Your base layer is not meant to keep you warm

You may already be ready for an argument about this: “but I am warmer with my base layer” you will probably angrily reply to the screen (though you can comment it at the bottom too…), but wait a second – I have a point here. Base layers will keep you warmer than being naked, or even wearing a t-shirt, but a real base layer (not so called “thermals“) is not meant to keep you warm. What base layers are meant to do is to prevent you from getting cold – this might sound like semantics, but it is not. If you can accept the basic rule that base layers are meant to prevent you from getting cold, not to keep you warm, you are much more likely to buy the right item, be more comfortable when active and stop supporting a vast false marketing system (which we are all against?).

base layers are meant to prevent you from getting cold, not to keep you warm

The question about which base layer is warmest and which one I should get to keep me warm is one of the questions I heard the most in my days in outdoor retail. This is not an accident – retailers and brands alike try to tell us that this base layer is super thermal (no such thing) and will keep us nice and toasty without any bulk. If you buy into this idea you will probably also accept that you should have several of those layers to “layer up” and other myths told on the floors of outdoors stores. To make a clear stop to that, go back to read my post about insulation to understand how your base layer can not keep you warm when you are static (hint: it is called conduction).

I don’t expect anyone to just take my word on this, so here is the full explanation why your base layer is not meant to keep you warm:

The heat loss quartet (or, why we use base layers)

Energy loss (in the form of heat) can happen in 4 ways:

1. Conduction – contact between two objects with different temperatures, like your body and the air
2. Convection – movement of air carries the energy away, i.e: wind chill
3. Radiation – transfer of heat without direct contact via infrared rays, like sun heat
4. Evaporation – energy loss from the transfer of liquid to gas, i.e: sweat

These 4 energy loss processes are dealt with in different ways when it comes to dressing for the outdoors and we use different tools for each:

• Conduction –  requires insulation to prevent direct contact with cold air, rain or snow
• Convection – wind and water proof layers help wind chill cool off the body even more, best used over an insulation in very cold conditions
• Radiation – requires using even thicker insulation to push the colder air away from the body
• Evaporation – using base layers to prevent sweat evaporation from the skin but to facilitate its evaporation from the base layer itself

And just like that, we have explained what a base layer does and how it fits with our layering system. As you can imagine, base layers will have some level of conduction or radiation prevention because they have some volume of fabric. The thicker and smarter the fabric, the more it will offer some form of insulation, but at their core base layers exist to prevent evaporation.

How base layers work and how to use them

Lets talk sweat

Sweating (perspiration) is one of those amazing biological mechanisms that nature created to make sure mammals don’t over heat and suffer from internal organ failure. Different mammals sweat in different ways (like the tongue for dogs) but humans sweat directly from their skin via sweat glands.

The main idea behind sweating is to use evaporation to cool the body when we are warm – whether overly dressed, being active or just in a warm environment. The body aims to stay at an average temperature of 98ºf (36.6ºc) and will use all means necessary to do so: shiver to warm up muscles (while burning calories), raise hair to trap more warmth in the air or sweat to cool the skin and so the rest of the body. The actual evaporation is the movement of the liquid sweat into a gaseous vapour that is released into the air; this movement also takes a lot of energy and cools the surface it evaporates from rapidly.

The base layer as a second skin

Now that the concept of sweat is clear, you can easily see where a base layer fits into the story: in cold conditions or in situation where rapid cooling of the skin is not welcome, sweat can evaporate from the base layer instead of the skin. For this to work, the base layer needs to have as much contact with the skin as possible (to avoid “pooling” spots), which means very, very tight. This is also the reason why many times base layers are described as a “second skin” because that is what they are meant to be: the layer that gets cold instead of your skin.

Base layers have two more essential characteristics beside being skin tight:

• Absorb sweat very well: the fabric needs to be hydrophilic (absorbent) on the inside to “pull” the sweat away from the skin as quickly as possible.
• Dry as fast as possible: if the sweat lingers in the fabric too long it will start cooling your skin and increase the chill we are trying to prevent. For that, the fabric needs to be hydrophobic.

To get the most from your base layer you need to pick one that will be tightly fitted, even if you are self conscious about your body, as it will not be effective otherwise. An effective method is to wear your base layer under a light hiking shirt or with shorts if you feel a bit self conscious – just make sure it is tight.

Training apparel falls under the same category as base layers, but are just meant to also be seen, unlike base layers. If you have a range of training clothes that are tight you can easily use them as base layers; and vice versa.

Full sleeves or shorts?

The decision on what kind of garments to use are very much about preference: you can use long sleeve or short sleeve items in any mix. I personally prefer using a short sleeve top and thigh length underwear in all conditions, adding a longer shirt on top when it is chilly and long tights when really cold. The idea is to protect the core more than anything else, so if you know that your arms and legs tend to be on the warmer side, stick with short sleeve base layers.

