Vapor Barrier Liners: Theory & Application

This article is reprinted here with permission from Backpacking Light Magazine.

Introduction

Occasionally during the FAQ portion of my slideshows, and frequently at the start of every winter, I receive questions about vapor barrier liners (VBL’s). The content and tone of these questions suggest a general misunderstanding of and slight mystery about them, so in this article I’ll attempt to offer a comprehensive review of VBL’s based on my understanding of and experience with them – basically, what they are, how they work, and when to use them.

I believe that VBL’s can be a critical and pivotal component of winter clothing and equipment systems—and, to a lesser degree, shoulder-season systems. Unfortunately, there is not much information available on VBL’s—an internet search returns information that is mostly outdated, incoherent rambling, or mistaken. My hope is that this article will result in (1) a greater understanding of VBL’s and (2) increased use of VBL’s by those who recreate outdoors in winter conditions, especially those who go on extended multi-day endeavors. This includes backpackers, snowshoers, skiers (Nordic, backcountry, and alpine), alpinists, ice climbers, mountaineers, and even ice fisherman and hunters.

What are vapor barrier liners?

A VBL is a non-breathable material that does not permit the transmission of moisture through it. They are typically made of fabrics like silicone-impregnated nylon, polyurethane-coated nylon, or Mylar; and there are at least two multi-layer proprietary fabrics too. In a pinch a VBL could be made from a plastic trash bag or a foil balloon—I have done both; in essence, any material that does not “breathe” will work. VBL’s are available as articles of clothing—including socks, gloves, pants, jackets/shirts, and vests—and as sleeping bag liners. Later in this article I will address the pros and cons of the various VBL fabrics and forms.

To avoid any question, it should be noted that VBL fabrics are fundamentally different than waterproof-breathable fabrics or treated-breathable fabrics (e.g. acylic-coated nylon, or any fabric with a durable water repellent finish). It is fair to question how “breathable” such fabrics really are, but even the poorest performing will still have some degree of breathability, whereas VBL’s do not permit any transmission of moisture, i.e. zero breathability.

Commercial Availability

No major outdoor manufacturers—not even technical mountaineering companies like Mountain Hardwear or Arc’teryx, whose core customers could arguably benefit most—offer VBL products. There are just a few mom-and-pop cottage manufacturers: RBH Designs has the most “extensive” product line; and other manufacturers like Stephenson’s Warmlite, Integral Designs, Forty Below, and Western Mountaineering also have a limited assortment of VBL products.

The limited commercial availability of VBL’s, I believe, is a function of two factors. First, VBL’s are optimal for just a narrow range of conditions—namely, multi-day outings in frigid temperatures—and so the potential customer base is very small. After all, how many people do you know who go on winter trips for a week or longer? And second, the defining characteristic of VBL’s—their lack of breathability—completely contradicts what consumers are regularly told they want in outdoor performance equipment—breathability—and thus the lack of intuitiveness keeps organic demand low.

I believe that there are a few excellent lightweight VBL products—most notably the Backpacking Light FeatherLite Vapor Mitts (manufactured by RBH Designs) and the RBH Designs Bonded VaprThrm Liner Socks—but that overall consumers are vastly underserved. In fact, I resorted to making my own VBL pants, jacket, and balaclava because I was not satisfied with what is commercially available.

A case study: Why I began using VBL’s

In the winter of 2004-05 I snowshoed 1,400 miles of the North Country Trail through both peninsulas of Michigan, northern Wisconsin, and northern Minnesota as part of my 7,800-mile 11-month Sea-to-Sea Route trek. With temperatures as low as -20 F and a consistent snowpack of 2-4 feet, this was unequivocally the most difficult part of the entire hike. It was my first serious winter experience, and a problem that became immediately clear was that my clothing and sleeping system failed to adequately manage perspiration. For example, my sleeping bag (a top-of-the-line model rated to -5 F with premium 850-fill down) would become more damp—and less lofty—with each long night curled up in it. My running shoes and Forty Below Light Energy overboots were frozen stiff each morning due to trapped foot-sweat from the days before. And sometimes I would perspire so much at night—without noticing it—that my clothes would steam when I emerged from my sleeping bag in the morning.

