Last Updated on 27/08/2023
In this guide to the best ultralight synthetic sleeping bags, we share the most lightweight and compact synthetic fill sleeping bags for backpacking. Avoid getting a heavy and bulky synthetic insulation sleeping bag that won’t fit in your backpack, and get one of the down-like alternatives you’re about to learn about.
Sleeping bags that use a man-made insulation material are often referred to as synthetic sleeping bags. This differentiates them from down sleeping bags which use a natural filling and offer contrasting advantages.
The main downside of synthetic fill sleeping bags vs. down sleeping bags is that they are heavier and bulkier, which isn’t great for backpacking. Down sleeping bags are generally much more compactable. However, new types of synthetic fibers have been developed to mimic the qualities of down, which are becoming more and more ultralight all the time.
5 Best Ultralight Synthetic Sleeping Bags
SITKA Gear Sleeping-Bags Kelvin Aerolite 30
- COMFORT RATING: 30° F / 0° C
- TOTAL WEIGHT: 38 oz / 1077 g
- MATERIALS: Synthetic PrimaLoft Gold Insulation, 20D Polyester Shell and Liner
The SITKA Gear Sleeping-Bags Kelvin Aerolite 30 is the best ultralight synthetic sleeping bag this year, in our opinion. What makes this such an awesome sleeping bag is that it has been designed so that it can be used fully clothed with your boots still on. This is not only useful for military personnel and hunters but for campers too; it means you can really use this sleeping bag all year round with the help of a down jacket and down pants.
You can almost wear this synthetic sleeping bag like an ultralight parka jacket. There are two zippable arm holes, so you don’t have to sleep with your arms inside the bag. The front zip runs down the center just like on a jacket, the hood is more like that of a jacket, and the bottom unzips so that you can walk around unhindered.
It is super lightweight and warm for its compact pack size and has a comfort rating of 30° F, which is good for 3 seasons in most places. When you wear warm clothing inside the bag or use a sleeping bag liner, you could use this all year round in many places. One thing we like about this bag is that it moves with you, so if you roll over, it doesn’t all twist up. It just flexes with you.
The insulation material is PrimaLoft Gold which is known for being incredibly breathable and having a four-way stretchiness that makes it ideal for outdoor clothing. It is extremely lightweight for the warmth it offers and has qualities that are very close to those of down insulation.
VERDICT: The Kelvin Aerolite 30 Sleeping Bag from SITKA Gear is our favorite ultralight synthetic sleeping bag for the cold evenings when you want to get in your sleeping bag as soon as it gets dark. Hunters, photographers, campers, and anyone who has to sit still for long periods of time outdoors need one of these in their life.
Nemo Forte Ultralight Synthetic Sleeping Bag
- COMFORT RATING: 32° F / 0° C
- EXTREME RATING: 20° F / -7° C
- TOTAL WEIGHT: 46 oz / 1300 g
- SEASON RATING: 3 Season
- PACK SIZE: 11.5 x 9 in / 29 x 23 cm
- MATERIALS: Synthetic Primaloft RISE 80% PCR Insulation, Polyester Shell, Nylon Lining
The Nemo Forte Ultralight Synthetic Sleeping Bag is one of the best ultralight synthetic sleeping bags for 2- 3 seasons of the year. The reason it works so well for multi-seasonal use is that it has two ventilation gills on top that really help to regulate the temperature in summer. When the vents are zipped shut, it is like any other ultralight synthetic sleeping bag, but when open, you don’t spend all night sweating.
It has a comfort rating of 35° F and an extreme rating of 20° F, which can be a little misleading if you don’t know what you’re looking at because they use the extreme rating in the name. To make things more confusing, they have the same bag available in a 35° F version as well as a 20° F. In both cases, these temperature ratings are the extreme limits, the comfort rating is very different.
Apart from the ventilation gills on top, there are some other nice features to improve comfort and convenience. The first is the wide shape which has a knee girth of 60 inches, well above average mummy synthetic sleeping bags. Another feature is the fold-out fabric which doubles as a neck baffle and is a nice place to tuck your hands as you sleep. The last point worth mentioning is the pillow pocket on the inside which will fit an inflatable pillow or can be used to stuff a jacket in as a pillow.
