Do Sleeping Bag Liners Actually Work? And Which Ones?

A lot of hikers want to push their sleeping bags a bit further into colder weather, and a liner looks like an easy answer. But do sleeping bag liners actually work?

The opinions out there swing wildly. Some people call liners a scam, while others swear they work like magic. So what is the truth, and should you spend money on one?

Do sleeping bag liners work? Yes, but the effectiveness depends on the type of material. Sleeping bag liners also keep your bag clean and help it last longer.

How much warmth does a sleeping bag liner add?

The honest answer is "it depends," and the biggest factor is the material. Shape and model matter too. A snug, mummy-shaped liner leaves less dead air to heat than a loose, rectangular one, and a thicker fabric holds more warmth than a thin one.

Some manufacturers claim as much as 32°F of added warmth, while others sit at the low end, around 5°F. Plenty of users dispute the bigger claims and say real-world gains are closer to half of what is advertised. Either way, you can expect at least a little warmth, and you keep your bag cleaner.

It helps to understand why those numbers vary. A sleeping bag keeps you warm by trapping a layer of still air around your body, and a liner simply adds another thin layer of fabric and a little more trapped air. It is not a replacement for insulation. If your bag is rated well above the temperature you are sleeping in, a liner will not close that gap. Think of it as a modest adjustment, not a fix.

Temperature ratings are also not a guarantee of comfort. They are a guide based on standardized testing, and your own metabolism, tiredness, and hydration all change how warm you feel. There is a general overview of how sleeping bags are built and rated if you want the background.

Which sleeping bag liner material is best?

If you are looking to add warmth to your bag, pick a liner made from thermal synthetic, merino wool, or microfiber/microfleece. These fabrics either trap air in their structure or hold a bit of loft, which is what actually slows heat loss.

If you are just looking for a liner to keep your bag clean and something that will feel comfortable, then silk will be fine as well. Silk is really compact, which makes it great for travel. It also feels smooth against the skin, which many people prefer to the slick feel of some synthetic bag linings.

Silk liners are really meant to keep your bag clean and aren't designed to actually keep you warm. Any extra warmth is a bonus.

If you are after the absolute warmest option, the Thermolite Reactor Fleece Liner is the one that stands out, adding 32°F, which is huge. Head over to Amazon to compare prices and reviews for all Thermolite Reactor products.

Thermolite fabric works so well because it uses hollow core fibers which trap in air, making it a very lightweight and warm insulator. Trapped air is the whole game with insulation. The more of it a fabric can hold without adding bulk, the better it performs per ounce.

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Are sleeping bag liners worth it?

A lot of campers want to increase their sleeping bag warmth without having to buy another bag. If you're car camping at a campground, you can shove whatever else you want in your car, so you might be better off bringing extra clothes and blankets. Weight and space simply don't matter much when the trunk is right there.

But if you are a backpacker, carrying whatever you want isn't a luxury you have. On longer routes like the Appalachian Trail, a liner adds extra weight and every ounce counts. The Appalachian Trail Conservancy is a good place to start if you are planning that kind of trip. Thru-hikers really need to weigh the pros and cons of carrying a liner.

It's my opinion that you shouldn't rely on a liner to stay warm during cold weather. Staying warm isn't the only benefit, though. There are other reasons to use one, and they are probably just as important.

Reasons to use a sleeping bag liner

Yes, sleeping bag liners can add a little bit of warmth, but there are other benefits too:

• Adds extra warmth to your sleeping bag
• Prolongs the life of your sleeping bag by keeping it clean
• Is easier to wash than a sleeping bag
• Can be used by itself during the summer and warm weather
• Can be used as a sleep sheet in hostels to avoid dirty beds

Sleeping bag liners keep your sweat and body oils out of your sleeping bag. Without a liner, your bag can become dirty and lose its effectiveness. Oils and grime coat the insulation and the inner fabric, and dirty insulation clumps and traps less air. If you're using a down sleeping bag, the liner helps keep sweat from ruining the down loft. That makes your bag last longer and keeps you warmer.

Washing your liner instead of your bag will prolong the life of your sleeping bag by a lot. A liner is much easier to wash, and it saves the wear and tear that repeated washing puts on a bag. Insulation, shell fabric, and zippers all take a beating in a washing machine, so the fewer times you do it, the better.

The summer use is worth a second look. On a warm night, a full sleeping bag can be far too hot, and a light liner on its own gives you a cover without the heat. It also packs down small, so it is easy to carry as a backup.

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Base layer vs sleeping bag liner

If you're wearing clean base layers to bed, like long johns or thermal underwear, you probably don't need a liner as much. Clean clothes won't make your bag as dirty, and you're going to stay warm.

I'd rather wear extra clothes to bed as needed. It also helps on a cold morning, when you don't have to do much to get dressed. Getting out of a warm bag into freezing air is the hardest part of the day, and already being half dressed takes the sting out of it.

Another advantage of base layers is that a lot of campers get tangled up in their liners, or the liner ends up scrunched into a ball around their feet. A liner that twists as you turn over can be more annoying than the warmth is worth. Clothes stay where you put them.

That said, the two aren't mutually exclusive. A liner and a clean set of sleep clothes together do the most to protect your bag, and the choice mostly comes down to what you want to carry.

Staying warm without a liner

If you're wondering whether a liner is necessary, it isn't. There are worthwhile benefits to using one, but when it comes to warmth, there are plenty of other ways to stay comfortable. Getting cold at night is more than uncomfortable, and the general background on hypothermia explains why it is worth taking seriously.

The first thing to get right is your sleep system. That means a properly rated sleeping bag and good ground insulation from a sleeping pad. The ground pulls heat out of your body faster than the air does, which is why the pad matters as much as the bag. Doubling up or getting a thicker sleeping pad can help a lot.

Another big factor in staying warm inside your bag is wearing clean, dry clothes. After a day of getting dirty and sweaty, changing into fresh dry clothes at bedtime will make you feel better and keep you warm and dry. Damp clothes only rob you of body heat.

Other small habits add up as well: eating something before bed, going to the toilet before you settle in, and making sure your bag is fully lofted rather than crushed. If you aren't sure how to pack it, here is why stuffing beats rolling.

I put together a list of 27 ways to stay warm inside your tent, some of which can make a big difference.

DIY sleeping bag liners

If you are crafty and want something extra warm, you can make your own liner. Fleece is a good choice. The challenge, if you're hiking, is that a fleece liner takes a lot of room in your pack, possibly even more than your sleeping bag.

If you're just looking to keep your bag clean, you can get a travel sheet for as little as $10. If you want to sew your own, it isn't too difficult. Measure the fabric you'll need, fold it together, and sew the bottom and sides. You could even use a regular bed sheet. Leave enough width to roll over comfortably, since a liner that is too tight will pull and bunch as you move.

Another option some people like is a military surplus poncho liner, commonly called a Woobie. Woobies are like blankets, with a 100% nylon outer shell and 100% polyester batting.

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So, should you buy one?

Reviews are mixed on whether sleeping bag liners work, and that is fair. Some people say they work and others don't. Besides adding at least a little bit of warmth to your sleeping bag, they also keep your bag clean, make it last longer, and can feel more comfortable than the synthetic bag material.

Liners aren't necessary, but they are nice to have. If you decide to go without one, make sure you are staying warm enough and wear some base layers.

Whatever you decide, practice good winter safety and test your gear before any big trip. A cold night in the backyard teaches you far more than a spec sheet will.