Cold at Night? This Warmest Sleeping Bag Liner Keeps You Toasty

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A sleeping bag liner is a part of the sleep system that hikers almost always overlook. Many think it adds unnecessary weight and extra cost with little benefit. However, if you’re an old geezer like me you know that isn’t the case. Getting a restful night’s sleep on the trail is difficult enough. Being cold at night is no laughing matter. Sleeping outdoors, you will appreciate the extra comfort of the warmest sleeping bag liner. 

Made from lightweight fabrics, these thin pieces of material will extend the life of a sleeping bag. A sleeping bag liner is like adding a top and bottom sheet to your bag. It prevents dirt, sweat, and body oils from soiling your sleeping bag.

When camping in warm weather, sleeping bag liners are good standalone gear. You can leave the heavy sleeping bag at home and stick with a sleeping liner instead. When sleeping in cold weather, you can take a lighter weight warmer sleeping bag, knowing that if you get cold at night you have a sleeping bag liner to crawl into as well.

man hiking in the forest with sleeping bag attached to backpack

Sea to Summit introduced its innovative Thermolite Reactor Extreme to make colder weather conditions more bearable. It is the warmest sleeping bag liner you can get, with all the features especially crafted for outdoor enthusiasts. Thermolite is best for backpacking and camping weighing no more than 14 oz.

What Are Sleeping Bag Liners?

Sleeping bag liners are an accessory piece of gear made of lightweight cloth. It is inserted into the sleeping bag, adding an extra layer of insulation. This is most appreciated when outside air temperatures drop below the temperature rating of your sleeping bag.

The insulation and warmth of a sleep system comes not only from the sleeping bag, but also from the sleeping pad. Uninsulated sleeping pads often fail to keep you warm enough regardless of the sleeping bag’s temperature rating. The cold from the ground migrates upwards through the pad to your body. You know how messed up that can get during frigid weather. That is why having a warm and excellent sleeping bag liner becomes essential. It helps insulate you from the cold coming up from below.

Adding a traditional sleeping liner to your sleeping bag typically adds 5 to 15 degrees additional warmth to the sleeping bag itself. If you are a cold sleeper, you may find this added feature beneficial. Remember, the air temperature inside your tent is the same as that outside. If it is cold outside at night, it is going to be cold inside your tent.

What Is the Sea To Summit Thermolite Reactor Extreme Liner?

man sleeping in the warmest sleeping bag liner the Reactor Extreme Liner by Sea to Summit

Thermolite Reactor Extreme Liner is an insulation technology adopted by Sea to Summit in its sleeping bag and related products. The Thermolite fabric is made up of a hollow core fiber that increases the insulation without compromising the weight and packability.

The Extreme Liner is built to add about 25⁰F warmth to a sleeping bag. The Geez takes his when backpacking with his 35 degree sleeping bag. For example, there may be snow on the rim of the Grand Canyon. Yet down at the Colorado River, the temperature can be in the mid 90s. Temperature extremes like this make a sleeping bag liner an essential piece of gear without having to backpack with a heavy zero degree bag.

It is also good to use as a standalone piece of sleeping gear if you do not want to be bothered by a bulky sleeping bag. You can take it without carrying a sleeping bag when camping in warm weather. The weight difference in your pack and comfort of carrying less weight are beneficial for warm weather outdoor adventures.

Key Features

  • +25⁰F warmth
  • Excellent insulation
  • Soft
  • Stretchy
  • Easy to wash
  • 110 g/m² Thermolite fabric
  • Warmer than the standard Thermolite Reactor
  • Comes with an Ultra-Sil stuff sack

Benefits of Thermolite Reactor Extreme

Sometimes a sleeping bag is not enough to keep one warm during cold nights. The temperature ratings advertised for sleeping bags are guidelines only, and not applicable to every backpacker. Each hiker responds to cold differently, some needing more warmth. To increase the bag’s thermal rating, an insulated sleeping pad and sleeping bag liner complement the sleeping bag’s stated temperature rating.


Thermolite fabric consists of engineered hollow core fibers that provide insulation to keep the temperature up when the ambient temperature goes down. It is essential for campers during drastic weather changes and temperature extremes.

The fabric traps insulating air and increases body warmth by around 25⁰F (15⁰C). It is by far one of the warmest sleeping bag liners for cold-climate camping and unpredictable weather intervals.