The insulation is PrimaLoft RISE which was introduced in 2020, but it wasn’t until early 2022 that we started to see more products using it. The fibers are meant to mimic down and provide maximum warmth through lofting and synthetic clusters. They claim to have cracked the issue of compression resistance which can be an issue if you are stuffing your sleeping bag into a tiny stuff sack every day.
VERDICT: The Forte Ultralight Synthetic Sleeping Bag from Nemo is perfect for hot sleepers who prefer extra ventilation for summer. It is also well suited to active sleepers who roll around a lot at night because of its shape and wide girth. The comfort rating is accurate, and it feels warm to around 30° F. Go check it out.VIEW ON REI
Big Agnes V Notch UL 40 (Primaloft) Mummy Sleeping Bag
- TEMPERATURE RATING: 40° F / 4° C
- TOTAL WEIGHT: 22 oz / 620 g
- PACK SIZE: 4 x 5 in / 10 x 12.7 cm
- MATERIALS: 70% recycled PrimaLoft Hi-Loft Ultra Silver insulation, Nylon ripstop shell fabric with a water-repellent finish, Polyester Taffeta lining
The Big Agnes V Notch UL 40 is an ultralightweight sleeping bag with Primaloft Ultra Silver synthetic fill designed for summer use when the temperature doesn’t quite get to freezing. There are no baffles to separate the insulation into compartments which helps to improve the loft on such a small amount of fill but does risk forming some cool spots.
I would not want to use this sleeping bag in cold weather with another means of staying warm, but in summer, it is perfect for taking the edge of cool nighttime temperatures. It is favored among thru-hikers ad backpackers because of its lightweight and compact pack size but also because of how well it deals with moisture. The insulation will not only keep you warm when wet but dry out in just a couple of hours n the morning sun or a spring breeze.
To be honest, it really doesn’t feel like there is very much filling in this at all, but it lofts up about an inch, and you can feel it trapping warmth on a cool night. I met a guy on the trail this year who loves his V Notch UL, but he uses it with base layers, down pants, and a down jacket on cold nights.
VERDICT: The V Notch UL 40 Sleeping Bag from Big Agnes certainly falls into the category of ultralight with synthetic fill. The problem is it is so lightweight that it is only really suitable for summer camping unless you couple it with another sleeping bag or some other form of insulation. If you just want something lightweight for warm weather camping, then this is excellent value and performs just like down.VIEW ON BACKCOUNTRY
Elite Survival Systems Recon 5 Sleeping Bag
- TEMPERATURE RATING: -4° F / -20° C
- TOTAL WEIGHT: 5 lbs / 2250 g
- PACK SIZE: 10.6 x 10.2 in / 27 x 26 cm
- MATERIALS: DuPont Thermolite Micro insulation, 210D Nylon Oxford with PU2000mm DWR finish outer bottom, 210T Ripstop Nylon Teflon finish top outer shell, 210T Ripstop Nylon Lining
The Elite Survival Systems Recon 5 Sleeping Bag is not considered ultralight compared to other summer sleeping bags, but that is because this will keep you warm down to well below freezing. For a -4°F rated sleeping bag, this is the lightest we could find. It is even lighter than most 5° F synthetic sleeping bags.
Used by military services around the world for its reliability and durability, this is the most lightweight synthetic sleeping bag for winter and freezing temperatures. There are features like Teflon and waterproof finishes to the outer shell to keep you dry and protected. There is also a 210D durable internal foot box lining so you can sleep with your boots on inside the bag (more important for military personnel than campers).
The pack size, while double many others in this guide, is pretty good for such a warm bag, and when you stuff it to the bottom of your backpack, you still have space for a sleeping pad or your tent to fit down the side of it. The Thermolite DuPont insulation is laid flat and doesn’t have any baffles as a down sleeping bag would. Instead the insulation
VERDICT: The Recon 5 is the warmest in the Elite Survival Systems range of lightweight synthetic sleeping bags, but it weighs five pounds, so not exactly ultralight. It is one of the lightest synthetic sleeping bags for winter and 4-season use though. You will not be disappointed with its durability, warmth, and comfort.
