Extreme weather conditions require extreme clothing, and nothing beats top quality thermal underwear when it comes to keeping warm. Modern thermal underwear can be composed of multiple thin layers of fabric to trap more body heat and still allow the skin to breathe. Polyester, Wool, and silk are all commonly available materials, but what is the best thermal underwear for extreme cold outdoor conditions? Here we have a head-to-head comparison of the three main fabric types.

Polyester is a man made fiber that is fairly inexpensive, and is becoming more and more commonly used in thermals. It does a pretty good job of imitating natural fibers like wool, but unfortunately it has a habit of trapping odors. The combination of ammonia and bacteria that live on your skin quickly results in smelly clothes. Poly-cotton and polyester-wool blends can be a reasonable compromise, being comfortable and fairly breathable. However, when the polyester ratio in thermals goes above 20 or 30% you will find it has a distinct plastic quality. My verdict: Good for a budget solution, and great for short term sports or running underwear for women or for guys, but not for the serious outdoor enthusiast. Although synthetic fibers are constantly advancing, these are not the best thermal underwear for extreme cold situations. Still, this might change in the next few years. We’ll see.

Silk is not usually associated with hard wearing underwear, but silk long johns for men and women are not a bad choice. Silk breathes well, and has a delightfully smooth and cool feeling when worn under heavier outer clothes. In a single layer silk is not terribly effective as insulating underwear, but thanks to its very fine fibers and resulting light fabric it can easily be fashioned into 2 or 3 layer composites. Quite often silk is combined with wool or a cotton layer to increase its weight and thermal properties. Although expensive, silk composite is often regarded as the best thermal underwear for extreme cold where you can stay fairly dry.

Fine wool, most often from Merino sheep farmed in Australia and New Zealand, is probably the ultimate material for thermal underwear. In my opinion pure wool is simply the best thermal underwear for extreme cold hands down. Superfine wool is very smooth and light weight (nothing at all like a scratchy woolen jumper!) and keeps you amazingly warm. For people like me who are into outdoor sports it is ideal because you don’t ever get soaked in sweat, and if you do get wet through you still don’t freeze. Wool doesn’t hold water like a sponge like cotton does, so you still end up with air pockets trapped in the fabric even when it is wet – the result is that it still insulates you. If you are into skiing, hiking, or any other adventure sport then woolens are the best thermal underwear for extreme cold no matter where you are. The only downside to pure wool thermals is that they cost a lot more than polyester or cotton, but once you have worn them you will appreciate it as money well spent. Be wary of really cheap wool-polyester blends that advertise “contains pure wool” or “real wool” and similar – have a look just how much wool they actually contain, since quite often the fine print reveals that they might only have 10% wool with the balance being a cheap and nasty polypropylene filler. A small percentage of synthetic fiber can make woolens more elastic, but above that 20-30% threshold you lose effectiveness.

Remember that you generally get what you pay for, and super cheap outdoor clothes are rarely worth it. Online store like Amazon are a great place to shop for the best long underwear for cold weather and other great camping gear, so keep an eye on the “featured products” list down below to snap up the bargains that come up from time to time.

Thinking of getting some thermals?  Good move.  Whether it’s for cold-weather sports, keeping warm during exam time, pajamas for lounging in the winter holidays, or just to keep you warm in those in-between months where it’s a bit on the cool side…

The big question is, of course, which is better – natural or synthetic thermals?  Some people automatically choose one or the other – “crunchy” people (known for their habits of a big bowl of crunchy granola for breakfast!) will probably grab the natural thermals without any hesitation, whereas people who have lots of faith in the abilities of modern science to improve our culture will no doubt be choosing the synthetic thermal underwear every time.

However, it might pay for both parties to take a look a the benefits of each type of thermal underwear.  The boundaries between natural vs synthetic thermal materials are now so often blurred that to say one is better over the other is to argue over which is better, apples or oranges.  Before the natural crowd holler that thermals made with natural fibres are better because anything natural is better than anything synthetic, let’s remember that a lot of new textiles considered to be “natural” are actually processed so far as to render them no longer purely “natural”.

