Do You Wear Undies Under Thermals? (Long Johns, Flannels, Thermal Underwear)

When trying to figure out what to wear while adventuring in the cold, experts always say to dress in layers. Thermals (also known as long johns) should be worn under regular clothes but are they the bottom layer or does something need to be worn under those too?

Wearing underwear underneath your thermals is socially acceptable, safe, will not disrupt the thermal properties of the garment and also has some advantages including lengthening the time in which you can wear the thermal without washing the garment.

Dressing for cold winter temperatures can be confusing, especially for those of us from warmer climates. Dressing in layers can help keep us warm and also prevent horrible things like hypothermia and frostbite, but it can also be overwhelming if you are not sure what to layer. Let’s dive in and try and alleviate these concerns:

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Do You Wear Underwear Underneath Thermals?

Wearing underwear under your thermals is socially acceptable, normal, and safe to do while out in cold weather.  

Thermals are designed to act as a barrier between the cold air and your body while helping your body retain heat. Wearing underwear will not reduce the ability of your thermals to insulate you or disrupt the barrier between the base layer and your skin.

Additionally, wearing underwear can also provide comfort and support in the areas you need it most.

So knowing that wearing thermals doesn’t hurt is one thing, but there are actually advantages to wearing underwear underneath your thermals:

Underwear Helps Thermals Go Longer Without Washing

Many people do not wash their thermals after each wear, so having a barrier between them and your skin is also important for hygiene reasons.

If you are going on a long trip and only have one pair of thermals–wearing undies can allow you to go multiple days before washing your thermals which is great if you are in an area without access to a washing machine.

When I’m backpacking, I only take one set of thermal underwear and I have to make them last for several days.

The underwear I wear, on the other hand, is much more lightweight and easy to swap out.

It’s very important to keep your underwear areas clean. You can get away with not changing your clothes for a while, but make sure to wipe clean with a camp towel or with whatever you have.

Allows For Wearing Thermals As Outer Layer

Dressing in layers is a practical approach for cold weather outdoor activities. If it’s too warm, you can always peel off layers until you hit the sweet spot where you’re not sweating too much. If it’s too cold you can add back those layers.

If you are wearing your underwear underneath your thermals, and you are too warm, you could strip down to your thermals if you really wanted to. Since thermals are form fitting and leave little to the imagination, wearing undies would probably be appreciated by your hiking mates.

Tips For Selecting Thermals

Thermals are available in many different fabrics and come in different weights which are suitable for different outdoor temperatures. They are available as “long johns” which is a one-piece suit or they can be worn separately like a shirt and pants.  There is not really any functional reason for choosing one over the other, it is just a matter of preference.

It is important to find a material and thickness appropriate to your weather conditions, so you do not get too hot or too cold while outside.

Being too hot is a thing? Well, if you’re sweating a ton then that actually can be just as dangerous as not having enough insulation. If you’re sweating a lot you’re losing more water and you’ll be tempted to take off layers when you might not want to.

Striking the balance of layering where your inner layers aren’t too hot and you can take off your outer layers is the key, here.

Additionally, finding thermals that are comfortable, stretchable and able to manage moisture is extremely important to ensuring you enjoy your outdoor adventure. 

Thermals should be tight fitting and close to the skin–but regular underwear can still be worn underneath.

Being comfortable in the brisk outside air is so important if you want to enjoy your outing to the fullest while staying healthy. If you are planning to be out in extremely cold conditions, synthetic fabrics are good choices:

  • Spandex
  • Lycra
  • Nylon
  • Polyester

These are just a few examples of some suitable synthetic thermal materials.

Wool is another good option–although wool does absorb water, it keeps the water off your skin and doesn’t lose as much of its insulation abilities as do many other organic fabrics.

Cotton thermals aren’t the worst idea. You’ll hear the phrase “cotton kills” a lot, but it truly depends on your circumstances. If you are hiking in a remote area with any weather condition being possible, then it’s probably best to leave the cotton at home. If you’re car camping and it’s going to stay dry, then cotton thermals will work just fine.

If the weather turns while car camping, don’t be afraid to bail. Even if you’re at a campground it can be dangerous to be outside during cold weather conditions. See our guide here to see what to wear while car camping in 30 degree weather.

Now, if you’re going to a remote area away from help and where staying warm is important for survival, cotton does retain moisture against the skin. This actually disrupts the garment’s ability to insulate and furthermore can cause evaporative cooling. This is very dangerous in cold conditions since the excess moisture could potentially lead to disorientation, hypothermia, and potentially death. 

Fabrics like rayon, corduroy and bamboo also fall into the cotton category and are not advisable for the extreme cold winter conditions.

What To Wear Over Thermals

Now that we have figured out what to wear under our thermals, let’s figure out what to put on over the thermals.  The number of layers you wear depends on how cold it is and what activities you are planning on doing.  As much as you do not want to be cold, you do not want to overheat either. 

Making sure to pick a base layer that is appropriate in weight and material is extremely important and can dictate the rest of your clothes.

Here are some examples of what you wear over your thermals:


You can wear a short or long sleeve shirt over your thermals depending on the climate conditions. 

As mentioned before, some fabric choices, like cotton, are not ideal for being worn during extreme winter conditions because it will retain moisture, including perspiration and takes a longer time to dry. So, it is best to choose one of the other available fabric choices. You may want your t-shirt to be a bit less snug than usual so that the thermals fit comfortably underneath.


If you plan on being in a very cold area, it would be best to wear a sweater over your thermals and t-shirt layers. 

If the area is not extremely cold, you could skip either a t-shirt or sweater, it is up to you. Just like your thermals, the sweater should be stretchable and not prevent your full range of motion. You also want to make sure the sweater is comfortable, there is not much worse than wearing an itchy wool sweater when you are on a long, strenuous hike or other adventure.  

You could also opt for a fleece pullover, a vest or something similar to keep you warm and comfortable on top of your other two layers.

In fact my favorite option is to wear a fleece sweater over my shirt and base layer. Fleece is a great material for the middle-layer. You can learn more about fleece at our post here.


This is the easiest layer to put together. It is best to wear waterproof and windproof hard-shell pants with full-length zippers on the sides which you can unzip and will serve as ventilation when you start to get warm. Make sure you are able to comfortably squat, run and climb in them depending on the activity.

For me, I wear a heavy weight base layer thermal during cold weather that’s 100% polyester. On top of this I wear a pair of pants that has a fleece lining with a hard-shell exterior that does a great job as a windbreaker.


There are so many types of jackets to choose from, and again, it depends on just how cold it is outside.  In extreme cold, it is best to wear a long, hooded parka, that is insulated with goose down.  In more mild temperatures you could get away with a wind breaker or lightly lined jackets. Also, be sure to find a jacket that is water resistant if you will be out in rain, snow or ice, the last thing you want is a soggy top layer that drenches the rest of your body.


Don’t forget your hat, scarf and gloves, these can be easily removed if you start to get warm but will provide extra warmth when you are feeling cold. Also, a good warm pair of socks is essential and should be changed if they get wet. These accessories will take your warmth and comfort to the optimal level.

Dressing for activities in the cold winter air is not as challenging as it seems.  The thermals go under your regular clothes of pants and a shirt; they are just an added layer of warmth and protection from the harsh cold air. A big jacket goes on top of everything else. Easy!

Now that you know what to wear under and over your thermals, you are ready to go. Good luck on your cold weather adventure, stay warm!


Peter is a software developer who loves to take every opportunity to go outside that he can get. Peter grew up going on long backpacking excursions with his family every Summer and now enjoys staying at the beautiful Texas State Parks and swimming in the amazing Texas Rivers.

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