We often write about the importance of layering your hiking clothing, especially when backpacking in winter conditions. Using a layered clothing system with a moisture-wicking base layer is an effective way to move moisture away from the skin and transport it to the outer side of your clothing. However, in very cold conditions, classical layering may not work that well. In this article, we’ll try to clarify what stays behind the term “vapor barrier liners” (VBLs) and why you may want to try this concept yourself. It’s a completely different concept that challenges accepted wisdom about “breathability” and cold weather protection.
VBLs are popular among people who travel in extreme cold conditions
What are the vapor barrier liners?
Everyone’s body constantly produces liquid, both when having rest and when exercising hard. The former is called insensible perspiration and it comes in the form of a vapor that’s evaporated from the epidermis, while the latter is called sensible perspiration and it comes in the form of sweat that must be evaporated from the body surface. In three-season conditions, thanks to the breathability of the clothing, the humidity produced through perspiration is allowed to escape through the fabric.
However, in winter conditions, some of the advantages of breathability can become serious disadvantages. Take the mechanism of evaporation that reduces overheating and perspiration buildup inside the layered clothing system in warmer conditions. In very cold conditions, this mechanism causes heat loss and – what’s worse – wetness inside the layers due to the fact that the water vapor begins to condense within the layers and not outside of the layered clothing system. The reason is that the dew point is now inside the clothing system and not outside of it as is the case in three-season conditions. The vapor barriers offer a solution to this problem.
The VBL fabrics are fundamentally different than waterproof breathable fabrics as well as any fabrics that claim to be breathable. The main difference is that VBLs are made of nonbreathable waterproof materials that restrict the transmission of moisture. Any material that has zero breathability is suitable for making vapor barriers. For example, polyurethane-coated nylon is among the most widely used fabrics for producing VBLs. The concept of how vapor barriers work is counterintuitive; so don’t worry if you can’t grasp it initially. If that’s the case, you’re like most people – they are often able to understand the principles behind this theory just after they try it themselves.
There is a popular misconception that anyone using vapor barriers would be soaked in sweat. Everyone knows that a body dripping with sweat means chilling and possibly hypothermia. Right? No, not really. Though this may actually happen in certain cases, here’s why this doesn’t happen when you use VBLs properly.
How do they work?
Theoretically, the human body is trying to achieve and maintain a preferred level of humidity. That’s one of the main reasons why it is constantly producing moisture even in frigid conditions when temperatures are way below zero. The vapor barrier theory states that you can reduce the evaporative chilling by keeping the moisture next to your skin. This urges your body to stop or to significantly reduce producing more perspiration ceasing the process of heat loss. One of the best ways to achieve this is by creating an artificial barrier between your skin and the atmosphere. This will make you lose less moisture through perspiration (high humidity levels next to the skin mean that it can’t hold any more water vapor), which will raise the temperature next to the skin. As a result, you will stay warm and your clothing will stay dry because the vapor barrier liner will trap more than 90% of the moisture produced by your body. There are several other positives that come out as a result of using VBLs:
- Less clothing needed
In general, instead of several layers, you just need an insulating layer over your nonbreathable waterproof layer to stay warm. And this can be a big plus for your winter hiking expeditions.
- Increased awareness of your perspiration rate
An increase in your perspiration rate gives you an advanced warning of the raised chance of overheating. Once, there is an indication of possible overheating of the body, you basically have two options: 1) to remove extra clothes to get cooling; 2) to reduce your effort or walking pace.
- Lower risk of dehydration
Less perspiration means that you’re less likely to suffer from a condition related to dehydration such as frostbite, heat exhaustion, hypothermia or altitude sickness. This is very important, especially in dry-cold conditions.
- Reduced loss of warmth
VBLs protect the upper layers of your hiking clothing and your sleeping bag from becoming damp or soaked because all the perspiration stays within the VBL product. This way they remain dry and warm for a longer time.
What kinds of VBLs exist?
There are various types of vapor barrier liners. Some manufacturers focus on the most challenging parts of the body to keep warm (such as feet and hands), while others concentrate their efforts on producing vapor barrier sleeping bag liners or jackets. What’s common to all of them is that they are made of nonbreathable waterproof fabrics that don’t allow any transmission of moisture. Apparently, this feature makes them very different from the garments we discussed in our article about waterproof breathable clothes. Knowing that our hands, feet, back, and forehead are some of the sweatiest areas of our body during high exertion, it makes sense to address this through the application of VBLs. Several types of vapor barriers stand out among the rest:
Vapor barrier clothing is used to prevent outside moisture and perspiration from entering and saturating the insulation layer of your clothing as the application of VBLs will allow you to stay warm and dry.
VBL clothing systems are especially effective in very cold conditions
There’s VBL clothing intended to keep the extremities warm. For example, glove and mitts are exceptionally warm due to their ability to prevent the insulation from wetting out and to share collective warmth throughout the fingers. It’s pretty much the same with vapor barrier liners (VBLs) in footwear. Vapor barrier socks are commonly used to prevent the socks as well as the insulation layers in the footwear from becoming saturated. Moreover, keeping the feet dry is essential for the feeling of warmth. There are also complete clothing systems available, including VBL suits (shirts, trousers, and jackets) as well as balaclavas. As a general rule, VBL clothing is more functional than VBL bag liners since you can also use it and during the day.