Thermolite fabric is super soft and stretchy. Bag liners made with this fabric fit many sleeping bag shapes and sizes. The sleeping bag liner itself is mummy shape for a snug fit within your bag. But, the fabric is not restrictive given its ability to stretch as you toss and turn during the night. It gives you freedom of movement and is great for restless sleepers.

Keeps the sleeping bag clean

More than keeping the campers warm, the Thermolite Reactor also serves as a barrier between your body and the sleeping bag. It prevents excessive wear and tear due to body oils and sweat. A sleeping bag liner prevents dirt and moisture from attaching to the bag’s surface. So no matter how sweaty and dirty you are outdoors, the inside of your sleeping bag stays clean, dry, and odor-free.

Easy to clean

After an exciting outdoor adventure, nobody looks forward to the cleaning of their gear. Cleaning tents, sleeping bags, backpacks, and other backpacking equipment is labor intensive and tedious work to rid items of dirt and grime. But with the help of a sleeping bag liner, it reduces the number of tasks you have to do. A sleeping bag liner can just be thrown in the washing machine and it is as good as new when it comes out.

Summit Thermolite Reactor is machine washable for your convenience. Any detergent is compatible with the Thermolite Reactor, but the manufacturer warns against using fabric conditioner. It diminishes its wicking capacity, which draws moisture out of the fabric.


Okay, Thermolite may not be the lightest of all the sleeping bag liners, with silk fabric owning the top spot. But when it comes to the warmth-to-weight ratio, Thermolite is one of the best.

Warmth to weight is the evaluation of the effectiveness of an insulated product. The Thermolite Reactor Extreme offers 25⁰F more warmth for an extra 14 oz (399 g) of backpacking weight.


You can compress the Thermolite Reactor Extreme in its stuff sack to a size similar to a red solo cup. A compression carrying sack comes with it when purchased. It compresses the liner to its smallest size. With its extreme packability, Summit Thermolite is a valuable accessory that deserves a space in every hiking backpack.


When it comes to material quality, Thermolite has an excellent rating. Its hollow core fabric works like animal fur, albeit in a knitted fabric.

It uses a heavier version of the Thermolite at 110g/m² that gives its warmth and degree of stretch. It is a reliable product that can withstand stringent outdoor gear and equipment requirements. It has a lifetime manufacturer guarantee against defects in the material and workmanship.


The two main reasons people buy sleeping bag liners are to increase the sleeping bag’s warmth and extend the life of the sleeping bag. Thermolite is versatile and warm enough to use as a standalone sheet when needed. You can use it as a light blanket in warm weather when using a full sleeping bag is unnecessary.

Other Special Features

This mummy shape bag is designed for more thermal efficiency. It has less dead air space since it fits snug against your body. This reduces heat loss. It also has a foot box to allow your feet to rest comfortably without getting cold. The drawcord hood is cinchable and has a mini cord lock. You can use it to cover yourself well in freezing temperatures.

Reactor Extreme has an Ultra Sil stuff sack that compresses its volume to 1.5 L for convenient carrying. It has two available sizes: Regular at 82 inches and Long at 92.5 inches.

Disadvantage of Thermolite Reactor

Thermolite Reactor Extreme has neither a full-length nor partial zipper on the liner. This is understandable for a mummy shape, compact and lightweight product.

No-zipper sleeping bags and liners have minimal weight and are designed to be ultralight. However, lacking a zipper means you have to crawl into the liner feet first. Entry and exit from the liner is more difficult than ones with a zipper.

The Thermolite Extreme is priced higher than other Thermolite products and comparable brands. But, it is well worth the price for its benefits and warmth.

Other Warmest Sleeping Bag Liner Choices

Here are more Thermolite products that add an extra layer of protection and warmth to your sleeping bag.

Reactor Liner offers a +14⁰F warmth difference to sleeping bags. It is the lightest Reactor product but has uncompromised insulating properties. It is very compact to fit anywhere and has better packability than fleece.

Reactor Compact Plus is a hybrid sleeping liner that uses two weights of Thermolite fabric in the torso and foot box section. It adds up to 20⁰F warmth to the sleeping bag and is reasonably priced.

Reactor Fleece Liner is the premium Sea to Summit Thermolite Reactor that cranks up the sleeping bag warmth to 32⁰F. It is the warmest sleeping bag liner, followed by the Thermolite Extreme Reactor. Unlike other Reactor products, the Fleece Liner features a quarter zip for easy entry and exit.