THERMAREST Space Cowboy 45 Synthetic Sleeping Bag
- TEMPERATURE RATING: 45° F / 7° C
- TOTAL WEIGHT: 28 oz / 780 g
- PACK SIZE: 8 x 10 in / 20 x 26 cm
- MATERIALS: EraLoft Insulation, GRS-certified 100% Recycled Nylon Shell and Lining
The THERMAREST Space Cowboy 45 Synthetic Sleeping Bag weighs under two ounces and packs down to about the size of a football. Designed for mostly summer use, this lightweight bag is perfect for backpacking or lightweight camping trips with minimal gear. Shape, size, and design are all perfect – no complaints at all.
I like the way the insulation is distributed, with 35% of the fill being on the bottom but the other 65% on top, where it matters the most. I also like that there is a baffle system, unlike the Big Agnes and Recon 5 sleeping bags, which have a flat-lying filling. There are other cool features too, like the sleeping pad attachments and YKK Anti-Snag Zippers, which I have yet to see fail.
Thermarest uses its own proprietary synthetic insulation called EraLoft, which uses hollow fibers, like polar bear fur, to maximize heat retention even if wet. The other advantage of this type of synthetic fiber is that it is ultralight and compressible, which helps when your backpack space is rationed.
VERDICT: The Space Cowboy 45 from THERMAREST is very much a summer synthetic sleeping bag that works best when nighttime temperatures don’t drop below 50° F. If temperatures get close to 30° F, then I have no doubt you will feel some chills in the night. In spring and autumn, you can still use it but with a sleeping bag liner and thermal base layers on the inside.VIEW ON MOOSEJAW
Types of Synthetic Sleeping Bag Insulation
Unlike down, which either comes from a goose or a duck, synthetic insulation comes in many different varieties. They all essentially do the same thing but may be better in certain areas. Like how Primaloft has an amazing warmth-to-weight ratio or how eraLoft uses hollow fibers to ensure it works even if wet. Here is a brief explanation of what each insulation claims to do:
Primaloft insulation is used in many of the best ultralight synthetic sleeping bags because of how similar it is to down. The clusters of synthetic fibers are composed to behave the same way as downy feathers by retaining loft and trapping heat. It works even when wet and compresses down very small with the help of a stuff sack.
After reviewing dozens of synthetic ultralight sleeping bags, Primaloft is constantly one of the highest performers.
3M Thinsulate is one of the most commonly used insulation for outdoor clothing like gloves, hats, and winter jackets. It is one of the best-value synthetic insulation types for ultralight sleeping bags and doesn’t often feature in higher-end synthetic sleeping bags. Thinsulate is moisture resistant and works even if wet. Like Primaloft, 3M Thinsulate is its own product that is licensed out to other brands.
Thermolite is a group of polyester fiber insulation designed by Lycra to be lightweight, warm, durable, and sustainable. Like other synthetic insulations, this will keep you warm if you get rained on or experience high condensation. Elite Survival Systems is one company that uses Thrmolite to make their military sleeping bags which have been proven to be very warm and ultralight in the winter synthetic sleeping bag class.
ThermoBall is The North Face‘s sustainable alternative to down, which uses 100% recycled materials and is incredibly warm, lofty, and compressible. The microfibers mimic the way-down clusters together to create pockets of warm air. The benefit ThermoBall has over down, though, is that it doesn’t all clump together when wet and will continue to keep you warm. One of my favorite North Face ThermoBall products is their camping slipper mules.
Climashield has five different variations of its outdoor clothing insulation, but the best for the warmth-to-weight ratio is the Climashield Apex. Climashield Prism is also very lightweight and keeps its composition and structure under sustained use and when wet. All five variations use the same microfiber composition, which makes them some of the lightest wearable insulation out there.