If not for being invited inside 1-2 times a week by generous locals and having the opportunity to dry my things, I definitely would have shivered through more nights than I actually did. The complete compromising of some of my most critical equipment was unstoppable with the system that I had.

Fast forward two years to January 2007, when I decided to revisit northern Minnesota in the depth of winter, but this time better equipped. Among the objectives of my 380-mile 16-day “Ultralight in the Nation’s Icebox” hike was to perfect my deep-winter gear list, or at least approach perfection. That meant bringing a VBL jacket, pants, socks, gloves, and balaclava. I intended to complete the entire trip without a night indoors, or at least feel that I was capable of doing so. (As it happened, I spent one night inside, about 5 days into the trip, with one of my favorite trail stewards, Ken Oelkers of Silver Bay.) After the trip I made a few small adjustments to my winter clothing and sleep system, but generally these systems were spot-on—they led to a tremendous improvement over my Sea-to-Sea experience.

Having realized the value of VBL’s, I also began experimenting with them in other situations, including during the shoulder seasons and on done-in-day skiing and snowshoe trips. In February and March 2008 I even used VBL’s while removing ice dams off of rooftops in Frisco, CO, including some windswept 7-story buildings at Copper Mountain.

In sum, I have become convinced of the value of VBL’s and have attempted to find the limits of their applicability. They are definitely most critical on multi-day trips in frigid conditions, but they are valuable for both shorter and warmer trips too.

Effects and Benefits

The principal effect of a VBL is stopping the transmission of insensible and sensible perspiration, i.e. sweat, away from your body, effectively creating a microclimate between the VBL and your body. (Without a VBL, perspiration would move away from your body and through outer layers (if applicable), and then hopefully evaporate into the atmosphere.) This entrapment of moisture has three benefits:

First, perspiration will not reach outer layers like a windshirt, insulated parka, or sleeping bag. This is hugely important because in cold conditions your perspiration will often stay in these layers: the dew point is somewhere between your body and the outside atmosphere, and your perspiration will condense from water vapor into actual water, thus wetting the layers. This will cause down and synthetic insulations to ultimately collapse. And it will cause unwanted evaporative heat loss with other fibers like polyester, nylon, and wool.

Second, the wearer is always keenly aware of their rate of perspiration, and they are better able to thermoregulate properly as a result. Without a VBL, you might begin to overheat and sweat profusely without fully realizing it. This will soak layers and cause dehydration, which will lead to poorer circulation and lower respiratory efficiency; you may also waste more time and fuel melting snow to get water. With a VBL, however, this scenario is far less likely to happen: you will notice the rainforest-like humidity level in the microclimate—or, if you really overdo it, the sweat dripping down your back—and you will react by removing layers or increasing ventilation.

Finally, evaporative heat loss is minimized. All forms of heat loss should be carefully managed in cold conditions, and a VBL is an effective way in which to manage evaporative heat loss. (The other types of heat loss are conduction, convective, and radiation.) To illustrate this point, imagine how it feels to work up a sweat while snowshoeing up a mountain and then resting for a few minutes at the cold, windswept summit. Brr…

Applicability: When to use VBL’s

There are no set rules, just guidelines, about when you might consider using VBL’s. In deciding whether to use VBL’s, and which exact items to use, I consider four factors:

1. RealFeel Temperature®. I do not necessarily use AccuWeather’s patented index, but I think the idea is useful—it is a measure of all of the environmental factors that affect how warm or cold I am. This would include ambient air temperature, wind, sun exposure, precipitation, humidity, and ground cover. I find that I can begin to wear VBL gloves in temperatures below 40 degrees F, a jacket and socks below 20, and pants below 10. If it is windy and/or cloudy, if precipitation is falling (particularly cold rain, sleet, or wet snow), and/or if I am walking on or through snow or ice, then I may be comfortable wearing VBL’s in warmer temperatures. If the conditions are opposite (no wind, lots of sunshine, no precip, and dirt or grass ground-cover), then it may have to be colder before VBL’s can be worn comfortably.

The maximum temperature at which a sleeping bag liner can be used is very dependent on the warmth of the sleeping bag. A liner will add about 5-10 degrees of warmth to a bag (not including the warmth preserved by preventing loft loss).

