Breathability differential in liner and shell material for quilts/bags

[Brendan Swihart]

Recently it seems that a lot of people are using 8d/nobull for liners due to it's nice hand and breathability and something more windproof (less breathable) with better dwr for the outer shell (m50, etc). I can remember multiple threads from a couple years ago saying that having a less breathable shell is a bad idea (especially with down bags) because it's easier for moisture to get in than out, therefore increasing the chance of condensation buildup and insulation failure. This makes sense to me. Seems like you'd want the shell equally or more breathable than the liner.

Thoughts?

[Brendan Swihart]

Any ideas out there?

[Jerry Adams]

I have a bag with Nobul liner, Primaloft polyester insulation, eVent shell which is way less breathable than M50

I weigh my bag after a trip and it weighs only slightly more – 28.3 ounces before, 28.7 after 6 day damp trip – I think any moisture evaporates from body heat

Next time I'll use M50 outer and Nobul liner because it weighs less

[Brendan Swihart]

I'm sure that most of the time it's fine, especially with synthetic fill. I'm just thinking that in cases where condensation is going to be a problem anyway, that a less breathable shell would magnify the problem.

Maybe the newer Nobul2 is the way to go for both liner and shell. I've made a bag for my wife out of the new M50 and it's definitely not the nicest feeling stuff and takes forever to loft.

[Jerry Adams]

Nobul1 feels good on the skin

[Sam Farrington]

Had already decided to use M50 class material for both inner and outer to save weight. For this reason, the question of using something heavier for the outer is moot. But if there were a .66 oz WPB material, I'd err on the side of caution and forego it (except maybe for a tent wall – dream on). Packing the quilt/bag to protect it from moisture seems like a lighter alternative than heavier shells that may or may not cause condensation inside.

But I don't use a shelter where the bag is exposed to condensation on the canopy inner. It must be double wall, or have good ventilaton and protective netting at least 18" high everywhere inside. If using a tarp, maybe there is a need for a WPB shell. Tarp users would know.

[Colin Krusor]

Brendan, I've had the same concern. I plan to make a down quilt for my wife and I some time this winter, and I considered using Nobul1 for the shell and Nobul2 for the liner, so water vapor would move out more readily than in. My wife dislikes clammy/plasticky liner materials, though, so I bought Nobul1 for the whole thing because I thought it might be more comfortable for her.

I'm considering using a strip of silnylon for the shell and liner along the edges (the outermost 6"), to prevent wetting of the down by contact with the damp walls of the shelter, but I worry about water vapor getting trapped in there. I did this with a synthetic summer quilt I made this year and it worked well.

[Brendan Swihart]

Colin, do you have any concerns about the Nobul1 for the shell? The quilt I'm currently using is 1st gen M50 (the bright orange stuff), which has mostly been excellent, and the high breathability has been great for fast drying/airing out. It's mostly been a perfect quilt; mostly I'd like to make a couple shape tweaks.

[Joshua Kronberg]

It seems to me that you could design your quilt with more supple liner and and less breathable shell if you were able to reverse it on occasion. In this case the more breathable material would be on the outside and you could vent off any accumulated moisture.

[Jerry Adams]

Maybe it doesn't matter if shell is a little more or less breathable, water will evaporate out regardless

unless its somewhat below freezing, in which there will be a place inside insulation where it reaches freezing level, and water vapor that hits that spot will freeze regardless of breathability of shell

[Colin Krusor]

Brendan, I'm not too worried about Nobull1 being too breathable for the shell. I think some convective heat loss is an acceptable price for reduced accumulation of moisture in the down. I think we might be glad for this in the shoulder seasons or on trips to the PNW, where the humidity will be higher and the sun might only come out intermittently.

[steven franchuk]

I can remember multiple threads from a couple years ago saying that having a less breathable shell is a bad idea (especially with down bags) because it's easier for moisture to get in than out,

Just an observation but I hae notice in multiple post people having issues with condensation in Bivys made wit DWR breathable materials such as (M50). But I have also seen comment from people with Event bivys (less breathable than M50 but waterproof) Stating that they didn't have condensation issues. Such zas Jerry's "I weigh my bag after a trip and it weighs only slightly more – 28.3 ounces before, 28.7 after 6 day damp trip".

I don't think we can say that more breathable is better always better. In a bivy made from M50 air flows in and out so fast the temperature inside will probably be very close to the outside temperatures. If the air temperature drops to the dew point. the air inside will quickly follow and condensation will inside will occur. In an Event bivy the flow of air in and out is restricted. As a result a persons body heat will start to influence the temperature inside the bivy. If a persons body temperature can keep air temperature inside the bivy above the dew point, condensation may form outside the Bivy while the inside of the bivy stays dry.

So I think there might be a optimal range of breathability for a Bivy. If you have too much you will get condensation when the outside temperature drops below the dew point. If you have to little the humidity inside will build up until you get condensation inside even it the outside air stayed above the dew point. Between those two values I think you might have a bivy that will stay dry even if the outside air is at the dew point.

I don't know if this is true but it is worth considering.

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