Bamboo, for example, is a textile which began its life as unfertilized and unsprayed bamboo canes, but once processed into today’s bamboo fabric, is now considered to be a rayon.  Rayon is a name for a type of man-made fibre which may or may not originally have been sourced from a natural plant.  You may find thermals and other clothes which are labelled “rayon from natural fibre”, which means that the fibres used once came from a plant.  It’s fairly safe to assume that fabric labelled simply “rayon” came from synthetic fibres in the first place – one example is recycled plastic bottles, an increasingly popular choice for textile producers, making a very soft fibre and recycled too thus saving on the environmental impact of the garments made.

However, for all this talk of bamboo and rayon and other fancy man-made things which it can’t be denied, are pretty clever – there is still a whole lot to be said for natural fibres and natural cotton or wool thermals. Caring for thermal underwear made of natural materials is not hard – just remember to put them through on a gentle (wool) wash cycle.

I say cotton or wool because these are the most common fibres that natural thermals are made from, but within these categories there are many variations.  Let’s start with cotton, since that’s pretty simple.  The cotton plant has been used for at least 7000 years, maybe more, for the production of textiles.  It’s easy for textile producers to get hold of, so it’s inexpensive, and it breathes well and doesn’t require extra special processing so it’s widely available. Cotton makes for comfortable and soft, and is generally used to line thermal gloves as well.

Thermals made from cotton are soft and comfortable, cheap and widely available.  However, they do have one major drawback.  They hold moisture against the skin, which makes them completely useless for outdoors sports and endeavours such as hiking, camping or hunting.  Anything which will cause you to sweat requires you to wear underclothes that wick the moisture off your skin, otherwise you are going to be a candidate for chafing, clamminess, and maybe even frostbite if are wet and in cold conditions.  You don’t want to get sick in the wilderness, so please, for outdoors endeavours, avoid cotton thermal underwear, not matter how inexpensive and tempting they may seem.

Thermals made from wool are considered among the best you can buy.  Wool thermals are very good at keeping the warmth in and wicking the moisture out.   That’s because wool has natural wicking properties and is also designed to keep a sheep toasty warm – so wearing wool will also keep you toasty warm!

Some new advancements have been made in the breeding of fleece animals in recent years, and some of the most highly prized wools are Merino wool (a breed of sheep), possum (they’re not bred, but New Zealand is getting into possum wool processing since possums are a pest there – beautiful warm wool!), cashmere (from goats) and alpaca.  Alpaca thermals are a luxury indeed, lending warmth and wicking and a touch of class as well.

Then there’s man-made or synthetic thermals.  Don’t forget that many synthetic thermals are actually a blend of synthetic man-made and natural fibres. bringing the two together to truly get the best of both worlds.  Although these may be the most expensive option, due to clever processing and smaller runs of the garments, you will undoubtedly be impressed by the qualities of these garments.

On the whole, thermal underwear is fairly tough stuff. It stands up to years of abuse, and you can generally expect many years of service life from good quality garments. However, there are some basic tips that can easily help you increase the life expectancy, and the comfort of the wear that you get from your thermals.

Basically, there are three main factors that will destroy your thermal underwear.

1. Shrinkage. It’s a big problem especially with polyester and other synthetic underwear, especially because so many people use clothes dryers these days. Yes, you might lose a few pounds and fit back into it, but generally once the fabric has shrunk it won’t be a stretchy and comfortable any more.

2. Staining: Mainly this is an aesthetic issue, since nobody likes to wear tops with yellow armpits, can also be accompanied by persistent bad smells … potentially cutting short the life of a garment.

3. Pilling. This is when the fabric develops little rolls, or little pills of fluff, fibres and other bits and pieces usually picked up in the wash cycle. Not only is pilling unsightly, it also makes the underwear very uncomfortable to wear when these little rolls are on the inside.

Thermal underwear is best dried outside in direct sunlight. This is environmentally friendly, and naturally removes stains and bad odors.

So let’s go through some basic care instructions for different types of thermal underwear.

