Thermal clothing or thermal wear is designed to provide insulation when it’s cold. It retains body heat to keep you warm, particularly during outdoor activities like camping, hiking and mountaineering. Thermal clothing is typically worn as a base layer underneath other clothing. 

In this post, we explain how thermal clothing works and how to shop for the best thermal wear. 

The Science of Thermal Wear: How Thermal Clothing Works

When you are outdoors in the cold, your body loses heat rapidly. If you are not warm enough, your body will not be able to generate enough heat to keep you warm and you could get hypothermia, a potentially fatal condition. 

In mildly cold weather, normal clothes are enough to keep out the cold. But when temperatures drop further or if it’s also windy or raining, regular clothing may not insulate you adequately. 

Thermal wear provides much better insulation compared to normal clothes. It’s made with special materials that trap body heat, thus keeping you warm no matter how cold it gets. 

Note that thermal clothing is designed to work in combination with additional layers of clothes. So you’d normally have thermal underwear as the base layer and then one to three layers of regular clothing like a shirt, sweater and waterproof jacket on top depending on the weather. 

How To Choose the Best Thermal Clothing for Outdoors

Here’s what to consider when shopping for thermal clothing. 

1. Material 

Thermal clothing should accomplish three things. 

• Keep you warm. 
• Keep you dry. 
• Prevent overheating. 

The type of material greatly determines how well a particular piece of thermal wear does at all three. Most thermal clothing is made from polyester or merino wool. 

Polyester is great at retaining heat. It also wicks away sweat to keep you dry, making it great for things like hiking and trail running. 

While polyester can be designed to be more breathable, it’s still prone to overheating. So if you opt for synthetic underweight, make sure it’s lightweight, especially for active adventures and activities like hiking, sports or outdoor work.   

The other downside of polyester thermals is the stink. Polyester attracts odor-causing bacteria. After just a few hours of wearing it, it develops an odor. 

Tip: To reduce BO when wearing synthetic thermals, wear the Hercleon antibacterial long sleeved HercShirt over them. The HercShirt is odor-resistant and can keep the thermals from getting too stinky.  

Merino wool is the gold standard of thermals. Wool has excellent temperature regulation properties, keeping you toasty when it’s warm and releasing heat when you start to heat up. So you don't have to worry about overheating. 

Woolen thermals are also great at transporting sweat off your skin, keeping you dry and comfy. Unlike polyester, wool doesn't stink. You can wear the same wool thermals for days without washing and they smell fine. That’s because wool is naturally antibacterial. 

What About Cotton?

Cotton is not very common when it comes to thermal wear. In fact some outdoor enthusiasts will go as far as warning against wearing cotton base layers. ‘Cotton kills’ is a common sentiment. 

The problem with cotton is that it loves moisture. It absorbs sweat and holds onto it. The dampness sucks away your body heat, leaving you cold. In windy or extreme cold conditions, wearing cotton base layers increases the risk of hypothermia. 

If you don't want polyester or merino wool, silk thermals are also great, though pricey. 

That said, there are situations where you can wear cotton thermals. For mildly cold weather and during non-outdoor activities (e.g going to work or a walk around town), a cotton thermal undershirt or pants should be fine. 

You can also try poly-cotton thermals. You get the softness and breathability of cotton, and the sweat-wicking and heat retention of polyester. 

2. Type of Clothing

There are three types of thermal clothing: shirts, pants or bottoms and long Johns, which combine a shirt and pants. 

Which type of thermals you wear depends on the weather and your preferences. If it is seriously cold such as during winter or at high altitudes, go for the full protection of long Johns. This ensures you don't lose heat from either your upper or lower body.

For milder weather, many people find a thermal long sleeved shirt to be enough to keep them warm.  

3. Fit 

Thermal underwear is supposed to fit snugly against the skin. This improves its insulation capability and allows it to efficiently wick sweat away from the skin. It also ensures you can wear it comfortably under other layers. 

But it shouldn’t be too tight either. You should be able to move freely.