Thermal R is Marmot‘s own proprietary group of polyester insulation. Using hollow fiber and multichannel filaments, Thermal R is hardly affected by perspiration, humidity, or moisture and continues to trap heat when damp. Marmot also has a sustainable version of the same material that is made using 100% recycled fibers called Thermal R Eco. Both have amazing warmth-to-weight ratios and are very durable for sleeping bags.
I cannot find much information about Thermic MX, but I remember having a Mountain Hardwear sleeping bag a good few years ago that used this insulation, and it was amazing. It appears that it is no longer being used, but you can still find some vintage synthetic sleeping bags that use it and are still ultralight.
Coreloft is the insulation of choice for Arc’teryx, who make THE best outdoor jackets in the world. I have never seen a sleeping bag made from Coreloft, but it would absolutely work. It was designed to replicate the properties of down and is one of the most compressible but lofty synthetic fibers out there.
EraLoft is one of Thermarest‘s many innovative products created for camping sleep systems. EraLoft is used in many of Thermarests lightweight synthetic sleeping bags for its ability to withstand being compressed and lofted without losing performance over time. As many synthetic sleeping bag fibers are, EraLoft is spun using a hollow core which means it will continue to trap heat even if soaked.
Guide To Ultralight Synthetic Sleeping Bags
When looking for the best lightweight synthetic sleeping bag, there are multiple factors that come into play. t is not just the weight of the sleeping bag that matters. Warmth to weight ratio is very important, as is pack size and other features. This is where we share what we looked for and how we tested the different sleeping bags to see why they perform so well:
There is no point in having an ultralight synthetic fill sleeping bag if it doesn’t keep you warm. This is why it is important to look at what temperatures the sleeping bag is rated for. There are two measurements to look for; comfort and extreme. The comfort rating shows the temperature in which the sleeping bag is designed to work well in. The extreme rating is the temperature at which the sleeping bag is designed to keep you alive.
To really compare how the different sleeping bags stack up side by side, we had to make a quick calculation to figure out how much warmth each sleeping bag offered per pound or kilogram of weight. To figure this out, simply divide the weight by the temperature rating, and you should be able to figure out how many degrees of warmth you are getting per ounce, etc…
What makes the biggest difference to synthetic insulation warmth is how much of it there is. The thicker and more fill that is used, the warmer the sleeping bag will be. We talk about thermal resistance here if you want to learn more.
When specifically looking for a synthetic sleeping bag that is ultralight, the weight is a key priority that is linked to the warmth of the bag. Again, the warmth-to-weight ratio is very important here.
You can create the lightest synthetic sleeping bag in the world if you only put an ounce of fill in it, but that won’t keep you warm. So you need first to find a sleeping bag that is warm enough for the temperatures you will be camping in, and then start comparing the weights to see which is the lightest. Here is a rough guide to how much an ultralight synthetic sleeping bag should weigh:
- For summer synthetic sleeping bags, we are ideally looking for ultralight options around or below the 2-pound mark.
- For 2-season synthetic sleeping bags, we would expect to carry around or less than 2.5 pounds.
- For 3-season sleeping bags that will keep you warm down to about 20° F, we would still consider a 3-pound pack weight to be very lightweight.
- And then, for winter sleeping bags rated down to 5° F and below, anything under 5 pounds would probably be the lightest option out there.
The lighter, the better, so long as your warmth isn’t compromised.
Pack size is important for backpackers and hikers who often need as much space in their bags for supplies as possible. Most sleeping bags can be compressed much more than stated by the manufacturer if needed, but when you do this, you risk compromising the loftiness of the insulation.
While synthetic insulation does not suffer from compression damage as much as natural down does, it should still be avoided wherever possible. To get a smaller pack size, a standard stuff sack won’t do; you need a compression sack.
Down sleeping bags are often measured next to a water bottle to show how small they are, but synthetic sleeping bags are far bulkier and less compactable. I have found that if you can get a synthetic-filled sleeping bag to compress down to the size of a soccer ball, then you are winning.
