Gore-tex vs. Pertex for bivy sack?

[Dana]

I'm looking for input on the difference between "Gore-tex Respiration Positive" and Pertex DS fabrics for a bivy sack. I'm leaning toward the OR Highland bivy, which uses the Pertex (all their other bivys use Gore-tex), but I know some other gore-tex competitors haven't been all that great.

Not interested in discussion on the merits of bivies... I've been using my old OR basic bivy, which is now worn out, for years... haven't carried, and haven't missed, a tent in all that time. The OR's basic design fits my style, only the fabric is in question.

 

[DougPaul]

A few refs on breathability of water resistant/proof-breathable fabrics that might be useful:

http://verber.com/mark/outdoors/gear/breathability.pdf
http://www.pitt.edu/~kconover/ftp/Clothing Materials.pdf
http://web.archive.org/web/20040719025505/http:/asc2004.com/23rdASC/summaries/g/GP-21.pdf
http://www.mtsanjacinto.info/viewtopic.php?p=10912sid=0e34612e0dc0876b65ddd90707d07622
http://www.sierratradingpost.com/lp2/waterproof-guide/
http://www.rei.com/expertadvice/articles/rainwear+how+it+works.html

Last I checked (late 2010), Event was the most breathable fabric. Unfortunately, it is expensive.

If you are simply worried about a bit of condensation and spilled water inside a tent, the requirements are not too demanding (eg Pertex). However, if you want to bivy out in the rain and (falling) snow then the higher performance fabrics might be of interest (eg Goretex, Event).

All of these fabrics pass water vapor from the inside to the outside (breathability) due to the temperature difference between the inside and outside--ie your body heat drives the water out.

Doug

 

[Dana]

Hmmm... If I'm expecting rain I use the bivy under a tarp so perfect waterproofness isn't so important, it mainly needs to control splash or blowing snow... but when I'm not expecting rain and I don't put the tarp up and it rains or snows anyway... then waterproofness is more important.

 

[Kevin Rooney]

I have a -40F down sleeping bag with Pertex. Am not too concerned about rain, because at that rating I'd only use it when the precip was frozen, and then all bets are off re: Pertex vs. Goretex because neither "works" at that temp. It's a good bag, but a little noisy compared to one covered in nylon.

I think you'll find the Pertex weighs less, so if you were using it as cold weather bivy, Pertex might save a few ounces. OTH, if all seasons use is planned, then if it were me, I'd go with Goretex.

 

[DougPaul]

http://web.archive.org/web/20040719025505/http:/asc2004.com/23rdASC/summaries/g/GP-21.pdf shows that permeability decreases with temperature above freezing. Presumably this is also true below freezing unless the fabric frosts up (which would likely reduce the permeability).

I too have a -40F bag with a Pertex shell--I have wondered if it retains more body moisture in the down than a taffeta shell would.

Of course, cotton is a still fine shell fabric if you can guarantee that it will stay well below freezing and not come in contact with liquid water. (I believe it is still used in suitable environments (eg Antarctica) and is less prone to icing than modern waterproof-breathable fabrics.) FWIW, I still have a 65-35 (polyester/cotton) shell that is perfectly useable in nice (non-precip or dry-cold) weather.

Doug

 

[DougPaul]

Perhaps I should also add that there is the more extreme approach used by the Stephenson Warmlite bags: built-in VBLs and waterproof shells. http://warmlite.com

Permeability of a bivy sack is a non-issue if used with such a bag.

Doug

 

[Dana]

Problem it's hard to get real numbers for comparison. The Pertex DS is supposed to be 7000 g/m²/day permeability and 10000mm waterproof, but Gore-Tex doesn't publish any specs and the GT "Respiration Positive' fabric seems to be a trademark of OR with nothing to compare it to. That said, the reviews of the Pertex bivy are pretty good.

Probably 90% of the time breathability would be more important, but on those 10% times when I get caught in heavy rain poor waterproofing would make for a l-o-o-n-g night...

At this point I may wait to see what OR does... somebody suggested contacting them about a warranty replacement and when I did they said they'll replace my old one, no problem... but whether I can upgrade at the same time (they no longer sell the same model as my old one) is yet to be determined. I would like to go to a single pole bivy (my old one has no pole) but the OR Alpine bivy (Goretex) is heavier and bulkier than their Highland bivy (Pertex).

 

[HAMTERO]

You might want to check out some Integral Designs bivis. They have a full line including Event.

 

[Dana]

You might want to check out some Integral Designs bivis. They have a full line including Event.

I looked at them (as well as all others I could find) but I prefer the design (particularly zipper location) of the OR bivies the best.

 

[DougPaul]

Problem it's hard to get real numbers for comparison.

The list of references that I posted gives a bunch of real numbers for a number of fabrics.

Unfortunately there are no standards for the tests and not all fabrics are included in each test--thus comparisons between different sources may not mean much. Comparing numbers from a single source is generally more meaningful. You also have to figure out how relevant each test is to your application.

Doug

 

[Dana]

Doug, those were useful links but I didn't see any specs for the Gore-Tex fabric used by OR.

The end result is that I'm going with the GT fabric. OR is replacing my bivy under warranty and letting me upgrade it, but the Highland (the only one using the Pertex) won't be in stock at OR until sometime next year, and I don't want to wait that long. The GT Alpine bivy is only a few ounces heavier than the Highland.

Outdoor Reasearch: Great customer support and they really mean their "Infinite Warranty".

Dana

 

[DougPaul]

Doug, those were useful links but I didn't see any specs for the Gore-Tex fabric used by OR.

Agreed that the refs don't list every fabric and of course cannot list fabrics that have come out post-publication. But they are still the best publicly available refs that I know and as good a base as any from which to extrapolate to other/newer versions of the fabrics.

They also contain the info that Goretex uses a polyurethane membrane (blocking those famous microscopic holes) in their current products. Thus all of its current products probably suffer from the resultant permeability reduction. (EVent's unique feature is that it doesn't need the polyurethane membrane and the microscopic holes are open.)

Doug

 

[Dana]

Just got this from OR customer service:

Great question, the Gore-Tex versions are going to be more waterproof and will have a greater waterproof durability over time too. Also they will breath better so condensation, while still always an issue with bivies, will be less. I would suggest the Highland if you do not plan on using your bivy very often. If it is going to be something you are using on a pretty regular basis I would go for the Alpine or maybe the Advanced.

Having already decided on the the Alpine, nice to see it confirmed.