.

G'luck and as always post pics, you make some fantastic gear.

I have a Outdoor Research aurora bivy and I use synthetic quilt with draw string bottom made with a Momentum shell. I get a little condensation in my bivy at the foot end in a synthetic. So a all down water proof breathable down bag would take quite a while to dry out compared to synthetic bag.
I have come to the conclusion all sleeping bag or quilts that are made out of breathable nylon after spending a winter in a bag/quilt because of no heat in the house and all my years backpacking/camping in a sleeping bag and quilt with sewn in toe box .

That it is the toe box that get condensation first because it is sealed in area.Because the condensation has to to travel the full length of the bag to be released. But in cold weather most people have the head area sealed off with just their face sticking out of the bag or quilt and the condensation does not get released in to the air because of the closed in to toe box area and sealed in head area.

The condensation problem is even more of a problem in a bivy so I have found synthetic to dry out faster for bivy use verse a down bag.

So I have found the draw string toe box to be be the answer to release condensation in to the air. Also I have been wearing sleep socks to cut down on foot box condensation in my sleep system. You could also try the small zipper in the bottom of the foot box like north face uses to release condensation.

If down is the way you would go I would suggest go the way of Golite sleeping bags. Use only the water proof breathable fabric in the toe box and the neck area of the bag with a small zipper in the bottom of the foot box like north face uses .

The cold feet problem is a dilemma that will plagued sleeping bag design year after year.I don't think it will go away till they a perfect sleeping bag insulation and outside fabric combination. I think the reason the feet become cold is because of it wetting out first. Not just because it gets cold.

Post script edit:
I also think sometimes especially in climates like Southern California or other like climates. We sleep in to warm of a Sleeping bag/Quilt for the condition. Because most hiker on a budget are looking for a all season bag. That can lead to wetting out of the sleeping bag.
It will be nice some day and wishful thinking when the manufactures come out with a smart insulation that can thermo regulate depending on the conditions of air temperature.

Edited by socal-nomad on 10/28/2010 12:17:52 MDT.

I have noticed the foot end gets wet before the torso

I assume it's because there's more warmth released at the torso area - more skin surface area at the torso and your skin is warmer there

So, the outer surface of the bag is warmer at the torso than the foot end

So, it evaporates water more at the torso so stays drier

There are a lot of variables here. So many that what helps
in one situation causes problems in another.

There is condensation coming from the outside (dew) and from
the inside (sweat).

I owned a NF Gold Kazoo sleeping bag for years. It was shelled with goretex and filled with down. After hundreds of
nights it finally started to de-laminate so I replaced it
with a standard dwr shelled model.

The difference I have noticed was that the gore shelled
model was more windproof and thus warmer sometimes.

I got condensation in both models at times, but could tell
no real difference between them.

When I was going to be in colder conditions than the bags
would handle, I covered them with a polarguard overbag
and this was very warm and condensation formed in the
outer synthetic bag and was easy to dry. Also it gave
a little more insulation underneath.

When I was going to be many days out
I also used a VBL bag.

Now when I am out, if I don't have a bivy, I pull my
Driducks or my Goretex Rain coat over my feet and this
helps keep my feet warm and seems to reduce condensation.

To really have dry warm feet, use a bottle of hot water
stuffed in a wool sock at the bottom of the bag. This
can even dry out damp clothes you put in the bag.

I purchased a GoreTex upgrade to a Marmot bag many years ago. I now regret it. Compared to my other bags the Marmot Grouse always feels clammy, and I have to take care to turn it inside out and air it out well.

It's longest use was 13 nights in mild to cool weather, well ventilated under tarps, warm days. No real problems, but these were great conditions for removing condensation.

Anyway, these days I only use breathable fabrics on quilts or bags.

Yes I think the condensation inside the layers of the bag may be the issue. I had hoped that the Bivy may attract the condensation rather than the bag. I suppose what has worked for many a years has worked for a reason.

Putting ANYTHING on the outside which could limit the ability of moisture to escape is a REALLY REALLY bad idea.

Yes, I know some companies sell bags with a WPB outer shell. That does NOT mean they know what they are doing - or posibly that the bags are really meant for warm maybe damp weather when there is no risk of condensation inside the bag.

However, if you put the WPB fabric on the inside it might (MIGHT) improve things slightly. A not-quite VBL lining. But not by much.

Cheers

Edited by rcaffin on 10/28/2010 23:38:32 MDT.

Then are you saying bivys, that have WPB outer layer, are a bad idea?

Some day I'm making a new bag and thinking about what to put as outer layer.

> Then are you saying bivys, that have WPB outer layer, are a bad idea?
The arguments over this rage on. :-)

Under many conditions the WPB layer will trap moisture which will condense and wet out your bag, before it can escape. Basic physics. It can also turn to ice ... This happens especially when the outside is rather humid, or very cold.

Under some good conditions the moisture manages to escape through the fabric before it condenses, so you can be lucky. For it to escape you want a moderate temperature, a little breeze and a low external humidity.

Cheers

I can say that even here in AZ where conditions are usually mild, low humidity, and exactly what Roger mentioned as ideal, my WPB bivy (REI minimalist) still gets condensation unless I have it fully unzipped and open. Granted the REI Elements material isn't the most breathable fabric, but I'm in the desert!

As soon as I can scratch the cash together I'm definitely converting to a tarp and breathable bivy with a good DWR and retiring the REI minimalist.

That's why I like my Outdoor Research aurora bivy and it clam shell design. I can leave the clam zipper open except the mosquito netting.
And it breaths or keep it down in bad weather and leave the clam partial zipped and breath out the opening am a side sleeper. But you still get some condensation in the bottom part of the bivy like a sealed off tent.

"Under many conditions the WPB layer will trap moisture which will condense and wet out your bag, before it can escape. Basic physics. It can also turn to ice ... This happens especially when the outside is rather humid, or very cold.

Under some good conditions the moisture manages to escape through the fabric before it condenses, so you can be lucky. For it to escape you want a moderate temperature, a little breeze and a low external humidity.
"
---

Physics didn't seem that simple in college. :^)

More truisms--

Moist air holds more warmth than dry air.

Outside moisture can quickly overwhelm a DWR coating.

Sudden heat loss can occur by evaporative cooling.

There are no "one size fits all" situations.

Sometimes I carry a DWR Bivy, sometimes a WP/B one,
sometimes an insulated overbag, sometimes a VBL,
many times nothing over my bag, sometimes when it
is hot and buggy I use just the bivy, sans sleeping bag.