VBL Hack image

VBL Hack

July 4, 2021

The use of a vapour barrier layer (VBL) within a clothing system, used to block moisture from degrading insulation, is well established in socks. Be they plastic bags or dedicated silicone nylon socks, VBL socks are a valuable tool in mitigating cold injuries, even in these days of high tech and super warm third generation mountain books, like the Sportiva Olympus Mons. Nevertheless, the use of a VBL in other clothing is less well established or even fully understood, which is unfortunate, as employing the concept for other vital body parts can really boost one’s survivability, doubly so in stop-go-stop and high energy output (sweaty) style activities. This short article aims to cover this topic and give some ideas on where and when it’s worth playing with.

How a VBL works

Not wanting to cover the subject of how to suck an egg, a VBL layer works in the same way that a condom works, or Gandalf on a bridge, in that it’s supposed to act as a 100% secure barrier to everything. If I was to be cheeky, I’d add that a VBL is an opposite of what a waterproof-breathable layer is advertised to be, which in reality is generally not so waterproof or so breathable, in that it’s 100% waterproof and 100% not breathable, being a 100% impermeable barrier to moisture, both from the outside and within.

This membrane can be any non-permeable fabric or material, but will generally be silicone or polyurethane (PU) coated nylon, rubber, plastic (as in plastic bag), Latex or Nitrile. The ideal VBL material is usually chosen to be thin enough to fit between layers, such as foot and sock, or hand and glove, although you can get some thicker, fleecy brushed VBL laminates (see RBH VaprThrm), or you can create a VBL that completely encompasses the insulation, as seen on the rubber/kapok boots used on the first ascent of Everest, or the US military Bunny Boots (as used on the first Winter ascent of Denali).

The VBL can either be worn direct to the skin or sandwiched between a thin layer that will reduce that clammy VBL feeling. When worn on the foot, a VBL is generally matched with a very lightweight polypropylene sock, although personally, I’ve worn VBL socks for days, weeks and even months at a time with and without an inner sock, and although an inner sock will make the system a little warmer and cosy, I think you also increase the risk of footrot unless you can replace the inner socks regularly. On very long trips, this is generally not an easy option, meaning just washing your feet every night (using any fungal powder) is easier than washing your inner socks.

One thing worth noting is that once you achieve 100% humidity in a VBL layer you should stop sweating, meaning at the end of the day you won’t be pouring out a litre of sock sweat! It’s also worth noting that men sweat more than women, and will sweat earlier even doing the same level of exercise. This means it can sometimes be harder for women to keep cool in hot weather, but that their clothing will stay a little dryer in cold conditions.

VBL socks have fallen out of favour due to the huge leaps in footwear technology, but they still provide an extra layer of protection when your boots insulation may not be enough, or when you’re pushing the insulation value of your boots. Dry socks always trump wet socks when you’re standing around below the summit of Everest, no matter how good your boots are.

Beyond VBL socks

The most obvious place for the VBL concept is in your handwear, as most often people will struggle with either their feet or their hands. As with the socks, keeping your gloves and mittens dry and fully insulating your hands can be a real lifesaver, especially if you’re using down mitts that can slowly turn into wet rags, and be tough to fully dry overnight in your sleeping bag. Remember that although it’s possible to get down off a mountain with frostbitten feet, doing so with frostbitten hands is far more perilous (it’s worth experiencing the early stages of hypothermia when you begin to lose dexterity, so as to fully grasp how debilitating it can be).

Of course, no climber is going to do a four-week trip wearing VBL gloves, but they are well worth testing out low down on cold days and then using on the summit day. Even if they only provide a marginal benefit, they will still provide a psychological boost (as will dry socks). They also have a place on technical climbs, worn under gloves, as keeping your hands warm in thinner technical gloves is generally asking a lot from a few mills of insulation, but add in sweaty insulation, and they will not perform at all. 

The best gloves for this job are medium weight disposable Nitrile gloves, which are tougher than Latex, and ideally, you want to look for Nitrile gloves designed for car or bike mechanics, as they’ll be a little thicker. When buying them, make sure you fit them oversized, as not to restrict circulation (don’t fit them like a surgeon or serial killer). If you want to try out the concept, then just nab some of those plastic gloves you get at a petrol pump. 

People will often ask about using neoprene gloves in the same way as a VBL, but generally, neoprene is not ideal, due to needing to be a tighter fit, meaning they’re restrictive, and that neoprene is not as good an insulating layer compared to fleece, pile or synthetic insulators.