2. Trip length. The longer the trip, the more critical VBL’s become in maintaining the integrity of my clothing and sleep system. On a weekend trip, for example, loft loss will not be significant. On a week-long trip (or longer), however, the loss of a few degrees of warmth each night—due to perspiration entering the sleeping bag and wetting the insulation—would be much more noticeable and consequential. Without VBL’s, I would either need to dry my things during the day or bring an excessively warm sleeping bag so that by the end of the trip it would still offer enough warmth.

While the use of VBL’s is most critical during long-term endeavors, they can still be very valuable during shorter efforts. For example, towards the end of a full day of alpine skiing, when the sun disappears and the temperatures begin to drop, many skiers feel chilled because their boot liners, gloves, and clothing have become damp with sweat during the day. By wearing VBL layers skiers could avoid the compromising of their insulation and the sucking of heat away from their bodies by this trapped moisture, allowing them to catch one more lift at 4pm.

3. Type of insulation. Down is more susceptible to loft-loss than synthetics when exposed to moisture. Synthetics are still vulnerable in the long-term too, but the rate of degradation is less. Therefore, it is possible that I can stretch an all-synthetic system a few days longer than I could an all-down system. Ultimately, the all-synthetic system will fail too, but perhaps not before I finish the trip. Since down is significantly superior in its thermal efficiency, it’s debatable whether the all-synthetic system would be lighter—for example, I could put together a lighter weight all-down system that is unnecessarily warm at the start but that would still be adequate by the end.

4. Effort intensity. In order to avoid over-sweating while using VBL’s, I must be attentive to body heat generation and be willing to regulate it. This is fairly easy during steady, low aerobic activities like hiking, snowshoeing, mountaineering, ski touring, snowmobiling, ice fishing, etc. The task becomes more difficult for activities like alpine skiing and climbing, when periods of intense exercise are followed by periods of sedation, e.g. leading a pitch and then belaying a climbing partner up to the anchor. For high aerobic activities like running, skate skiing, or alpine touring (AT) racing, I find it almost impossible to avoid sweating and therefore VBL’s are probably inappropriate in this context.

Developing your own VBL system: Insider’s Tips

By this point in the article you hopefully understand what VBL’s are, and why and when you should use them. In this final section I hope to explain how to integrate them into your clothing and/or sleeping system, and to point out the pros and cons of various VBL fabrics and forms. All VBL’s are not created equal, and I have developed preferences for what I think is my optimal VBL system.

Layering. VBL’s are typically worn directly against the skin or with a base layer between the VBL and the skin. Personally, I prefer the latter approach, which I believe has a few key benefits. First, the base layer creates a small buffer that minimizes discomfort (i.e. “clamminess”) but without reducing sensitivity to perspiration, which I need in order to make informed thermoregulation decisions. Second, by wearing a base layer I protect my skin from direct contact with the frigid air, which would happen otherwise if I needed to ventilate my “next to skin” garment by unzipping it. Finally, the base layer seems to keep my skin dry enough that moisture-related skin issues (e.g. maceration)do not arise. I like to pair VBL’s with lightweight, form-fitting polyester base layers (like those made by CW-X), not wool. Polyester can be knitted thinner and does not absorb moisture like wool does, which may result in decreased sensitivity.

Forms. An effective VBL system needs to consist of either a sleeping bag liner or a full multi-piece VBL clothing suit. It is redundant and unnecessary to use both a VBL liner and a VBL suit. Personally, I prefer to wear VBL clothing, which has a few advantages. First, I can use a lighter sleeping bag because I can sleep with all of my clothes on—a base layer between my skin and VBL, then all of my other layers outside the VBL. With a VBL liner I can only sleep in my base layer garments; otherwise all of my layers would get wet. Second, I already have all of my clothes on in the morning when I wake up, which saves time and body warmth. Even if I brought all of my non-base layer clothes into my sleeping bag but outside of a VBL liner, I will lose a lot of heat when I try to change into them. And third, I keep all of my clothing dry at night and during the day, except for my base layers, which may become slightly moist with perspiration. If I were to rely exclusively on a VBL bag liner, perspiration would enter and become trapped in my insulated jacket and pants while I wear them during rest stops or in camp. The one downfall with VBL clothing is that I need to have a complete VBL suit, which is heavier and more complex than a bag liner. In the long term, my sleeping bag could be compromised if I am not completely covered with VBL’s. A complete suit would include socks, pants, jacket, gloves, and a hat or balaclava.