Advantages of a Sleeping Bag Liner

Keeping a sleeping bag liner handy will make your sleeping bag serve you better in a wide range of temperature conditions. It also provides the following benefits:

Extra Warmth

The extra degrees added to the sleeping bag ensure you will get more warmth and reduce heat loss during sleep. This layer of insulation helps you stay comfortable throughout the night, giving you a better night’s sleep.

Light Layer

In warm weather, if you think your sleeping bag is going to be too toasty, pull out the liner and use it as a cover sheet. It should be enough to keep you covered and protected from cool drafts without you becoming overheated.

Keep It Hygienic

Sleeping bag liners work like your bed’s top and bottom sheets for the same reason. Sleeping directly in the bag introduces dirt, sweat, bacteria, and ultimately odor to the fabric on the inside of the bag.

Having a barrier between your body and the bag keeps it more hygienic. It is easy to change the sleeping bag liner and it can be washed and dried before your next backpacking adventure. What a luxury to be able to crawl into a clean, fresh liner sheet at night.

Protect the Sleeping Bag

The removable sleeping liner reduces the need to frequently wash your sleeping bag. It helps extend its longevity and prevent degradation of its down or synthetic insulation.

Insect and Bug Protection

In the wild, silk sleeping bag liners may be used as a covering to protect you from insects. If staying in a hostel, bed bug infestations have been known to be a problem. The extra protection a sleeping bag liner offers is invaluable. Hotel travelers love the handiness of a sleeping bag liner to get them through the night, not hounded by bug issues.

Types of Bag Liners

Sleeping bag liners are made of different fabrics. Each offers a different level of comfort you need in different weather conditions.


Silk is a comfortable fabric for a sleeping bag liner, although costly. Silk bag liners are compact and lightweight, with good thermal insulation in cold weather. In warm weather, silk is breathable, absorbent, and soft on the skin.

Fleece Liners

Fleece is one of the warmest and best sleeping bag liners for extremely cold weather. No wonder, since it is made from the natural fibers of sheep and yak. Fleece liners are soft, but also bulky and heavy.

Cotton Liners

Cotton is the most popular fabric and is preferred by most backpackers. Cotton is comfy, durable, and affordable. However, it is also medium-heavy and not the most compact. Another fact, cotton is not the best fabric for humid weather. 

Synthetic Insulation

Synthetic liners like Thermolite fabric are well-known for their insulating qualities. They use a hollow-core fiber that adds about 25⁰F more warmth. Some other synthetic liners like CoolMax are breathable and ideal in humid weather.

Related Article: Sleep Easy On The Trail: What is a Backpacking Sleep System 


Can you use a sleeping bag liner in a camping quilt?

Yes, sleeping bag liners add extra comfort and eliminate the drafts in quilts. Some hikers use liners outside the quilt to effectively resist draft and cold.

How can sleeping bag liners improve the loft of sleeping bags?

Oil and sweat build up on the sleeping bag’s surface and migrate into the down or synthetic fill. Over time, it prevents the bag from properly lofting, whether made of down or synthetic insulation. A sleeping bag liner prevents this oil and dirt on the surface to effectively protect your sleeping bag from damage.

How do I clean the Thermolite Extreme Reactor liner?

Regular washing is necessary to keep your sleeping bag liner clean and fresh. Check the label on how to care for the product.

Some liners can be washed in the machine with a gentle cycle and common detergent. Fabric softeners are often prohibited as they compromise the fabric’s moisture-wicking properties. It is also not advisable to dry clean them because it may damage the fabric.


Thermolite Reactor Extreme is one of the best-selling liners of Sea to Summit. It is the lightest and best sleeping bag liner that is badass in extreme weather. Thermolite is equipped with hollow core polyester fibers that trap heat well but is lighter than fleece and is more packable.

The Thermolite is very stretchy and soft with a mummy shape that fits well in any type of sleeping bag. It is light, easy to use, and clean, making it the best hiking and camping companion.

Sleeping bag liners do not have any disadvantages, except maybe their extra weight and cost. Realizing the amount of protection it gives you and your sleeping bag, it sure is a worthy investment. If you are that backpacker who hates cleaning, this sleeping sack can save you from added elbow grease.

Instead of cleaning the entire sleeping bag, throw the bag liner in the washer. As far as cleaning is concerned, your sleeping bag liner is to a sleeping bag as a tarp footprint is to a tent. Both keep your backpacking equipment clean and protected.

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