Vapor Barrier Liners: Theory & Application

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

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I believe that VBLs 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 Unfortunately, there is not much information available on VBLs; an internet search returns data that is mostly outdated, incoherent rambling, or mistaken. My hope is that this article will result in (1) a greater understanding of VBLs and (2) an increased use of VBLs by those who recreate outdoors in winter conditions, especially those who engage in extended multi-day endeavors. This includes backpackers, snowshoers, skiers (Nordic, backcountry, and alpine), alpinists, ice climbers, mountaineers, and even ice fishermen and hunters.

What are Vapor Barrier Liners?

A VBL is a non-breathable material that does not allow moisture to pass through it. Typically crafted from materials like silicone-impregnated nylon, polyurethane-coated nylon, or Mylar, they can even be fashioned from easily accessible items such as a plastic trash bag or foil balloon in a pinch. The essence is any material that does not breathe will work effectively. VBLs can be found in various clothing forms that include socks, gloves, pants, jackets/shirts, vests, as well as sleeping bag liners. I will elaborate on the advantages and disadvantages of different fabric and form options later in this article.

To clarify, VBL fabrics differ fundamentally from waterproof-breathable fabrics or treated-breathable fabrics (like acrylic-coated nylon or those with a durable water repellent finish). While one might question the actual breathability of these fabrics, even the least efficient ones offer some breathability, whereas VBLs permit no moisture transmission whatsoever – hence, they maintain zero breathability.

Commercial Availability

Notably, major outdoor manufacturers – even renowned mountaineering brands such as Mountain Hardwear or Arc’teryx, which could significantly benefit some customers from VBLs – do not produce them. Only a few small manufacturers produce these specialized products: RBH Designs offers the most extensive product line, while others like Stephenson’s Warmlite, Integral Designs, Forty Below, and Western Mountaineering have limited options available.

I believe that the limited commercial availability of VBLs stems from two primary factors. First, VBLs are most effective in a narrow range of conditions – essentially, during multi-day outings in extremely cold temperatures – which shrinks the potential customer base significantly. After all, how many individuals do you know who venture on winter trips lasting a week or longer? Secondly, the defining characteristic of VBLs – their non-breathability – stands in stark contrast to what consumers are frequently told they desire in outdoor performance equipment: breathability. This contradiction contributes to low organic demand.

Despite this, I consider a few lightweight VBL products, particularly the Backpacking Light FeatherLite Vapor Mitts (manufactured by RBH Designs) and the RBH Designs Bonded VaprThrm Liner Socks, to be excellent choices; overall, however, consumers remain largely underserved. In fact, I took matters into my own hands and created my own VBL pants, jacket, and balaclava, as I was not satisfied with the offerings available commercially.

A Case Study: My Journey with VBLs

Winter 2005 was particularly memorable when I tackled 1,400 miles of the North Country Trail across both peninsulas of Michigan, alongside northern Wisconsin and Minnesota, during my larger 7,800-mile Sea-to-Sea Route trek. Encountering temperatures as low as -20°F and a continuous snowpack of 2-4 feet, this truly tested my endurance. It was my first serious winter experience, and swiftly, I recognized that my clothing and sleeping systems struggled to effectively manage perspiration. For instance, despite having a top-tier sleeping bag rated to -5°F with premium 850-fill down, I noticed it grew damper and less lofty with each successive night spent curled up inside. Moreover, my running shoes and Forty Below Light Energy overboots froze solid each morning due to the foot sweat trapped from the previous days. On some nights, I perspired so much it steamed the air around me once I emerged from my sleeping bag.

Had it not been for a few generous locals inviting me indoors to warm up and dry my gear, I likely would have endured more nights of discomfort than I did. My essential equipment was undeniably compromised due to the system I had in place.

Fast forward two years to January 2008, when I ventured back into northern Minnesota during the thick of winter, this time better prepared. My objectives for this 380-mile, 16-day "Ultralight in the Nation’s Icebox" hike included perfecting my deep-winter gear list — effectively utilizing a VBL jacket, pants, socks, gloves, and balaclava. I aimed to complete the entire itinerary without spending a single night indoors or, at the very least, feel capable of doing so. (In reality, I ended up sleeping inside just once about five days into my journey with one of my favorite trail stewards, Ken Oelkers of Silver Bay.) Post-trip adjustments were minimal, but overall, my winter clothing and sleeping systems were superb; they significantly outperformed my earlier Sea-to-Sea experience.

Realizing the value of VBLs, I began using them in other scenarios, including shoulder seasons and day trips involving skiing and snowshoeing. In February and March 2009, I even donned VBLs while removing ice dams from rooftops in Frisco, CO, even tackling some windswept seven-story buildings at Copper Mountain.

Ultimately, I have become an advocate for VBLs and explored their limits of applicability. While they are indispensable during multi-day trips in frigid conditions, I have also found value in using them for shorter and warmer trips.

