Sea To Summit Reactor Extreme

Priced: $64.95 - $66.95 Rated:   - 4 stars out of 5 by 66 reviews.
Sea To Summit Reactor Extreme
Zoom In
Color: Orange
Available Colors:
Sea To Summit Reactor Extreme -
Sea To Summit Reactor Extreme Thermolite Mummy Liner Features.

Sea To Summit Reactor Extreme Thermolite Mummy Liner.
The Sea To Summit Reactor Extreme Thermolite Mummy Liner is the ideal way to get a great night s sleep in the biting cold. This liner comes from 110g./m2 Thermolite, a hollow core fiber that is extremely breathable. It ensures a great increase in warmth by adding over 14 degree Celsius inside the sleeping bag. Its mummy shape design provides your body with optimal mobility, while the box foot contours to your shape for added comfort. The draw cord hood and mini cord lock are easily operable and make sure a snug fit. This liner also keeps the inside of your bag clean, prolonging its life. It is lightweight (14 oz.) and easily portable with its own Ultra-Sil stuff sack. Go ahead, camp outdoors in any season with the Sea To Summit Reactor Extreme Thermolite Mummy Liner.

Key Features.


  • Lighter weight and more packable than fleece
  • Constructed of insulating Thermolite® fabric, a hollow-core, breathable fiber that provides extraordinary warmth for its weight
  • 110g/m2 Thermolite® construction throughout

Wash and Care:

  • Liner helps keep the inside of your bag clean, saving it from the wear and tear of washings


  • Draw cord hood with mini cord lock
  • Drawcord hood with mini cordlock cinches around face to hold warmth in


  • Adds up to 25°F of warmth to a sleeping bag; can also be used as a summer ultralight sleeping bag
  • Color of liner is orange
  • Comes with a lightweight Ultra-Sil™ stuff sack
  • Mummy shape with a box foot
  • Packs into its own Ultra-SilT stuff sack
  • Weighs 14 oz / 399g
  • Contrasting color along the top edge makes it easier to find the opening
  • Mummy shaped liner features a 3D footbox providing plenty of room for feet
The Sea to Summit Thermolite Reactor Extreme Sleeping Bag Liner is a great way to get an extra season out of your sleeping bag or stay warm in unpredictable weather. Lightweight and highly breathable Thermolite fabric will add up to 25 degrees Fahrenheit to your bag’s temperature rating at a low cost of 399 grams added to your pack. The Reactor Extreme has a mummy shape to mesh with your sleeping bag and a draw cord hood that can be synched up when it’s really cold.

The Sea To Summit Thermolite® Reactor Extreme Mummy Liner Increases sleeping bag performance by up to 25°F (14°C) with super-insulating 110g/mý² Thermolite® fabric. Thermolite® is a hollow core fiber that provides extraordinary warmth for its weight and remains extremely breathable. Spacious design allows for a comfortable night's sleep, even when the mercury plummets. Perfect for extending the season of your sleeping bag.

Sleep soundly even as the temperature drops with Sea to Summit's Reactor Extreme Sleeping-Bag Liner. Super-insulating Thermolite® fabric adds up to 25˚F of extra warmth to keep you comfortable all night long. Mummy shape features box foot and drawcord hood with mini-cord lock. Packs into its own stuff sack. Imported.
84”L x 36”W.

Appalachain Outdoors
The Sea to Summit Reactor Extreme Sleeping Bag Liner is made from Thermolite, which is a breathable hollow core insulating fiber. The liner adds up to 25 degrees F of warmth to a sleeping bag.
Eastern Mountain Sports (EMS)
The lightweight Sea to Summit Reactor Extreme liner adds up to 25°F of warmth to a sleeping bag. Or, it can be used alone for warmer conditions.

Fiber Content: Thermolite


This Sea to Summit Reactor Extreme mummy bag liner adds a few degrees of warmth and extends the life of your sleeping bag.


Mountain Gear
Sleeping out in unpredictable conditions can be pretty miserable.

Half-Moon Outfitters
110g/m2 Thermolite® construction throughout
Want it cheaper? Set your own price.
Enter the price you want to pay and we'll email or text you if we find a store that will sell it for that amount:
Price: $
Your email:
Cell number: (optional, for text message)
Learn how offers work...
22 people have already set their own price.
Average Price History: Price History
Review RatingNumber of Reviews
Added warmth (C):Up to 14 degrees
Added warmth (F):Up to 25 degrees
Bag Shape:Mummy
Country of Origin:Australia
Dimensions:84" x 36" / 210cm x 90cm
Shell Material:Thermolite
Shoulder Dimension:90 cm
Sleeping bag shape:Mummy
Storage Sack Included:yes
Stuff Sack:Yes
Stuff sack size:3 x 5 inches
Temperature Rating:14 C
Weight:14 oz. (399 g)
Weight - metric:0.39 kilograms
Width:36 in. (90 cm)
Compare specifications to related products.

