We’re all more than capable of turning up and looking the part for a day in the field but it is what lies beneath that counts. Are your underpinnings less Bear Grylls and more threadbare and chills? Are your thermals, in fact, pants?

Done correctly, layering can keep you as cool, warm, dry or supported as the situation demands. Shopping for new base layers, however, can feel like going back to science class without having done one’s homework. To make sense of it all, it’s best to return to the basics. “Most of these base layers are about surface level thermal regulation,” explains Simeon Gill, senior lecturer in fashion technology at the University of Manchester. “In essence, how well they can support the local environment close to the skin.”

This is important because humans are heat-producing machines – at wakeful rest we produce about the same amount of heat as a 100-watt lightbulb – and the more we work the more heat we produce. As we heat up we all produce moisture that cools us when we’re warm – but continues to chill us once we’re not. Clothing, therefore, needs to maintain our ‘thermophysiological comfort’.

Ratings systems for garments on this basis can be complex and flawed, so the best way to choose a base layer is to determine how the garment’s characteristics meet your unique needs. Age, sex, fitness and other factors all affect how much heat you produce and how much your skin reacts to both the cold and to different fabrics. Knowing your body shape is also important for finding base layers that can achieve a good fit – something that, no matter how technical a garment’s specifications, is crucial to its effectiveness.

In wintry conditions, warmth is a key consideration

To this end, some manufacturers have invested in ‘body mapping’ technology. “They start to zone the body,” explains Gill. “You can change a garment’s structure and material relative to the individual areas it might be covering.” Unfortunately, however, there are limitations to this for ‘off the peg’ items due to no two bodies being the same. “The ability to plan for body mapping versus actually achieving it are probably a little further apart than people would like to say.”

“Everyone says ‘layer up’,” says Chris Dewbury, Sitka brand ambassador, former pro skier and avid fieldsportsman. “But they often go out in a great big unnecessary jacket, and more often than not end up incredibly hot and stressed, which doesn’t produce the best outcome.”

For most of us, therefore, a little trial and error is required. “Finding quality thermal layers for women in fieldsports can be challenging,” says Hayley Varney, relations manager at The Country Girls UK. “I used to depend on Marks & Spencers thermals but they often felt bulky under my clothes. Thankfully, I discovered Foxy Pheasant leggings, which are perfect for fieldsports all year round, whether I’m out shooting or walking the dogs.” (Read more on how to stay warm in cold houses.)

Aside from your individual physiology, the other factor that should influence your choice of base layer is what level of physical activity your sport will demand, and where it’s taking place. This will not only help you understand what to prioritise in your fabric selection (breathability, wicking, weight, warmth and so on) but also how to build your layers so they don’t counteract the wondrous properties of your new investment.

Sitka’s Core Lightweight Crew with odour control technology is a go-to for stalkers pursuing keen-nosed quarry

Breathability

Breathable fabrics have gaps between their fibres to allow moisture to evaporate, properties you might see measured in terms of Thermal Evaporative Resistance or g/m2/24hrs – how much moisture passes through a square metre per day. The most important consideration with breathability, however, demonstrates why attention to your layers is important: where is the moisture going to go from the garment?

“For stalking I tend to stick with a vented cotton shirt as the base layer; I personally wear Seeland or Jonsson (a South African workwear brand),” says Defence deer manager Major James Cook. “I get the warmth from my mid-layers in the form of a down or fleece jacket. The reason for this is that gralloching and extracting larger deer can be warm work, so I can strip down to the shirt while I do it. Because it’s cotton it will dry quickly, and I can chuck on my warm kit when the hard work is done.”

If you can’t ensure moisture leaves the garment, either because of your external layers or the humidity, fabrics like cotton can become saturated quickly. For these situations, such as rough or walkedup shooting, a base layer that’s not only breathable but also absorbent is optimal.

Garments made with 100% ethical merino, such as Seeland’s Climate Base Layer, offer a variety of benefits, as Woolmark’s Angus Ireland explains. “Wool is the most breathable of the common apparel types: it can absorb large amounts of moisture from your skin (up to 35% of its weight) and release it to the atmosphere. During exercise in cold environments wool maintains a drier microclimate next to your skin, keeping you less clammy”.

Will Skjøtt, who travels all over the world to shoot in a wide variety of climates, swears by Icebreaker’s 175 and 200 merino base layers. “I’ve used them in the UK as well as elk hunting in Sweden in snow, sitting still for hours on end,” he explains. “They’re breathable and comfortable and very thin, and I use them for skiing as well.”

The level of activity your sport demands will influence your choice of base layer

Wicking

Wool keeps skin dry by moving moisture away by a capillary action process known as ‘wicking’. This is distinct from breathability, which concerns the garment’s structure, and is dependent on the properties of the individual fibres themselves.

