Thermals, also known as long underwear or long johns, are used asbase layers for maintaining a warm body temperature during winter activities. But, how tight should your thermal be?
For maximum insulation, thermals should fit close to the body and have no gaps around the waist, neck, or wrists. Overly tight thermals produce discomfort, but if thermals are too loose, you risk allowing cold air in through your layers. Loose thermals are appropriate for warmer conditions.
But is that it? Just wear tight-fitting thermals? Well it turns out there is more to the story. You may be surprised to find out that there are cases where loose thermals are actually desirable! Read on to find out more.
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Should Thermals Be Tight or Loose?
So, what’s the scoop? When is it better to use tight-fitting thermals or loose-fitting thermals?
When it’s Good To Wear Tight-Fitting Thermals
If it’s cold, for max comfort and effectiveness, your thermals should be tight-fitted, not loose. A good rule of thumb that you’ll hear all over the internet is to think of your thermal as a second layer of skin. Generally, you should not have any gaps between your long underwear and skin.
On the other hand, you should be able to breathe comfortably and not feel as though your thermal is cutting off your oxygen supply or preventing you from squatting or doing other movement.
When thermals fit snugly, they help the body retain warmth by evenly distributing heat across the body. This trait allows your body to stay warm in even the coldest of conditions. A fitted, quality thermal also pulls moisture and sweat away from the body, which keeps you dry and warm.
Because the purpose of the base layer is to stay warm and dry, the material, weight, and fit should all impact your decision to buy a specific thermal. After each of these factors have been carefully considered, it’s time to choose the size of your thermal.
When Loose-Fitting Thermals Can Be Better
So, it turns out there are times when loose-fitting thermals may be the best option! What?? Why?
While doing research for this article, I found an interesting bit of information in this study. Tight-Fitting thermals keep your skin warm in cool and in warm weather!
Okay, that sounds obvious, but think about it for a second. Most people say that your thermals should fit snug (not too snug), and that even in warm weather conditions a tight-fitting base-layer will help your body sweat and wick away moisture. This is not necessarily true! In the case of this study and the clothing they were using, loose clothing keeps you cooler in warmer weather.
This matches up with other studies I’ve found on similar subjects. (go to our article here and look at the section entitled: “Does Outerwear Matter” for more information on the subject).
So in short, if you’re in a situation where it gets warmer during the day (think 60 degrees Fahrenheit) but it gets cold at night (think mid 40s), then looser thermals may be the optimum solution.
That being said, everybody is different, like I’ve said. Every garment has a specific blend of materials that make this a very complicated question to answer. So, the only thing for it is to experiment yourself and find the optimum fit for each situation.
How To Choose The Right Fit Of Thermal
Thermal sizes are sized in a way to match your body. So instead of choosing a smaller size because you want the thermal tight, or larger size because the thermal looks too small, try your normal size first. You will likely find that is has the desired fit.
Then again, every human body is different—a good tip is to take a peek at the reviews and search for “fit”, “too big”, “too small”… you’ll for sure find people in all camps, but if you see a ton of people saying it’s too small than you can adjust the size from there.
I find I do this with any clothing I buy online, anyway.
Can Thermals Be Too Tight?
In many cases, thermals that are too constricting can cause limited mobility and chafing. At the very least, you will likely just be uncomfortable. Which, if you’re camping or hiking, or adventuring in general, that discomfort can wear on you and sour your trip.
However, in some cases, thermals that are too tight may cause health issues. If your thermal shirt is too tight in your chest, you may experience shortness of breath. This issue is something you would probably like to avoid, especially if you plan on hiking or skiing.
If thermal leggings are too tight, men may experience lower sperm count and women may experience yeast or urinary tract infections, according to healthline. Mostly, these conditions are caused by increased heat and friction in the genital area, and can be avoided by wearing appropriately fitted long underwear.
What Happens if Thermals are Too Loose?
If there are gaps between the thermal and your skin, air will come through. When you have gaps between your neck, wrists, and waist, the cold air has multiple areas to access your skin.
There are multiple reasons why this isn’t optimum. Air is actually a good insulator because it doesn’t conduct heat very well, but moving air means your body will be subject to evaporative cooling (like when you get out of the pool and you’re freezing cold when the wind picks up, even in the summer).
The air gaps I mention are not where the air should be captured, though, but rather in the fibers of the fabric. So, in short, if the cold air is allowed in, your thermal cannot do its job to trap your body’s heat. As a result, you’ll get cold.
When participating in active winter or cold weather sports, you are going to work up a sweat. A fitted thermal will transport this sweat away from your body and keep your body warm and dry. However, if your thermal is too loose, sweat will remain on your body. If any cold air comes through the gaps between your skin and thermal, you will become uncomfortable and at risk for health concerns.
That’s the theoretical knowledge on base-layers, at least. There is actually controversy to how effective this moisture-wicking actually is.
Also, our bodies can adapt pretty well. If you’re worried whether your thermals are too loose, experiment by going outside and hanging out in a camp chair or other low-intensity activity–you may find your body does just fine.
What Weight Should My Thermal Be?