Similar Products:

Sea To Summit Reactor Thermolite LinerSea To Summit Reactor Thermolite Liner$54.95 - $56.99

Subcategories of Sleeping Bags & Pads:

Sea To Summit Reactor Extreme Reviews:

Positive Reviews:

Nice liner

I bought this liner for winter solo backpacking to beef up my 15F Wenger Rhone 800-fill down bag. The bag is very warm for a 15F but I tend to be a very cold sleeper, so I need to have something that beefs up my temperature rating for nights below 20. I used it on a humid night that got down to about 15 or 10, and stayed very warm with only a thin base layer on. Without the liner I would have been cold. The improvement in temperature rating I would put at about 15 degrees, realistically. Be wary of people who say it does not work, especially if they do not know the limits of their sleeping bag well or are using insufficient ground insulation. For winter backpacking I use a R-5 big Agnes Q-Core with a closed-cell ThermaRest foam mattress underneath for added insulation. No sleeping bag will protect you from the cold ground. The liner is comfortable, easy to use, and packs up small. Also plenty long and roomy enough for me, I am 6'2" plus and 220 lbs. Overall very happy. Don't really expect the liner to convert your bag into a full-on winter bag, those have special face baffles and other jazz to keep you warmer, but on a budget it will keep you warm below your sleeping bag's temperature rating.
Cy the Guy at REI on 01/01/2013

Great addition to camping gear.

This is a great product. We got my wife one because she is always cold. She has a nice REI bag but just sleeps cold sometimes. She has been a huge fan of this and it has kept her warm.
I got myself one after because she liked it. I sleep warm generally so I usually carry it when we go camping or backpacking but never really had to use it. It's very lightweight and easy to pack. For its size it makes it ideal for many different situations. I have the North face Cat's Meow bag, went on a camping trip a few days ago in Colorado. Colorado camping in November can get cold up in the mountains. We were just above the 0 degree mark with wind pounding away at around 9,300ft elevation. Once I got warmed up in my sleeping bag it actually got a little too warm to where I unzipped part of my bag about 3am. It worked great. I think as long as you have a good base bag this will help, if you have a cheap bag not much is going to work for you. For the weight and warmth this thing offers I have already recommended it to several people.
Skrimz at REI on 11/11/2011

Nice! But Needs A Zipper!

Nicely made with ample room for a big or tall guy, no problem with being too tight in the liner.
But you need to do the "Worm" in order to get inside the liner. Then if you use it inside your sleeping bag, you can't unzip that bag without getting your arm and hand outside the liner. Poor planning there.
So, I had a 36-inch long YKK zipper sewn inside my liner on the right edge. That modification cost me $10 for the zipper and the sewing. For the price you pay for this, it should include a zipper. Heck, an economy model fleece liner bag with a zipper is only half the full price of this product----and adds more warmth.
But if you really want to cut down on bulk and weight for a liner that will add 10 to 15 degrees more warmth to a good sleeping bag, then you will pay a lot for this convenience, and still have to wiggle into your sleeping bag.
Dad Who Buys Lots of Gear at REI on 04/04/2013

Adds Warmth, but not 25°F worth...

I bought this to replace my sleeping bag on warm (>60°F) trips due to its extreme compactness and lightness. Used it thus on one trip. and learned that the slightest breeze (<5mph) is enough to negate its value. But if you cover yourself with a tent fly or something similar, it works like a charm (I was in a relatively open hammock).

I became suspect of the 25°F rating. I have a 15°F bag that I've tested successfully for its rating. Last night I spent the night in the bag and liner in ~0°F environment (digital thermometer said -3.2°F). It was slightly chilly - but for full disclosure, the cold was coming up through my mattress. Putting on thermals did the trick. I'd say an optimistic rating is closer to 15°F. It works, and while probably not quite as warm as the manufacturer's rating, it also is nice to have an easily-washable liner.
jrCranstonRI at REI on 01/01/2011

Its a must have

I got this liner almost on a whim but have since really used the heck out of it. In Summer is makes a great great light sleeping system. In Winter with temps dropping to single digits where I live I include it in my zero back for that extra warmth that makes it so much easier to deal with. I typically will pull the sack up about chest high so i have some foot wiggle room in it and stay very very warm. It is one product that truly does help the rating on your sleeping back improve. The regular bag liner works great with Big Agnes sleep system matress bags. I also use this as a tiny sleeping bag during the summer months when you really dont need one but in higher elevations it gets a bit of night chill on. I found this to be an excellent product.
push at REI on 12/12/2010

Great fix for a beat bag

I have only used product in a cold condition, but feel after using it that it is so sheer and lightweight that it seems perfect for summer as a "light" bag.