There are multiple types of polyester and nylon microfibre insulations out there, we have listed many of them further up in this guide. Without downplaying how amazing each one is, they are pretty much all the same, and the way the insulation is distributed and layered is what makes the real difference.
We found that Primaloft is consistently among the top performers and that Thermarest has done a very good job at optimizing its fill. Sustainability is a real focus at the moment, and so many of the fill materials are now being made from recycled products, which is great and something we should all get behind wherever possible.
When manufacturers focus on making outdoor gear as light as possible, one of the biggest sacrifices you have to make is that of durability. Thinner and more lightweight materials are prone to rip or get damaged easily. All the ultralight synthetic sleeping bags in this guide are durable enough for camping with, but if you see an outer shell that has less than 10D material, then be prepared to apply extra care not to damage it.
Loft and Design
Loft is the word we use to describe how much the insulation fluffs up and expands, and something that can influence loft is how the baffle system is designed. Baffles are the stitches and pockets used to separate and compartmentalize the insulation so that it can be directed and held in position long term.
Some ultralight sleeping bags with synthetic fill choose not to use baffles in order to maximize loft and minimize weight. This is true, but over time you can end up with thin spots that let heat escape.
One company that understands how sleeping bags degrade over time is Thermarest which uses insulation mapping to ensure you get the most warmth in the places you need it most and the least insulation where it would only be compressed anyway. They do this by distributing the fill for about 65% on top and 35% on the bottom.
Shape and Size
To be as lightweight and warm as possible, a mummy sleeping bag is the shape and style that you want. A mummy sleeping bag tapers down from a wide shoulder to a narrow foot box. This saves weight on excess materials and also means that any heat generated is contained in a smaller area for better efficiency.
Rectangular sleeping bags are great if you prioritize leg space and the ability to turn your sleeping bag into a quilt. But synthetic sleeping bags are notoriously bulky and difficult to pack inside a backpack without taking up half the space.
Synthetic sleeping bags are often very good value, and unlike down sleeping bags, the price doesn’t skyrocket just for saving an ounce of weight. You will struggle to find a synthetic sleeping bag over the $200 range, whereas, with down sleeping bags, your basic summer bags start at $200 and go up from there.
How Do Ultralight Synthetic Sleeping Bags Compare to Down Sleeping Bags?
You may have been told that if you want an ultralight sleeping bag, down is what you need. This is true to some degree, but there are many times when down sleeping bags are not the best choice, and synthetic sleeping bags that are ultralight are much more suited. Here are some of the pros and cons of synthetic sleeping bags for ultralight backpacking.
Benefits of Synthetic Sleeping Bags Vs. Down Sleeping Bags
The main benefits of synthetic sleeping bags are that they work even if they are exposed to moisture, are easier to care for, and are much cheaper. Here are a few others:
- Work When Wet
- Easy To Wash
- Fast Drying
- Better Value
Times, when synthetic sleeping bags are the better choice to down, are when you are expecting to be in wet or humid environments or when you really need to be able to rely on your sleeping bag to keep you alive.
Disadvantages of Synthetic Sleeping Bags Vs. Down Sleeping Bags
The compromises you have to make when choosing a synthetic sleeping bag over a down sleeping bag are that they are heavier, bulkier, and don’t loft up as much. If you can live with these downsides, then the choice should be fairly easy for you.
- Less Loft
Do Synthetic Sleeping Bags Lose Their Warmth?
Synthetic sleeping bags will lose warmth at the same rate as they lose their loft. This generally happens from compressing the sleeping bag as you lie in it and every time you stuff it into a small sack. After many repetitions, the insulation becomes more and more intertwined and unable to expand as much to trap heat.
Synthetic sleeping bag insulation fibers won’t decay for hundreds of years, but over time, they will lose their loft, migrate to leave cold spots, and appear much flatter. This happens far slower than with down, and generally speaking, synthetic fill is the more durable of the two.
We hope you found this guide to the best ultralight synthetic sleeping bags useful and that you now feel like you have all the tools you need to decide between a down sleeping bag or an ultralightweight synthetic one.