When considering the weight of your thermal, first consider your metabolism and general activity level. If you’re preparing for a winter camping trip and your main activity is lounging around a fire, then go for a heavier weight thermal; if you’re planning a cross country ski camping trip, a lighter weight thermal may be best.
Typical options for long underwear will include:
- Heavyweight: Best for below-freezing temperatures, heavyweight long underwear should be used when you expect to experience extreme cold.
- Midweight: The most versatile of the thermals, midweight thermals should be worn for moderately cold to cold weather.
- Lightweight: The lightweight thermals are best for mildly cold to moderate weather. You should also wear this if you are participating in heavy aerobic activity, such as snowshoeing, cross-country skiing, or winter hiking.
You may also come across ultra-lightweight, microweight, or featherweight thermals that attract those who are more interested in wearing an extra layer in moderate temperatures. These thermals can be worn as an extra layer under everyday clothing or as the base layer when participating in outdoor activities. You may find a looser fit works better in these conditions so you don’t overheat.
Alternately, you may also see terms like expedition weight that promise extra heat in the coldest conditions. However, keep in mind that the main purpose of your thermal is to manage moisture and keep your body dry. While adding a heavier base layer can add some warmth, the middle layer is meant to insulate your body’s temperature, not your base layer.
If your base layer absorbs too much moisture that can impact its effectiveness. Your outer layers can be shed so maybe the super-heavy weight isn’t the best thing. It’s really hard to know since every situation is different.
What Materials Are Thermals Made of?
When it comes to thermals, material matters. You want to choose a base layer that has durable fabric designed to wick moisture away from your body. Generally, a denser fabric retains more heat, and a porous fabric will lose more heat.
Additionally, you want your base layer to be resistant to odors. After all, this layer is the one against your skin and will likely come in contact with the most sweat and body odor, especially if your thermal is worn for multiple days in a row.
I mean… who wears their thermals multiple days in a row… besides me?
Whether you choose a thermal that is made from natural or synthetic fabric, you can find something that fits your needs.
Synthetic fabric is most commonly made from polyester, but some thermals may be made from nylon, rayon, or a mixture of fabrics. When mixed with spandex, synthetics have an extra flexibility that allows for a comfortable, yet snug fit.
Likely the most durable of the fabrics, synthetics will succeed in keeping you dry by expertly wicking moisture from your body. Synthetics also have an extra finish that prevents excess odor-causing bacteria from building up, so you can likely wear synthetic thermals for a couple of days with only moderate funk.
My thermals are 96% polyester and they do okay, but they definitely get some funk after a couple days of use.
Unlike traditional wool, merino wool is soft and comfortable. Like synthetics, merino wool can be combined with spandex to add flexibility and a close fit. When used as just a base layer, merino wool can have decent lasting power. If you plan on just wearing this thermal with no other layers, however, you may experience more wear and tear.
Merino wool wicks moisture well, but it does retain some moisture in its core. While the moisture won’t cause extreme discomfort, merino wool thermals will likely take longer to dry than synthetics. In some cases, this retained moisture can have a cooling effect when the external temperature is warmer than the body. The moisture at the core will be released, thus keeping the body cool.
Wool is naturally resistant to odor-causing bacteria, so you can expect little to no odor even after numerous sweaty days.
Silk is breathable and very comfortable. It doesn’t have quite the strength of wool when dry, but silk actually can work decently as a base layer. Check out our post about whether silk is good for outdoorswear here if you are curious.
Cotton, unlike silk, merino wool, or synthetics, retains moisture. While it may be one of the more inexpensive options, cotton will feel cold and wet against your body. The fabric will also likely stretch over time, causing gaps to form and cold air to enter.
I wear cotton when I go camping (even when it’s going to be very cold) when it’s going to be dry. If I’m not sweating a lot, then moisture absorption is not a big deal. Cotton can be very warm and comfortable and will work well in dry situations.
If it’s going to be raining or if you are going to be sweating, you should consider other base layer materials.
How Tight Should a Base Layer Be?
For optimal warmth, your base layer should fit snug against your body, even while you’re moving.
For sweat control (which many people consider the point of the base layer), snugness isn’t as crucial and a looser fit will still help absorb the sweat from your body. Some might say that snug is necessary for proper wicking, but there really hasn’t been a lot of good science on that.
If your base layer is constricting, uncomfortable, or feels like its cutting off blood supply in any way, your base layer is too tight.
Cons of tight underwear in men: https://academic.oup.com/humrep/article/33/9/1749/5066758
Pros and cons of wearing tight underwear: https://www.healthline.com/health/tight-underwear#takeaway
Prevent yeast infections by avoiding tight panties: https://www.mayoclinic.org/diseases-conditions/yeast-infection/symptoms-causes/syc-20378999
General info: https://www.rei.com/learn/expert-advice/underwear.html
General info: https://www.realmenrealstyle.com/thermal-underwear-guide/
Fabric types and thermal retention; tighter models retain and disperse more heat: http://www.fibtex.lodz.pl/59_25_98.pdf
Fabric characteristics: https://www.hoac-bsa.org/Data/Sites/1/media/districts/pioneer-trails/documents/PT%20How%20to%20Choose%20a%20Base%20Layer-mar11.pdf