Effects and Benefits

The primary function of a VBL is to inhibit the transmission of perspiration, thereby creating a microclimate between the VBL and your body. Without a VBL, perspiration typically moves outward through outer layers – hopefully evaporating eventually. The entrapment of moisture yields three benefits:

First, moisture doesn't reach outer layers such as windshirts, insulated parkas, or sleeping bags. This matters significantly because in cold climates, perspiration often stays trapped in those layers. The dew point lies somewhere between the body and the outside air, causing perspiration to condense into liquid water, which dampens the layers. This inevitably leads to the collapse of insulation such as down and synthetics, producing undesirable evaporative heat loss as well with fabrics like polyester, nylon, and wool.

Second, wearers become acutely aware of their perspiration rate, enhancing their thermoregulation capabilities. Without a VBL, it is easy to sweat profusely without recognizing it, saturating layers and leading to dehydration, which negatively affects circulation and respiratory function. Carrying enough snow to melt for hydration can also be a nuisance. However, with a VBL, such scenarios are less likely; users can detect the increasingly humid microclimate or observe perspiration dribbling down their back and respond by shedding layers or boosting ventilation.

Lastly, evaporative heat loss is minimized. Managing all forms of heat loss is essential in low temperatures, and VBLs serve effectively in managing evaporative heat loss (alongside conduction, convection, and radiation). To illustrate this, think about snowshoeing uphill and then resting in the cold wind at a summit – the chill can be quite shocking.

Applicability: When to Use VBLs

While there are no rigid rules, guidelines do exist regarding when to consider utilizing VBLs. Here are four crucial factors I evaluate:

1. RealFeel Temperature: Though I don’t rely on AccuWeather’s proprietary index, I find its concept valuable as it measures all environmental aspects affecting comfort. I begin using VBL gloves below 40°F, jacket and socks below 20°F, and pants below 10°F. If conditions include wind, cloud cover, precipitation (especially frigid rain, sleet, or wet snow), or snow and ice underfoot, I could comfortably wear VBLs at higher temperatures. Conversely, in calmer conditions (no wind, considerable sunshine, no precipitation, and dry ground), lower temperatures may be necessary to wear VBLs comfortably.

The upper temperature limit for using a sleeping bag liner depends on the sleeping bag's warmth. A liner typically boosts a bag’s warmth by 5-10°F (not factoring in warmth preserved through loft maintenance).

2. Trip Length: The longer the outing, the more critical VBLs become in preserving clothing and sleep systems. Loft loss may not be impactful on weekend trips, but on week-long excursions, the gradual decrease of warmth each night caused by perspiration within the sleeping bag necessitates consideration of VBLs. Without them, I'd either need to dry my belongings during the day or bring a significantly heavier sleeping bag to retain sufficient warmth by the end.

Although most essential for prolonged trips, VBLs also lend value on shorter outings. For instance, towards the end of alpine skiing days, many skiers chill due to dampened insoles, gloves, and clothing created by sweat. Using VBL layers could prevent insulation compromise and loss of warmth due to trapped moisture, enabling them to catch one last lift at 4 PM.

3. Type of Insulation: Down insulation is more susceptible to loft loss from moisture than synthetic options. Although synthetics experience degradation over time, the process is slower. Hence, it’s plausible that an all-synthetic system could last a couple of days longer than a fully-down system. Nonetheless, down provides far superior thermal efficiency, leading to debates about weight; for example, a lighter all-down system may seem excessively warm at the start yet remain efficient by the end.

4. Effort Intensity: To prevent excessive sweating with VBLs, I remain vigilant about body heat generation and regulation. This is manageable during lower-intensity activities like hiking and snowshoeing, but with activities like alpine skiing or climbing – where intense exercise alternates with periods of rest – maintaining regulation is more challenging. For high-intensity activities, such as running or alpine touring racing, I find it very difficult to bypass perspiration, which makes VBLs unsuitable.

Developing Your Own VBL System: Insider's Tips

By now, you should grasp what VBLs are and the rationale behind their use. In this conclusive section, I aspire to detail how to integrate them into your clothing or sleeping systems, outlining pros and cons of various VBL fabrics and forms. Not all VBLs are made alike, and I have developed preferences for what I consider my optimal VBL setup.

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Layering: VBLs are generally worn directly against the skin or with a base layer in between. I favor the latter approach, as it provides several advantages. First, the base layer acts as a buffer, reducing discomfort without sacrificing sensitivity to perspiration – key for making informed thermoregulation decisions. Second, it protects skin from direct cold air contact when ventilation is necessary. Lastly, a base layer helps keep the skin dry enough to prevent moisture-related skin issues (like maceration). I prefer pairing VBLs with lightweight, tight-fitting polyester base layers (similar to those made by CW-X), rather than wool, as polyester does not absorb moisture like wool may.

