Vapor Barrier Fabrics

Hi all,

I'm interested in sewing a few vapour barrier items and I'm searching for fabric. I've read that Warmlite uses something called 'fuzzy stuff' which is a polyester polyurethane laminate (PUL). I've found a few different sources for similar fabrics but they are all listed as being 'breathable' which isn't quite what I'd like. For example:

https://www.wazoodle.com/fabrics/pul-fabric/1mil-pul/pul-fabric.html

https://www.saharafabrics.com/what-is-pul-fabric/

How 'breathable' should a vapour barrier fabric be?

A vapor (vapour for our Canadian neighbors) barrier fabric needs to be impervious to water vapor (that's the "vapor" for which you want the barrier). Therefore, "breathable" fabrics are out of the question for this use, even though some are not very breathable.

Any fully waterproof, nonbreathable fabric will work. I use silnylon (silicon impregnated nylon), and seal the seams. PU coated nylon or polyester will work but is heavier and not as durable. Plastic is the ultimate.

Note that the vapor barrier layer needs to be underneath your insulating layers. If it's on top, the insulating layers will become soaked with your body moisture and not only useless but dangerous.

There is lots of controversy about the use of vapor barriers. Some cannot use them at all. I use them successfully at night (worn over a thin base layer) when it's below freezing--keeps me warm and my sleeping bag dry. But whether or not using a vapor barrier works appears to be a very individual thing. I suggest that you test it out (using cheap plastic) before investing in anything expensive

As OM said, the vapor barrier must be non-breatheable...that's what makes it a barrier to vapor.

Andrew Skurka has an in-depth article on the subject:

https://andrewskurka.com/vapor-barrier-liners-theory-application/

Thanks everyone. I'm familiar with the theory and use of VBs. I have plenty of silnylon and have used it in the past as a VB. However, I'm looking for a fabric that feels better against my skin. Warmlite's FuzzyStuff® fabric is a PU (polyester-urethane) laminate but is also Teflon coated. Warmlite will sell it by the yard, but I'm not a Teflon fan and I'm searching for alternatives. RBH has their VaprThrm® laminate bonded to Polartec 200 but I don't believe this is available resale, by the yard.

I suppose I have two questions:
1) In terms of the moisture vapor transmission rate (MVTR), what constitutes, good-enough to function as a vapour barrier? Is the MVTR of Warmlite or RBH's fabrics actually zero?
2) Does anyone have a source for comfortable/fuzzy but non-breathable fabric, sold by the yard?

Probably difficult or impossible to find. Since you sew, though, you could make your own by using silnylon and a thin layer of wicking material. Or Perhaps use ordinary PU coated nylon with the coated side out, which seems as though it would feel similar to the material of the sleeping bag, at least as long as you didn't overheat. OM's idea of wearing a thin base layer sounds like the most logical to me.

Incidentally, Warmlite has always had some questionable information on their web site, so I'd be reluctant to consider them an authority. For instance, they say:

"It has been reported that you lose up to four pounds of water each night through evaporation of insensible sweat when sleeping in a porous, breathable sleeping bag. Weighing of such bags in the morning shows 2 to 4 pounds of increase, confirming that statement..."

Other, IMO better sources put it between 40 and 600ml per day, much of it through exhalation.
Also, insensible perspiration is not sweat. There is no "insensible sweat" as referred to in the Warmlite quote.

With Sil-Poly becoming available, I have mostly switched to that from Sil-Nylon. Since you are in Canada, I don't know where to look for vendors. I have been getting fabrics from Rip-Stop By The Roll, but I don't know whether they ship to Canada or what might be involved in getting through the border.

The polyester base layer that I wear under a non-breathable silnylon rain suit works fine to keep my skin away from the silnylon. I keep it on after removing the vapor barrier. Because it's body temperature, the base layer dries really fast from my body heat when the silnylon rain suit is removed--maybe 5-10 minutes.

I usually use Patagonia Capilene 2 but in really cold weather have used Capilene 4 ("expedition weight"). Any synthetic fabric (which absorbs less moisture than wool) will do. The heavier weight base layer still dries in 10 minutes, even faster if I'm doing my "cold morning getting warm" dance. It helps if I've kept my hiking clothing inside my sleeping bag (in a plastic bag if damp) so that it's also body temp when I put it on.

If I need extra insulation at night, it goes on over the rain suit. There have been some 10*F/-12*C nights in which I have worn everything I own inside my 20*F/-7*C sleeping bag, and not changed into my hiking shirt and pants until well after sunrise. Here in the US Pacific NW, that's about as cold as it ever gets ( if colder, I stay home).

Whatever you use inside the vapor barrier will need to be dried every day. If the lining is attached to (or part of) the vapor barrier, the drying process will take quite a bit longer. If the temp stays below freezing, the lining will freeze when you take it off and stay that way. That's why I suggest two separate layers, the inner of which doubles as your base layer.

You'll have to experiment to see what works for you in the conditions you'll be in (presumably far colder than here in the Pacific NW, especially if you go out in winter).

Edit--Sorry, I forgot you're a Canadian neighbor, and you shouldn't have to convert our antiquated Fahrenheit scale!