Whenever someone asks me what type of sleeping bag to get for camping in the spring and fall, I always tell them to look into a quality 15-degree bag. My 15°F sleeping bag is my favorite piece of cold-weather backpacking gear; it has the perfect amount of insulation to keep me warm during the shoulder seasons, and has some of the nice features you would expect to find on a high-end winter bag.
Cold-weather sleeping bags are rated for the lowest temperature to keep you alive; a 15°F bag will allow you to sleep comfortably at about 25°F, which makes it a great choice for spring and fall camping. 15-degree bags feature quality insulation, a slim-fitting shape, and other features to retain body heat.
15-degree sleeping bags represent the perfect middle ground for camping in the spring and fall; they will keep you warm when nighttime temps dip below freezing, while staying lightweight and packable enough for the average backpacker or climber to carry anywhere.
Read on to learn more about the temperature rating system, what goes into the construction of a cold-weather sleeping bag, and some tips on how to stay warm if temperatures drop lower than expected.
Understanding Temperature Ratings
For decades, sleeping bag manufacturers have used minimum temperature ratings to define the products they offer on the market. The idea is that a “0-degree” bag will keep you alive down to 0°F, while the average person will sleep comfortably in that bag at about 10-15°F. The same rule of thumb applies for other temperature ratings: a 15-degree bag will keep you cozy at 25-30°F, a 30-degree bag at 40-45°F, etc.
The concept behind this system is still in use, but it has a couple of inherent flaws which have been addressed by the outdoor gear industry. The first problem was that each manufacturer had their own definition of what a “30-degree” bag was; this meant similarly rated products had different insulation types and different features, which resulted in big differences in the temperatures they would keep a person warm in.
The second problem with this system, or really an extension of the first problem, is that staying warm when sleeping in the backcountry is far from being an exact science. Every person has their own sleeping style; from side sleepers to back sleepers, “hot” and “cold” sleepers, and even people who have to have their feet warm but heads cool, or vice versa.
Fortunately, the industry recognized the risks of these variations and settled on a reliable, scientific temperature-rating standard known as the International Organization for Standardization (ISO) 23537 in 2016. This standard uses a human model equipped with temperature sensors to determine the thermal limits of a piece of equipment:
- Tested Lower Limit — the lowest temperature at which the bag will keep the average “warm sleeper” comfortable. This is the rating featured on most men’s sleeping bags.
- Tested Comfort Limit — the lowest temperature at which the bag will keep the average “cold sleeper” comfortable. This is the rating featured on most women’s sleeping bags.
All that this means is that a 15-degree men’s bag, at least one made by a reputable brand that follows the ISO standard, will keep a physically fit 25-year old man, who sleeps “warm”, comfortable at 15°F. Most people, myself included, will probably find that bag comfortable at temperatures down to about 25°F.
If you are concerned about getting cold at night, you might choose to err on the side of caution and get a warmer bag; just keep in mind, you will have to carry it.
Bottom Line: Buy a sleeping bag with a rated comfort level at the lowest possible temperature you expect to encounter in your outdoor adventures. This way it will keep you alive in case temperatures drop significantly lower than expected, and you can always unzip it if temperatures are warmer. A 15-degree bag is a happy medium that many backpackers agree on.
What to Look For in a Cold-Weather Sleeping Bag
Besides the temperature rating, there are a few features you will want to be sure a sleeping bag has if you’re planning on using it to sleep in chilly weather.
There is More to Insulation Than the Cost
First, determine whether you want synthetic or down insulation. A down sleeping bag will be significantly more expensive than one filled with polyester fibers, but that isn’t the only deciding factor:
- Synthetic insulation is made from thin-spun, hollow polyester fibers that provide loft, or airspace that retains heat. Synthetic-filled bags are not only a great choice for budget-minded backpackers, they also maintain heat when damp, dry quickly if they do get wet, and are non-allergenic.
They are generally heavier and less compressible than similarly rated down bags. Many brands have a proprietary name for their polyester filling material, so look for the term “high-loft”.
- Duck or goose down provides superior loft to synthetic fibers, while also being very lightweight and easy to compress into a stuff sack. The down used in high-quality sleeping bags is usually treated with a hydrophobic coating, and sourced according to the Responsible Down Standard (RDS).
Even when treated, down bags present enough of a risk from getting soaked that they are considered unsafe for use on some big wall climbing routes. For cold-weather camping, make sure you buy a bag with at least 650 fill-power (FP), and up to 800 or 900 FP for extremely cold temperatures.
Bottom Line: Choosing synthetic or down insulation shouldn’t be a contest to see who can spend the most money. They both have their benefits and drawbacks, so make sure to buy only what you need.
Mummy Bags Aren’t the Only Option Anymore
For decades, the “mummy” bag shape has been the go-to design for lightweight, packable cold-weather sleeping bags. This tried-and-true design gets its name from the simple fact that when you’re wrapped in a mummy bag, you are in a restrictive cocoon with limited freedom of movement.