Forms: An effective VBL system should offer either a sleeping bag liner or a full multi-piece VBL clothing outfit. There is no need for both a VBL liner and a VBL suit. Generally, I prefer VBL clothing for several reasons. First, wearing a full ensemble allows for a lighter sleeping bag since I can sleep in all my clothes (with only a base layer between my skin and the VBL). Second, I wake up already dressed, resulting in time-saving and warmth retention. Lastly, the majority of my clothing remains dry overnight and throughout the day, save for my base layers, which may dampen slightly from perspiration. Conversely, relying solely on a VBL liner means that perspiration could penetrate my insulated jacket and pants while resting. Of course, the disadvantage of VBL clothing is that it requires a complete outfit, which is bulkier and more complex than solely a sleeping bag liner. Moreover, failing to cover one’s entire body in VBL can jeopardize the sleeping bag’s integrity in the long term. A comprehensive outfit should include socks, pants, a jacket, gloves, and a hat or balaclava.

Fabrics: The ideal VBL fabric would ideally be a lightweight, non-slip material that offers four-way stretch without sacrificing comfort. Unfortunately, such a fabric currently does not exist. Until it does, we must resort to available alternatives. Silicone-impregnated nylon and reflective nylon (like Mylar) can be slippery, noisy, and crinkly. The RBH Designs VaprThrm® fabric consists of three layers and is intended for direct contact with skin; it resembles softshell fabric without breathability. However, its weight and limited adjustable features prove less effective than a three-piece setup comprising a thin base layer, VBL shirt, and an outer garment (like a windshirt or ultra-light insulated parka). Without stretch, these fabrics become impractical for pants due to their constricting design, necessitating creating looser-fitting pants that fail to create an adequate microclimate against the skin.

Features: Sweating while wearing VBLs proves uncomfortable, which compels me to manage body temperature constantly. Regulating temperature becomes straightforward and efficient through the addition of features like zippers (front chest, abdomen, pit, arm, and full leg zippers), removable parts (sleeve attachments), and easy-to-adjust components (like integrated hoods and mitt cords). During rapid temperature swings or immediately after rest stops, micro-adjustments may become insufficient, warranting complete removal or addition of layers.

Conclusion

Vapor barrier liners stand to be a crucial and transformative element in winter and shoulder-season clothing and equipment systems, particularly for those engaging in extended outdoor activities under harsh cold conditions. VBLs foster insulation retention, optimize thermoregulation, and curtail evaporative heat loss. While confusion and mystery may surround VBLs, I hope to have clarified their purpose through this article, prompting greater understanding and inspiring wider adoption by illustrating what they are, how they operate, and when to utilize them.

Vapor Barrier Liner Slideshow: The Technology in Action

My initial encounter with winter conditions was during my 7,800-mile Sea-to-Sea journey, which involved snowshoeing 1,400 miles through Michigan, Wisconsin, and northern Minnesota within the first three months of 2005. It quickly became evident that my clothing and sleeping systems — essentially just a warmer version of a traditional lightweight setup — struggled to handle perspiration effectively, leading to loft-loss.

In January 2008, I once more traversed Minnesota intending to refine my winter system, which culminated in a full VBL ensemble: jacket, pants, socks, gloves, and balaclava. This comprehensive system marked a substantial leap in efficacy compared to my Sea-to-Sea experience, as it eradicated loft-loss, improved thermoregulation, and mitigated evaporative heat loss. Notice the accessory carabiner on the shoulder strap, which enables fast and efficient adjustments to my layering strategy.

During my Ultralight in the Nation’s Icebox hike, I was joined one night by Backpacking Light staffer Sam Haraldson, who lacked VBL in his clothing arrangement. After several hours of hiking, Sam unveiled a layer of frost inside his waterproof-breathable jacket due to perspiration condensing upon reaching the dew point. Had he been out for longer than a single night, moisture saturation within his system would lead to failure of his insulated jacket.

Without VBL, frequent drying of clothing and gear is mandatory. This can prove challenging in frigid conditions, though once in a while possible. Despite utilizing VBL, I capitalized on a relatively warm, sunny day to dry two sleeping bags and a bivy sack, slightly dampened from snowy ground exposure and condensation from breathing.

VBLs are critical for long-term trips in extremely cold environments, yet I've also realized their usefulness during transitional seasons and day-long winter projects. An exemplary case of the latter occurred while I removed ice dams from rooftops in Frisco, CO—the VBLs effectively reduced evaporative heat loss and kept insulation dry, preventing chills by day’s end.

Steady, low-aerobic activities, including hiking, snowshoeing, mountaineering, and ski touring, lend themselves well to VBL usage due to consistent heat output. Meanwhile, stop-and-go activities like alpine climbing and backcountry skiing pose challenges as heat output fluctuates.