I used product in a chilly 30 deg. F enviro with a cheap 40 deg. F bag. We had a stiff breeze and I would have truly froze if not for the use of this product. By fully enclosing myself in it and closing down exterior bag, I was able to resist wind penetration and survive the night in relative comfort and warm feet! I would have to say that claims of adding another 10-15 deg. F seem very true.

It is a simple addition when questions arise to a bag being a low enough cold rating on a trip. Weighs around a pound or so- but better than carrying a bigger bag instead.
Nature's Wilde Photgraphy at Eastern Mountain Sports (EMS) on 11/11/2011

Thin, Light, Easy to pack, and Warm

I only have one bag that's rated at 40-45 degrees. Going on a mountaineering trip I needed a warmer bag, but money was tight. Camping on 3ft plus of snow at 5,000 ft in early May on the way to Mt. Stuart was very cold. Other people had between 20 degree and 0 degree bags. This liner helped me a lot. With my bag it performed almost as well as the 20 degree bag. It's very light and thin but does add around 25 degrees. It is very impressive. The value is pretty good too. I am gonna keep it in the car for travel and emergencies. It is nice to add range to your sleeping bag setup. It is hard to get the liner and bag to line up and fit well. Moving around can twist things up a bit. All in all it's pretty great.
zootrees at REI on 05/05/2012

I'm happy to own this Thermolite Reactor sleeping bag liner. It's surprisingly warm, and served me well by itself while sleeping at friends' houses while traveling with just a day-pack. I can't compare the additional warmth it provided while out hammock camping, but it undoubtedly helped.
It's sewn all the way up the sides, which is warm, but when combined with a mummy bag in a hammock - getting into this liner can be difficult! I found the best way was to pull the liner over your entire body first, before entering sleeping bag. Just sitting up in the hammock with feet dangling, then pulling the liner up like a pair of pants and then all the way up over the shoulders. Finally, rotating into hammock and sliding feet down into the sleeping bag.

Sangpo at on 05/22/2013

Good Stuff

I tend to buy a lot of gear- my wife says we have a mini REI in our basement. That being said, I sometimes buy gear that I use once, don't really like it and rarely use it again. I feared this liner would fall into this category, but I was wrong. It works great to add extra warmth to a sleeping bag, stands alone for a summer bag in warm places, and acts as a great liner when staying at a friend's cabing or home and don't want to have to wash all the sheets after the visit. It is a very soft, stretchy material that makes it feel larger than it is, making it very comfortable. It was pricey, but I am very pleased with the purchase.
Godzilla dislikes Mothra at REI on 09/09/2011

great add on for cold weather and summer

I have used this bad as a liner and by itself on many occasions. In extreme cold it will lock in heat and compliment the best down bags when you have cold near zero temps. It will improve your bags rating about 10 to 20 degrees. This bag is not confining either. There is plenty of room for moving around in it as well. In warm summer months its just right to use by itself with temps above 60f or as a sleep sheet with a thin bag. It packs down to about the size of a soda can and takes up little space. I think its a good product and has kept me nice an warm when waking to ice condensation on the inside of the tent cold.
push at REI on 03/03/2011

Negative Reviews:

A Dud

I've tested this twice and it didn't do much either time.

The first was test was with a new REI 40 degree bag. I was a little skeptical at the +25 degree claim so i went to a campground near my house. Temps got down more than I thought into the mid 20s. I ended up leaving during the night because I was so cold.

Second time I really wanted to give this a fair chance so I used my North Face Cat's Meow bag (20 degree). The temps got down into the high teens so i knew I was pushing past the limit of the bag. I was car camping so I brought TWO sleeping pads to make sure that I had enough between me and the ground (about 3.5 inches of padding). About 2am I was so cold I couldn't take it so I grabbed an extra sleeping bag from the car and was toasty the rest of the night.