VBL sleeping bag liners
Keeping the insulative characteristics of your sleeping system over many days in unfavorable conditions is no less challenging than walking in very cold conditions. Sleeping bag vapor barriers are simple and lighter than VBL clothing. However, sleeping bag liners are less often preferred to clothing. Wearing the vapor barrier in your sleeping bag can add up several degrees of warmth to the bag, and since the barrier is thin and has a slippery surface, it won’t restrict you or make you uncomfortable. Be careful when using sleeping bag vapor barriers because a misunderstanding or misuse of the VBL liner can be much more dangerous than a mere compromising a bit of your sleep comfort. Sleeping bag soaked with external moisture is the last thing you need in extreme winter conditions.
VBL sleeping bag liners such as the HotSac VBL by Western Mountaineering significantly reduce the heat loss from the body
Vapor barrier sleeping bag liners are especially useful for ultralighters who are not afraid of long-term endeavors in extreme conditions.
DIY vapor barriers
Though you can find certain VBL products in some stores and hypermarkets like Walmart, VBLs aren’t very popular among the general population. Probably, this is why few companies take advantage of the VBL technology leading to limited commercial availability of such products. Only a few manufacturers such as RBH Designs, Stephenson’s Warmlite, Rab, and Western Mountaineering produce VBL clothing and gear, which makes it difficult for everyone interested in using VBL layers to be satisfied with what is commercially available. Thus you probably won’t be surprised to learn that many experienced hikers prefer the DIY approach to vapor barriers.
Making your own vapor barriers certainly has at least two advantages: 1) you are less dependent on the commercial availability of these garments (which is often too low because VBL’s are optimal for just a narrow range of conditions such as multi-day trips in severe cold conditions); 2) you can make these products according to your own preferences. The most widely used DIY vapor barriers include polyethylene bags, bread bags, polyethylene food-service liner gloves, nitrile gloves, plastic socks, and rubber gloves.
You can wear your DIY VBLs in various combinations according to your preferences. For example, wearing your vapor barriers over dry thin socks with thicker socks over that helps your feet warm up faster in addition to the not-so-clammy feeling. On your hands, you can use your DIY vapor barriers just under your liner gloves and mitts. The biggest disadvantage of the DIY vapor barriers is that they slide down as they don’t have elastic closures.
Experienced outdoorsmen often use the DIY approach to vapor barrier liners
Keep in mind that VBLs take some experimentation to get used to using them and, what’s more, they may not be for everyone. Moreover, wearing a moisture-wicking layer (like a jacket, glove or sock) between the skin and the VBL fabric will minimize the clammy sensation.
It’s redundant to use both a bag liner and vapor barrier clothing because when using a liner, your clothing can become damp by your own perspiration. It’s also more difficult to notice moisture accumulation inside the VBL bag liner as it could have roomy chambers, which is not typical for the VBL clothing.
When to use vapor barrier liners?
VBLs can be beneficial to all outdoor winter enthusiasts in case they are familiar with the several factors that can determine when to use vapor barriers. The weather conditions together with the insulation type (when exposed to moisture, down loses its loft and degrades faster than polyester and other synthetic fabrics) and how often you will have an opportunity to dry your insulating layers and your sleeping bag on the trail stand out among them (the more frequently you can dry them, the less moisture buildup inside and thus the loss of warmth would be less worrisome). In general, vapor barriers are most efficient in dry cold conditions and in long outdoor trips. Keep in mind that they can be of benefit for both shorter and warmer trips too so don’t limit their use only to hiking in freezing cold conditions.
In the end, your own knowledge, personal experience, and preferences will be essential for maintaining the insulation capacity of your hiking clothing and sleep system at an optimal level for many days.
Photo by Ilana Beer
Using vapor barriers doesn’t come without certain problems, and you need to know not only about the positives of using VBLs but also about the negatives. So, the main problems with the VBLs are the following:
- Waterproof fabrics don’t feel comfortable next to the skin
Most people claim that using a vapor barrier next to the skin makes them feel clammy. This problem can be solved by wearing a thin base layer between your body and the VBL as a synthetic base layer feels more comfortable next to the skin than a vapor barrier and it doesn’t make you feel clammy.
- Psychological issues
Although VBL clothing may perform reasonably well, some people don’t feel confident in it. This is a mainly psychological problem as this type of clothing is ultralight and it makes you feel warm. However, most people perceive down jackets or wool sweaters as warm clothing and not thin nylon suits. Hence the latter doesn’t look as attractive as the former.
- No major manufacturers produce vapor barriers
VBLs aren’t very popular and there are many reasons for this. For example, their use is counterintuitive (too opaque to understand); their best use is in cold winter multi-day hikes (too narrow target group), and as a result, no major companies produce VBL products.
- Your hands and feet will look like you’ve stayed in the bath for too long
This is certain to happen after a long winter day on the trail. There are two positive things, however: 1) your hand and feet will be warm all day; 2) your hiking clothing will be dry.
The use of vapor barrier clothing and sleeping bag liners can be of vital importance for winter hiking in less than ideal conditions. VBLs can be of benefit not only for hikers but also for other outdoorsmen who spend many days in harsh winter conditions such as skiers, mountaineers, alpinists, hunters, and ice fishermen. As surprising as it may seem, it takes a minimal amount of clothing to maintain your equilibrium of output and environment and keep the chill at bay even in subzero conditions.
VBLs are perfect for cold conditions with very low temperatures and multi-day trips, though they can be used for shorter trips and in warmer environments too. They will keep you warm even in snow and bitter cold. However, you need to learn how to use them properly to trap all the excess heat coming off your body and seal it in to stay warm. If you need to be further convinced of the applicability and value of vapor barrier liner clothing and gear, put them to the test and decide for yourself.
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