Wool’s not, however, the only fabric that wicks moisture – or indeed the best at it. Synthetic fabrics such as polyester, Gill explains, can’t absorb any liquid but do wick away moisture far more quickly. So quickly, counters Ireland, that the resulting convection cooling is detrimental: “When you stop exercise in very cold conditions, you could experience three times more chilling wearing synthetic garments (such as polyester) than you will wearing wool,” he explains.

Given their different strengths, combining synthetic and natural fibres often produces positive results. Woolpower’s 200g Long Johns, for example, are made with a blend of 60% merino, 25% polyester, 13% polyamide and 2% elastane, providing the wearer with a mix of absorption, wicking, softness, durability (wool is less abrasion-resistant than synthetics) and stretch.

Combining fabrics also, thanks to the glut of polyester on the market, means that the price point of the garment can be more affordable – though do look for a high percentage of mulesing-free merino and that the company either has verifiable expertise or uses a reputable company, such as Polartec, for its fabric.

One of the best examples of the benefits of blending fabrics is in socks. Unlike base layers for your torso or legs, socks are usually – for obvious reasons – worn next to an unbreathable outer layer of boot or shoe. Mixing layers of different fabrics into socks allows firms to create an inner layer that removes moisture and an outer layer that stores it, keeping your feet dry in the process. While Sealskinz uses the wicking process to move water to outer layers, Drymax uses a hydrophobic inner layer to ‘mechanically lift sweat off the skin like a squeegee’.

One of the best examples of blending fabrics is seen in socks

Odour control

Any odour is less than ideal if you’re after a keen-nosed quarry. “Odour is a huge thing in the field of hunting,” affirms Chris Dewbury, who recommends Sitka’s Core Lightweight Crew as a go-to base layer for a variety of situations: “Polygiene odour control technology is incorporated within the fabric as it’s produced, and it basically neutralises odours.” Polygiene uses silver chloride to stop bacterial growth and is proving popular with fieldsports brands: Härkila’s Forest Hunter long-sleeve T-shirt, launched for this autumn, also incorporates it, while its equally new Base All Season tee uses 80% merino for its natural antimicrobial properties alongside greater insulation.

Edmund Hillary and Tenzing Norgay during their ascent of Everest

Warmth

In the UK we often demand warmth from our base layer. This can be achieved in two ways: either it preserves your core temperature or increases it by way of a heating mechanism. For preservation, some brands, such as Uniqlo with its HEATTECH base layers, will discuss the garments in terms of CLO – a measure of thermal resistance – while others rely on fabric weight to indicate how warm they may feel.

Weight, however, can be less important than the fibre structure. “It’s the quantity of air trapped in the fabric that does the insulating,” says Angus Ireland, “and for that you need crimp.” Crimp is the 3D crinkle pattern that traps the air pockets, and merino is wonderfully warming thanks to being the Walkers Max of the wool world.

Silk is also excellent at preserving an insulating layer of air, as passionate huntswoman Selene Obolensky attests. “I wear a long-sleeve 100% silk undershirt: very thin, very light, very warm. Frankly, a little tipple and the sheer joy of just being in a field is enough for me.”

Synthetics, however, can also engineer an excellent warming structure. Brynje’s innovative mesh, for instance, invented by fusing together fishing nets and worn by Edmund Hillary on his ascent of Everest, is superb at trapping air pockets and comes in lightweight union suit-type garments that are favoured by the Norwegian special forces.

As for providing an external heat source, options that help not hinder in the field are growing by the day. The Härkila Heat range, for example, offers USB-chargeable, washable synthetic base layers with a range of temperature settings, while Under Armour and Celliant (used by Kymira) have created mineral-infused fabrics that reflect heat back to the body and, they claim, improve overall function. Scientific studies on these technologies are still sparse, but watch this – breathable – space.

Stockings worn by explorer Sir Joseph Hoooker on an Antarctic expedition (1839)

What not to leave the house without on a cold day

1. Reusable heat packs. These clever, reusable pouches are filled with a sodium acetate solution that heats up once you click the metal disc inside. They’re perfect for popping in pockets and deploying when you need an instant burst of warmth.
2. High-calorie snacks. Your body needs energy to keep you warm, and in times of need something like a bar of Kendal Mint Cake can be the petrol to your stuttering bonfire.
3. A hat or scarf. Contrary to received wisdom, you lose no more heat from your head than anywhere else on your body. But a hat or scarf will still reduce heat loss from those areas, plus add an extra dash of pizzazz to your ensemble.
4. Lip balm. Cold weather dehydrates your lips, causing them to crack and become painful. Lip balm locks in vital moisture – and might also help you entice someone to snuggle up to you for warmth…
5. A foil blanket. These might be a bit extreme for a lot of activities in the UK but they’re also tiny and only about £1.50. Sling one in a pocket or bag if you’re headed anywhere particularly chilly and it might just save a life should the worst happen.