I don't think this adds more than 5 degrees to a bag. I could be wrong, but if you do buy it, make sure you test it out before you're depending on it.
Jon Roth at REI on 02/02/2012

Meh at best

I couldnt squeeze one extra degree out of this thing if my life depended on it. [$] better add extra 20 some degrees right? wrong. I used this thing in 25f weather and felt no difference at all, was still freezing. Granted I have a old 20 degree coleman rectangle bag, but still. So I took the liner out and slept exactly the same. I tried it 2 nights at around mid 20's outside and with no success stayed cold. Liner itself is very roomy and actually pretty soft material. Honestly though its a bunch of t-shirts sewn together to make nothing useful. Keep your bag clean, probably. Worth [$] USD? Absolutely not. If you do feel a difference it's most likely the placebo effect( you just spent [$] dollars on a liner, so you make yourself feel a difference) nothing that thin made out of 100% polyester can add up to 20 degrees, unless of course you have a heater in the bag with you. RETURNED!
djr21589 at REI on 04/04/2012


I've looked at these bags at length with the idea of a stand alone bag, ultralight, with warm clothing as a booster for bike and ultralight hike trips.
So I finally made the plunge with the knowledge that I had the pickup to sleep in if all went wrong. Sigh.
I was out of the tent even before it was dark and into the truck cab for an awkward nights sleep. The bag could not keep me warm even in the truck.
I don't need a booster for my down bag. It does a wonderful job keeping me warm but its 2 pounds and this product is something less than a pound. So my experiment is at fail.

Get a light down bag and don't buy into this as a summer weight bag which is one of their claimed uses. Its not.

I returned this to my local REI. The staff there was friendly and their return service was great.
lou at poo at REI on 08/08/2013

Not worth the price

I bought this to give me extra warmth in my Kelty 20 degree bag for winter hiking. It is small and compact and packs easily inside the sleeping bag with minimal weight. I guess it would be good as a sleeping bag liner if you just wanted to protect your bag from dirt and sweat. It does not give you an extra 10 to 20 degrees warmth as advertised. It is difficult to get into it and your sleeping bag especially when hammock camping. I have an ENO hammock, Thermarest Neoair sleeping bad, and was sleeping in a Kelty Lightyear XP 20 degree bag, The nights got into the lower 30's and I was cold even sleeping in high quality thermals and smartwool socks. The liner is a quality liner just don't buy it expecting to gain warmth in your bag. I'll be returning it to REI.
kwd340 at REI on 04/04/2013

Did nothing at all.

I got this liner because I wanted something in addition to my sleeping bag. I have a sleeping bag of a comfort rating of 32F and I was a slighty cold in 40F. So I thought this liner would be a great addition to it but I tried it twice now and this liner adds no warmth.

To make sure, my boyfriend even tried it yesterday in the sleeping bag and the temperatures dropped to low 30's. We woke up and everything was frosty. He mentioned that the liner did not add any warm and that he was pretty much cold all night.

This is a lightweight (not ultra light) liner and would probably be a good addition when it is over 50F out but I do not recommend it below 50F. Thankfully REI has a wonderful return policy.
Outdoor Mel at REI on 10/10/2012

Not even 1/2 the warmth

I went car camping with the boys and brought my new Marmot 40 degree bag and this new insert. I was exited to try it out to see how it performed. I had hoped I would be able to bring it as a main bag during this summer but will be returning it. I was in 30 degree weather for 3 days and froze. I figure a 40 degree bag and a 25 degree insert would be more than ample. I ended up having to use my summer thermals and emergency fleece (top/bottom). This insert did not even provide 1/2 the warmth rated. I would have been happy with just 10 degrees.

Now I'll have to find something heavy and I don't have the ounces to spare in the mountains.
djester at REI on 05/05/2010

Adds 25 degrees???

I bought this product with a lot of skepticism because it is a very thin layer of polyester. It is really thin and my primary use for it was to add warmth to my 15 degree down bag for winter backpacking. Lucky for me I got to test this in -30 degree conditions in Wisconsin. It did not add 25 degrees to my bag. However I did not freeze, because I kept eating and doing sit ups all night long. For the money and what they rate this liner, it should be thicker. Do not expect this liner to add 25 degrees to your bag.

However this product is durable, compact, and lightweight and keeps me from washing my down bag in the machine.
WildernessInstructor at REI on 04/04/2012

Does not live up to its name

I believe that the only reason customers may have a positive experience with this product is that their sleeping bags were not being tested by the environment.

I gave this product 2 stars because it does function fairly well as a light summer bag. I would NEVER expect it to lower the usable rating on my bag, and in my experience, it has never increased the comfort of my sleeping bag beyond its own rating.

Please do not purchase this and expect it to make your trip more comfortable or 'survivable' if you are testing the limits of your bag's rating.
Deanomag at REI on 11/11/2012

I really hate being cold so got this liner to add warmth to my 30 degree bag. If It added 5 degrees warmth I would be surprised. Will be good at keeping the bag clean, and maybe okay on a warm Florida summer night when a bag is just too warm. Otherwise I found this useless for it's stated purpose.
It does fit nicely in the bag and doesn't restrict my movement.