Even if you’re not sure about VBL gloves, you should always carry several pairs of disposable gloves on any trip. These are not for dealing with medical emergencies, blood etc, but toilet emergencies, the other kinds of bodily fluids, especially if they’re not your own, or just picking up other people’s trash (good for building up brownie points with the mountain).

VBL clothing

Of course, the best way to stop sweat getting into your clothing is to dress appropriately, which generally means wearing as little as you can get away with, or “dress heavily for light work and light for heavy work”. One way to approach this is via the 15-degree rule, in which you dress for conditions 15 degrees warmer than they really are, which in winter would be moving in just your base layer and shell, and avoiding wearing your mid-layers and heavy (static) insulation until you stop.

Most novice climbers will begin the day dressing like they’re standing around at a winter belay, when they’re actually engaging in high aerobatic activity, carrying a load, uphill, often through deep snow. When carrying out this kind of exercise you want to dress more like a cross country skier or skimo racer, i.e. light and easy to dump excess heat and sweat (is there anything sexier than a skinny sweaty Frenchman, skinning uphill, with his lycra onesies unzipped to his belly button?). Once you get to your climb, and those icebox belays, that’s when all the layers need to be put on.

But what if you don’t have the luxury of going light? Well, this could be the case in stop and go activity, or when you’re forced to wear a lot of insulation even when doing heavy work. In such a situation your best bet is to go for a true softshell - or Inuit style system, where you wear one thick layer that can be unzipped and vented instantly before you overheat, allowing you to just dump all that heat and heat generated sweat. This of course is the opposite of the traditional arctic layering idea, layers of wool and fleece, which you’re meant to finely adjust, layer by layer, as you move. In reality, most people don’t have time to adjust and so just sweat into all their layers.

But what if you’re in really extreme weather, and stripping off clothing or unzipping your layers is not an option? Well, here you can try to adopt a VBL layer on your upper body, where sweat is a real issue, reducing, or at least restricting moisture entering your insulation layers.

There are three approaches to this concept, the full VBL top with sleeves, the VBL vest, and the VBL half vest.

VBL Shirt

The full VBL long-sleeved top, as sold by Warmlite and RBH in the US, although very effective at keeping everything dry, is probably too extreme for most people, and even worn over a lightweight polypro top (mesh or Helly style), it’s still going to feel very strange, and is perhaps best suited to some specialist end user.

VBL Half Vest

The VBL half vest is a very good approach to the problem, as it features a VBL layer over the back area, to stop sweaty back, but unproofed nylon or fleece front. This style of top is made by 40 Below, and is ideal for carrying big packs in sub-zero conditions.

VBL Vest

I used a full VBL vest on Denali, which was less of a single VPL style top, but more a synthetic insulted gilet (thin 2.5 oz Climashield Apex), with a breathable inner (Pertex) and VBL outer (silicone nylon). This was worn over the top of a Brynje polypro vest and a thin wool base layer.

This set-up was designed to maximise the hydrophobic and hydrophilic features of all the fabrics involved, with the mesh polypropylene being unable to absorb any moisture directly, but the secondary wool layer, acting like a sponge, able to take 30% of its weight in moisture. This moisture would then be picked up by the Pertex, which spreads it over its entire surface, avoiding clammy spots. The Climashield is hydrophobic and a more effective insulator than fleece, wet or dry, with the VBL acting as the final dead stop barrier. The VBL stops the moisture going into the core of my jacket, which being Inuit style, is the only layer about my VBL layer, and so is where 95% of my actual insulation layer (1.5kg synthetic parka).

Being a vest, I guess that my wool layer actually wicked the moisture away out via my sleeves, but my insulation being secured meant that as soon as I stopped, the insulation just dried out my inner layers, so I never got the usual cold chills even when working hard (unlike other super cold mountain trips, I never had ice inside my layers).

The system worked so well that I often slept in the VBL in order to reduce the amount of moisture going into my sleeping bag (or bags, as I was sleeping in a 3 bag system). The biggest benefit from this system was it seemed to pretty much stopping sweaty back syndrome, which could be a serious issue over a long period and in very cold temps (down to minus 40 or 50).

Although there are a few companies making this kind of clothing, a gilet style top is very easy to make (some good info here), and can be constructed with just a zipped front and back panel, either as a single layer top or an insulated version (make sure you get the lightest insulation possible).


99% of climbers and mountaineers will never need a VBL clothing system and will be able to just make standard non-VBL clothing work for them; overcoming any sweat issues by just toughening it out, just like we have done for 200,000 years. But the more we push the boundaries and performance of existing fabrics, new and old, natural and synthetic, the more necessary it is to know and understand little hacks – like VBL – in order to push these boundaries just a little bit more, and to make it back again.

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