If you tend to be very warm, you can use the base layer on its own, adding a wind layer only if very exposed and insulation only when stopping. Many times this is what I do on milder or very fast-moving trips.

When to use a base layer

Skin chills and its dangers

Skin chill (not the same as “chills”) is when your skin gets cold from sweat in cold or windy conditions, leading to a rapid drop of inner temperature that can end with hypothermia if you aren’t careful. Skin chill is not an issue in situations where you are not sweating, only when we are over dressed or very active in exposed conditions.

Base layer to prevent skin chill

As a second skin, your base layer is there to take that very efficient sweating mechanism that is meant to cool your body when active, and slow it down, allowing the body to cool in safer conditions. Using an over jacket during breaks when active and having an efficient base layer will help in slowing down the cooling process.

External conditions that demand a base layer

As a general rule, if temperatures are low and you plan to be active, a base layer is good to have. Your goal is to prevent skin chill so think about the conditions: will you reach a peak that will be windy and cold? Are you running on a ridge that is chilly?

Base layers are actually more vital for breaks than for the time you are active, as cooling down and muscle cramps are a bigger issue at this time than keeping warm while moving. That means that even if you are usually warm while moving, a base layer will be especially important to make sure you don’t get very cold from sweating buckets.

Bottom line: if it is cold and/or windy and you will be sweating, you should have a base layer on to protect your core.

Fabrics truths and falses

There are a few statements that come up often when it comes to base layers, mainly around which fabric to use; some are true and some aren’t, either from false ideas by users or marketing spins in the outdoor industry. To clarify, here are some of the main fabrics out there and a couple of sentences on each:

• Cotton – I’m putting this in as a warning: cotton is the worst kind of base layer you can ever choose: it hydrophilic in nature and dries slowly. Cotton should only be used at home/everyday or in hot weather environments.
• Merino wool – the biggest seller of base layers in the last decade or so, it is hydrophilic and dries only slightly quicker than cotton, but it is naturally antibacterial (so doesn’t stink after lots of use with no wash) and has some insulation properties at all fabric thicknesses. You can have a more in depth look at it here.
• Polypropylene – a Hydrophilic polymer (oil based) that is highly cosumizable but has one big drawback: it tends to retain body odour and will stink after only a few hours of use. This is a very useful fabric to “pull” sweat away from the body and so very useful in cold weather when we want to keep the skin dry.
• Polyester – a Hydrophobic polymer (oil based) that is highly cosumizable and robust and so dries extremely fast. Usually used for athletic clothing in combination with elastic threads to increase mobility.
• Bamboo – a relatively new fibre that was supposed to be a more sustainable alternative to Merino wool, with the same characteristics and properties. Bamboo actually is not as sustainable or as antibacterial as it has been claimed.
• Coconut – another new joiner, made from coconut husks that are being processed to carbon and produces a hydropobic, light, antibacterial fabric. The biggest downside of coconut fibre is how fragile it is.

Additional fabric compositions:

• Hybrid fabrics – the best way to get the most out of a couple of fabrics like Merino and Coconut (Rab’s Meco), or Polyester (outside) and Polypro (inside), to create a fabric that is “pull” and “push,” or polyester and Merino to get the drying and robustness of polyester and the softness and antimicrobial effect of Merino.
• Silver treatment (like Polygiene)- an odour resisting treatment that can be added at the factory level and helps prevent stench in base layers. Uses silver salt to prevent the growth of bacteria.
• Construction – synthetic fibres like polypro and polyester can be engineered to provide better evaporation or absorption with construction. Synthetic fibres can also be hollow fibres, making them a bit more insulating as they create a little air gap in the fabric itself. With synthetic fibres you really do get what you pay for in terms of finish and construction.
• Elasticity – most of the fundamental fibers for base layers are not elastic so many times elestane, polyamide or lycra are added to make a fabric that is elastic enough to be skin tight and comfortable.

My use and take on base layers

In general, I wear a base layer every time I go outdoors: hiking, backpacking, running, cycling, climbing, you name it – I get very hot and sweaty quickly and get cold from that very, very fast when stopping. I have had so many colds from cooling down quickly outdoors that I wearing base layers all the time.

I always wear my Under Armour thigh length (9″) boxer jocks and a short sleeve top: merino for long trips, or the Arcteryx Phase SL short sleeve shirt. When things are chillier, I wear a long sleeve shirt made from Merino on longer trips, or the Arcteryx Phase AR Zip Neck top and maybe throw a wind breaker on top.

On lighter trips I use leggings from a synthetic fabric (running tights) under a pair of shorts but besides that I very rarely need more than my underwear and socks on my legs. I do have a pair of compression calf sleeves that can also fall under the base layer category, and I do use them at times under my trekking pants or with shorts.

As a rule of thumb for me: on cold or fast trips I use synthetic, and on longer or slower trips I will use Merino; but I always, always wear a base layer on my torso.