Sheri at on 04/21/2010

Little Help

Reading the earlier reviews, I felt confident buying this product. However, I felt it contributed nothing to my sleep warmth. It probably does keep the bag a little cleaner. Also, it likely would not hurt to put this in your day pack with your emergency blanket. Just don't expect this to add any warmth to a cold night.
Lancelot at REI on 02/02/2011

Neutral Reviews:

Not a warm liner

I purchased the Sea to Summit Thermolite Reactor Extreme Mummy Bag Liner from REI with the idea of increasing the warmth in my well used three season bag (When it was new, I believed it was rated as 15 degrees.)
Even though I do not like cold weather camping I decided to spend last night in a SNP hut to see what, if any, warmth the liner could add to an older sleeping bag. I was sleeping on a REI Trekker pad, in med weight long underwear (top and bottom), a hat, and a fleece pullover. The temperature got down to about 20 degrees F and I was freezing.
I found the Sea to Summit Thermolite Reactor Extreme Mummy Bag added little or no warmth to the sleeping bag. It did feel good and I'll keep it to try as a summer sleeping bag.
The 3 rating is because it does keep the bag cleaner, it felt good, it did come with a stuff sack, you can use the draw string to close the bag around your head, and I am hoping it can be used as an ultra light summer sleeping bag.
Incidentally, after I put on some more clothing, I toss a couple of Grabber Mega warmers in the sleeping bag with the liner and as soon as the warmers started to activate my legs and feet were comfortable the rest of the night.
pristine ruins at REI on 12/12/2010

Pretty Good Liner

I bought this the spring sale of 2010 to supplement my very old and worn out synthetic sleeping bag so I wouldn't need to buy a new bag just yet. I also hoped I could use it as a summer bag. To give a person an idea, the old bag is cold at 40 degrees now, though I can get through a night with spotty sleep. This liner does add insulation to my bag nicely, though I'd estimate it at adding 15 degrees. I've slept into the mid-30s with this and my bag, though I've also slept in a light jacket the same nights.

I won't be using this as a summer bag on its own, however. At the very least I would need to add some sort of emergency bivy to it because the liner doesn't block any wind whatsoever. That means lots of heat loss from convection. Maybe with a bivy AND a jacket I'll try it.

Lastly, for the 15 degree addition, this liner is heavy at 13 oz (the sack weighs about 1 oz). A top of the line down summer bag will weigh close to the same and keep me warmer. Granted, said bag costs a lot more.

So this liner has stretched the use of my old bag, giving me time to save up, but it hasn't worked as well as I hoped.
Joslin81 at REI on 03/03/2011

Good but not great

My wife and I purchased a pair of these liners to use on a spring camping trip that was forecasted down to the 30s at night. We have 35-degree rated bags, so thought that these would be a good option to provide some extra comfort. While they added a small degree of warmth (it reached a low of 34 degrees F), it was certainly nowhere near the 25-degree claim on the label. We still needed to break out our heavyweight baselayers to sleep in.

On the positive side, the liner is soft, comfortable, compact, and easy to clean.
Rayro at REI on 05/05/2013

On the packaging it says it adds up to 25 degrees to your sleeping bag but i wouldn't recommend it for more then 10 degrees. It is extremely light weight at only 14 oz. It fits well into both my -30 and my 20 degree sleeping bags and doesn't add very much size at all to the stuff sac. I would say its better for summer camping to use on those nights where its chilly but not cold enough for a bag.

Sean Longley at on 01/10/2011

Glad i got it on sale

this item works well enough, but the price is far too high for what you get. if i didnt get this on sale, i would have been pretty upset. there are other similar items that work just as well, and are just abt as light & compact at a much lower price. maybe that's why it was on sale, huh?
aikobarron at Campmor on 07/07/2012

Just another liner

Tried it out on a winter camoing trip with my brother. It added no noticable warmth. Seemed like a good liner for liner sake. Bottom line is, that if you are buying it to make your bag warmer -don't-. If you need a liner, it's alright.
SeldomSeenSmith at Campmor on 01/01/2013

I like this liner although its not a 25deg increase to your bag's rating, feels more like 10-15. It has a nice feel & is comfortable alone on hot summer nights or in combo w 40deg bag. Price is a bit steep @ $65

Ry Ry at on 09/29/2010

Looks like quality material

Have not used this yet because the bag I bought has been sufficient so far.
Streetpreacher at Campmor on 02/02/2014