Fabrics. The ideal VBL fabric would be a non-slip, 1-layer, 4-way-stretch ultralight fabric with a good hand. To my knowledge this fabric does not exist. Until it does, we have sub-optimal options. Silicone-impregnated nylon and reflective nylon (e.g. Mylar) is slippery, crinkly, and noisy. RBH Designs’ VaprThrm® fabric is three layers and designed to be worn “next to skin”; it feels like a softshell fabric, minus the breathability. This fabric is heavy, and it offers less adjustability than a 3-piece system consisting of a thin base layer, VBL shirt, and an outer layer like a windshirt or ultralight insulated parka. Without stretch, these fabrics are impractical for pants because they are so constricting. The only option is to make baggy pants, which are not conducive to creating a small microclimate next to the skin.

Features. It is very uncomfortable to sweat while wearing VBL’s and so I am constantly trying to regulate my body temperature to avoid it. Regulating can be done quickly and efficiently via features like zippers (e.g. front chest, abdomen, pit, arm, and full leg zips), removable parts (e.g. arm sleeves), and easy on-off adjustments like integrated hoods or mitt idiot cords. During periods of rapid warming or cooling, like during or just after a rest stop, these micro adjustments may be inadequate and entire layers may have to be added or removed.

Conclusion

Vapor barrier liners can be a pivotal and critical addition to wintertime and shoulder-season clothing and equipment systems, especially for those who are outdoors for long periods of time in frigid conditions. VBL’s prevent loft-loss, encourage better thermoregulation, and minimize evaporative heat loss. There seems to be a good deal of confusion and mystery about VBL’s, and through this article I hope that I managed to improve general understanding and inspire more widespread use by explaining what they are, how they work, and when and how to use them.

Vapor Barrier Liner Slideshow: the technology put to use

My first winter experience was during my 7,800-mile Sea-to-Sea hike, during which I snowshoed 1,400 miles through Michigan, Wisconsin, and northern Minnesota (pictured) in the first three months of 2005. It became immediately obvious that my sleeping and clothing system, which was simply a warmer version of a conventional lightweight setup, failed to adequately manage perspiration and loft-loss.

I returned to Minnesota in January 2007 in order to perfect my winter system, which included a complete VBL suit: jacket, pants, socks, gloves, and balaclava. The system was an enormous improvement over the Sea-to-Sea experience – the VBL system eliminated loft-loss, improved thermoregulation, and minimized evaporative heat loss. Notice the accessory carabiner on my shoulder strap, which is one of the ways I make fast and efficient adjustments to my layering system.

During my Ultralight in the Nation’s Icebox hike, I was joined for a night by Backpacking Light staffer Sam Haraldson, whose clothing and equipment system lacked VBL. After hiking for several hours, Sam removed his waterproof-breathable jacket to discover a layer of frost inside it, due to his perspiration turning from vapor into water as it reached the dew point, which was inside his clothing system. If he had been out for more than a night, the moisture inside of his system would have caused his insulated jacket to fail.

Without a VBL, it is necessary to dry clothing and equipment frequently. This is difficult in cold conditions, but possible. Even though I had VBL, I took advantage of a relatively warm and sunny day to dry two sleeping bags and a bivy sack, which had become slightly damp due to snow-covered ground and frozen moisture from breathing.

VBL are most critical on long-term trips in frigid conditions. But I have also found them useful during the shoulder seasons and done-in-a-day winter efforts. My favorite example of the latter is when I used them while removing ice dams off rooftops in Frisco, CO – the VBL helped to minimize evaporative heat loss and kept my insulation dry, which prevented me from getting chilled by the end of the day.

Steady, low-aerobic activities like hiking, snowshoeing, mountaineering, and ski touring are most conducive to the use of VBL because your heat output is consistent and can be managed easily. Stop-and-go activities like climbing and backcountry alpine skiing are challenging for VBL use because your heat output is more erratic.