While they weren’t intended to restrict movement, mummy bags were designed to save on weight by using less material, and also fitting close to the body to eliminate the empty spaces that create cold zones. They are great for slim folks who sleep on their backs; which, as market testing has revealed, represents only one portion of cold-weather campers.
In an attempt to reach the side sleeping market, the innovative gear company Nemo released their revolutionary “spoon” sleeping bag design, which features extra space around the elbows and knees.
Having slept in mummy bags for over two decades, I am happy to report that there is officially another packable option for people who sleep on their sides, or toss and turn at night! I have tried a couple of these spoon-shaped bags, with both synthetic and down insulation, and I am really impressed with the comfort and level of warmth they provide.
Bottom Line: If you’re a slimmer camper, you will benefit from the close-fitting warmth that a mummy bag provides. If you have a broad profile or are a side sleeper, be prepared to lose some pack space to accommodate a bag that fits your sleeping profile. After all, a good night’s sleep is invaluable in the backcountry!
Choose a Water-Repellent Bag with a Draft Tube
Two other features to look for in a cold-weather sleeping bag are resistance to moisture, and a full-length draft tube along the zipper.
No matter if you’re buying a down- or synthetic-filled bag, be sure to verify that it advertises a water-repellent shell, and if at all possible, a water-proof footbox.
The reason for this is that even the most well-designed tents accumulate condensation from the ambient air and your breath as they cool. This condensation is not only a nuisance, it can be a deal-crusher if your bag repeatedly brushes up against the tent walls during a below-freezing night. A water-repellent shell can eliminate a lot of potential discomfort.
The next key component to any cold-weather sleeping bag is having a draft tube that runs the whole length of the zipper. A draft tube is simply a rolled section of fabric that is filled with a small amount of insulation, stitched along the edge of the bag right next to where the zipper runs. If a sleeping bag doesn’t have one, the entire length of the zipper will invariably allow freezing air to seep in.
Other Features to Look for in a 15-degree Sleeping Bag:
- A well-insulated hood that can be cinched down around your face.
- A lengthy, insulated draft collar to fill gaps around your shoulders.
- A two-way zipper that allows ventilation in the footbox.
- Zipper pulls that are easy to operate while wearing lightweight gloves.
- A slim pocket to keep your phone and eyeglasses warm and close by.
Bottom Line: Only buy a quality bag from a reputable manufacturer or retailer. At a bare minimum, any cold-weather sleeping bag should be constructed with premium insulation, a mummy- or spoon-shaped design, a water-resistant shell, and a draft tube that runs the length of the zipper. Look for products that build on these concepts by adding other useful features.
Ways to Increase the Warmth of Your Bag
No matter which cold weather sleeping bag you decide on, you will need to know how you can stretch its temperature rating: either by choice, or due to an unexpected turn in the weather.
An Insulated Sleeping Pad is a Necessity
Before you take your new bag camping, you will need to purchase an insulated sleeping pad to complement it. The main reason you need a pad is because the filling along the bottom of a sleeping bag is compressed by your weight, which makes it basically useless; until you roll over, or lean forward, or otherwise need it.
Besides insulating your backside from the cold ground, a sleeping pad also provides much-needed cushioning to give your bones and joints a break after a long day of hiking or climbing.
There are several good brands of sleeping pads on the market, and if you’re shopping at a decent retailer they will be able to point you in the right direction. For shoulder season camping, look for an insulated pad with an R-value of at least 3, and if you’re a cold sleeper, shoot for a pad that has an R-value of 4 or more.
Other Tips to Stay Warm:
- Pack a base layer, including warm socks, gloves, and a knit cap. Put these items on before you get cold. Take items off as your body heat regulates inside the bag.
- Close the vestibule and any other flaps on your tent, except those that are required for ventilation. This will increase condensation, but can salvage a precious few degrees on chilly nights.
- Eat a high-carb snack an hour before bedtime; your body will burn the extra calories to stay warm.
- Go pee if you need to. Taking a brief trip out into the cold is much more efficient than burning calories by keeping the urine in your bladder warm.
- Invest in a silk or polyester sleeping bag liner. This can add up to 10 degrees of warmth to your bag.
- If you sleep really cold, bring a hot water bottle, or disposable heat packs.
Bottom Line: On your first few nights out in frosty weather, pack some extra items to ensure that you can increase the warmth of your cold-weather sleeping bag. As you develop a sleep system that matches your comfort level, you will learn which items you can safely leave at home.
Anyone who is planning to go camping in the spring and fall should purchase a quality sleeping bag that is rated for below-freezing temperatures. 15-degree bags offer the best compromise between warmth and weight. Whichever bag you decide on, be sure that it features high-loft insulation, a close-fitting design, a water-repellent shell, and a draft tube.
While investing in a quality sleeping pad is essential to maintaining warmth, there are several other options to increase the warmth of your 15-degree sleeping bag, including packing extra layers, closing up the tent, eating a snack before bed, and using a silk bag liner.