Air Permeable Fabric for MYOG Bivy Sack

ripstopbytheroll.com has some.  I made a jacket with, I think it was 1.4 oz/yd2 W/B.  Pretty breathable.

I made bivy with M50 from thru-hiker.com.  0.7 oz/yd2.  That has been good except it wouldn’t be so good in direct rain.  Fine with dew/frost/mist

Thanks Jerry.  I actually have a remnant of that at home that I was planning to use for some rain mitts.

Is that fabric actually air permeable though? I seem to recall someone asking RBTR when the fabric was first released whether it was air permeable and they noted that they hadn’t tested the air permeability and that you could not exhale through it for example.  But maybe some testing has been done since then.

I’ve found that regardless of the MVTR rating, air permeable fabrics seem to generate far less condensation than non-air permeable fabrics.

All those breathability measurements are confusing to me : )

I made a jacket from some WPB fabric from some other vendor and I had a lot of condensation

With the RSBTR fabric it’s much better, I still get some though.  If I stop hiking and generating so much sweat, it dries after a while.

For a bivy, you generate a lot less sweat so I’d think condensation would be less.

Discovery Trekking has 2.1 oz/71gsm Power Shield Pro. 5000mm HH and 2 CFM, allegedly

The big concern I have with the Bivy in winter is a slow accumulation of moisture in my insulating layers.  So I wouldn’t expect to see a rapid build-up of condensation on the inner surface of the fabric like you might get with a rain jacket during vigorous exercise but more like a humid environment caused by insensible perspiration that eventually causes condensation to form inside my insulation.

I plan to use a vapour barrier suit but given how critical it is to protect my down in winter, I’m trying to go for a belt and suspenders approach.

Look up dew point in past posts. In sub freezing temps that will effect condensation more than how breathable the outer shell fabric is.

Thanks for the pointer.  For anyone that’s interested, the discussion can be found here:

https://backpackinglight.com/forums/topic/28956/

The gist of it is that in freezing temperatures, condensation will form on the first surface (even if highly breathable) that is below the dew point temperature, which could well be somewhere inside the sleeping bag.

From this, it seems the only real protection against condensation in winter is a vapor barrier liner or suit.

The only thing I find a bit puzzling though is how breathability does seem to make a difference in warmer weather – in rain jackets for example.  If condensation forms on the first surface water vapor contacts that is below the dewpoint, then it must be the case that the rain jacket is that surface when people experience condensation and in that case, it seems like breathability wouldn’t matter that much. The only explanation I can think of is that highly breathable rain jacket fabrics actually provide some ventilation or air circulation so that the user doesn’t heat up as much and so that vapour is actively moved out through the fabric before it can cool enough to condense.

Anyway, I plan to use my bivy mostly in winter but I expect I might use it in shoulder season from time to time as well…so the search continues.

-Patrick

Thanks for pointing that out.  They also have polartec neoshell, which has a higher hydrostatic head and is still very breathable and air permeable.

I’m always a little nervous about using PU-based waterproof breathable fabrics.  I’ve had several jackets in the past where the PU delaminated.  Also the cost is pretty high so with all the materials, I might be coming close to what it would cost to just buy an MLD eVent soul bivy.

I have had two PTFE jackets, a pair of pants and a sleeping bag delaminate. PU will decompose if stored wet. The PU is cheaper, the PTFE works a bit better in my experience. If you are using a VBL I would use either (or even just a lighter DWR top if you know it will stay cold for the trip).

Speaking of belt and suspenders, on one winter trip in Olympic National Park we were both on the coast in the rain and then inland in sub freezing temps. I use a VBL shirt AND a VBL bag liner so I could wear my down jacket inside the sleeping bags. They were what I had. To be more weight efficient I should have had a half bag vbl to pair with the jacket.

I want to have a highly water resistant top to guard against getting insulation wet from the outside (e.g., if there’s an unexpected warm-up or precipitation, etc.) also if I want to use a snow cave.

For my VB setup, I currently use Lightheart Gear jacket and pants made out of silnylon as my rainwear so I just wear them over base layer in the winter and they work well as a VB layer.  If it warms up enough to have wet snow or rain, then I can stand to put up my shelter before putting on puffy insulation and I just wear the rain jacket and pants on the outside until I’ve got my shelter up.

Sounds like a good system.

I seem to recall Oware having a neoshell bivy at one point?   Were there any issues with it or was it discontinued due to lack interest, material sourcing problems,etc.?

I have used Gore Products, eVent, Epic and Cuben Fiber in bivys when the manufacturers were trying to get new products figured out. Couldn’t order enough fabric to keep their interest tho. Sometimes have been able to split orders with other small makers. The grey market on such stuff seems to have dried up.

Just ordered a roll of the WPB from RipByThe Roll for some bivysack tops. Should make some very water resistant bivys at a light weight.

This stuff is the most breathable I’ve ever used – he sells the fabric.  He’s a cool guy too.

http://milesgear.com/

I was just cowboy camping.  Clear sky.  25 F.  The outside was frosty – too bad my phone didn’t work because it was too cold and couldn’t take picture.

Even though the fabric is breathable, there was also a lot of condensation inside the bivy.

I think if I had been in a tent there would have been no condensation – clear sky means radiative cooling so the bivy fabric got much colder, so more condensation inside.

The down underneath stayed dry.  The whole thing dried out the next day in the sun.  The next night I set up the tent because it snowed, and there was no condensation.