WOOLEN THERMALS: Merino wool is by far the best material available for robust and effective insulating underclothes. It is naturally antibacterial, and maintains good insulating properties even when it gets wet, and therefore makes the best long underwear for cold weather. It is very important to only ever wash woolen thermal underwear in a cold, or at most lukewarm wash cycle. Warm water causes wool to pull together and tends to permanently damage the garments. Additionally, you should not use normal washing powder or disinfectant to wash wool garments as this will cause the fibers to mat together like felt. Always wash wool garments on a gentle cycle, as excessive agitation will cause the fibres in the thermals to mat together, causing shrinkage and distortion. It will feel rough and uncomfortable, and won’t keep you as warm anymore.

If wool stains, hang it in the sun for a few hours, don’t use bleach or similar substances on it.

COTTON THERMAL UNDERWEAR: Cotton is generally a very forgiving fibre, and you can wash using normal detergent on a normal wash cycle, even in hot water if need be. Be aware that if you wash cotton in hot water it will shrink somewhat, but this is nowhere near as severe as for wool. Generally the biggest issue with cotton thermals staining, and Pilling. To avoid staining, the first thing you can do is wash your clothes on a regular basis. This might sound like common sense, but it really is the best way to avoid stainless. This includes not allowing a clothes to sit around for a week after you wear them before they go in the wash. Stains result from components in your natural perspiration which react with the air and other materials which are found already in the fibres. If you come home really sweaty, take your thermal underwear straight off and if nothing else drop them in a bucket of cold water to soak. This will immediately help draw the ammonia and other components of perspiration from the fibres, making them easier to clean later. The same goes for other stains from soil and dirt, on thermal clothing or otherwise. There are plenty of proprietary anti-staining products out there, or you can even use a mild bleach solution to get the garments white again. Alternatively, if you like a natural approach, you can simply wait for a sunny day and give the thermals a full day in the sun. The UV rays in natural sunlight are incredibly good at bleaching clothes to a bright white, and they are also able to kill any bacteria which may be lurking in the fibres.

In order to prevent Cotton thermal underwear from pilling, the best thing you can do is avoid washing them with any other clothes that are likely to shed a lot of fibres. Are the worst offenders tend to be bath towels and fluffy socks, both of which lose a lot of very fine fibres. Also, make sure you regularly clean the leaf filters on your washing machine. It’s amazing how to stuff accumulates in there, and once it overflows, it simply washes back and gets stuck on your clothes. While it is possible to shave the fine pills or fluff back off, it takes forever. Sometimes you can use strips of packaging tape to clean it off, and some people use electric shavers too. The to the inside out. Although washing things inside out is often recommended, this is to preserve the outer appearance. In the case of thermals, we are usually more worried about the inside since this will sit closer to the skin. So leave them the right way out when they go in the machine.

POLYESTER THERMALS: Caring for thermals made of polyester and other similar synthetic fibres is easy, because they are generally very hard wearing. They do not suffer from pilling or staining as badly as natural fibres do, but they certainly make up for it in their tendency to shrink very badly the greatest cause of shrinkage is excessive feet. It shouldn’t come as a surprise, since everybody has at some stage thrown a plastic bag or a plastic bottle in the fire. The first thing that happens? The item shrivels down into practically nothing, into a tiny ball of molten plastic. The same thing basically happens with fine synthetic fibres when they are heated, even to a low temperature of about 60°C. Never, never never put polyester thermal underwear in the dryer on a hot cycle!!! It will come out looking like dolls clothing, and you will never be able to wear it again. The best way to care for polyester thermal underwear is to wash them normally (they can handle warm water for normal detergent) and either air dry them, since they drive very quickly, or put them on a very low dry cycle. Acrylic and polyester thermal underwear and does not kill too badly, but you should observe the same rules as for cotton. Clean your lint filters regularly, and wash them the right way out.

What if your thermals smell bad? Don’t be embarrassed, it doesn’t mean you are a dirty person. Fabric worn close to the skin can easily develop BO smell even when well washed. As already mentioned, air drying especially in direct sunlight can be one of the best ways to overcome smells and stains in all clothes. You can also add a small amount of disinfectant (preferably a natural one like sphagnum moss) to the wash, and if you really want to nuke bacteria AND mold use some anti fungal product such as grapefruit seed extract. Personally I go for sunlight – Its still free, and leaves everything with a fresh fragrance and soft feel that you just cant duplicate.