Sleeping Bag Liner Guide: Add 10°F to Any Bag

A sleeping bag liner is the cheapest way to make a bag warmer, and the only way to make it cleaner. A sheet of fabric inside the bag adds a few degrees to a lot of degrees depending on the material, keeps sweat and skin oil out of the insulation, and doubles as a bed on its own in a hut or a hot tent. This guide compares silk, synthetic and merino liners on the numbers, explains why wool adds more warmth than its weight suggests, and covers sizing, use and care.

What a sleeping bag liner actually does

Three jobs, in order of how much they matter to most people.

• Warmth. A liner adds a thin insulating layer and, more usefully, a wind and draught barrier close to the skin. Silk adds a little, synthetic fleece adds some, wool adds about 10°F. That difference turns a 30°F bag into a 20°F bag on a cold-snap night without buying a second bag.
• Hygiene. You lose roughly half a litre of water overnight, plus skin oil and sunscreen. In a bag without a liner that goes into the insulation, and down in particular loses loft as it gets dirty. A liner takes the load and goes in the wash; the bag does not.
• Bag life. Fewer washes means more years. A bag that is laundered once a season instead of every few trips keeps its loft and its shell far longer.

A fourth use is the one that sells most liners: a liner on its own is a summer bag, a hostel sheet, and the clean layer between you and a hut mattress of unknown history.

Silk, synthetic or merino sleeping bag liner

Liners come in three materials that behave very differently. The table gives typical figures for each class; the merino column is the Lucky Sheep merino sleep sack, the others are category averages from makers’ published specifications.

	Silk liner	Synthetic “thermal” liner	Merino wool liner
Weight class	4–6 oz	8–14 oz	30 oz
Warmth added	A few degrees; mostly a draught barrier	Makers claim 10–25°F; brushed polyester fleece, warm when dry, clammy when damp	About 10°F, and holds it when damp
Feel	Slippery, cool, catches on rough skin	Fleecy; builds static and clings	Soft knit jersey, like a good base layer
Moisture regain (65 % RH)	9–11 %	0.4 % (polyester)	13–16 %
Odour after a week	Moderate	Strong; polyester holds odour	Low; wool releases odour when aired
Washing	Hand-wash or delicate, dries fast	Machine wash, dries fast	Machine wash cool, wool cycle, line dry
Price band	$60–120	$30–90	$200–235
Best for	Hostels, hot-weather travel, hygiene only	Budget warmth in dry cold	Cold and damp; extending a bag’s range; sleeping alone in it

The honest reading of that table is that silk wins on weight, synthetic wins on price, and merino wins on everything that happens at 2 a.m. in a wet tent. At 30 oz the merino liner is not a light item; it is a few ounces short of two pounds, which is more than the difference between a 30°F and a 20°F down bag. If the only question is grams per degree in dry cold, buy the warmer bag. If the question is what works in the damp, in a hut, and for a decade, keep reading.

Why wool adds more warmth than its weight suggests

A liner sits against the skin, so it is the first layer that meets the moisture you give off overnight. What that layer does with the moisture decides how warm you feel.

Polyester absorbs almost nothing (0.4 percent regain). Sweat vapour condenses on the fibre surface, the fabric feels wet, and evaporation against the skin pulls heat out of you. That is the clammy feeling in a synthetic liner after a few hours, and why a “thermal” liner that felt warm at bedtime can feel cold before dawn.

Wool absorbs 13 to 16 percent of its weight in vapour inside the fibre, where it does not feel wet, and releases heat as it does so: published textile data put wool’s heat of sorption at roughly 960 kJ per kilogram of water absorbed. A 30 oz merino liner taking in part of the half-litre you lose overnight gives back real warmth in the coldest hours, when the bag’s insulation is starting to load up with moisture. Silk sits between the two at 9 to 11 percent regain but is too thin to hold much. Hence the number in the table: about 10°F from a wool liner, and it does not vanish when the air turns damp. More on the mechanism in how wool keeps you warm with heat of adsorption and why damp weather makes us feel colder.

How to use a liner

Inside any bag

Slide the liner into the bag before you get in, seam side out, and get into both together. Pull it to your chin and leave the bag’s draught collar over it. Match the liner girth to the bag so it does not bunch under your hips, and do not pull it over your head; breathing into it is the fastest way to wet it from the inside.

A wool liner works in any bag. In a down bag it is worth the most: down is what suffers when it gets damp, and a wool layer keeps the moisture in the liner where it does no harm.

Alone in summer

Above about 60°F a merino liner is the whole bed. It breathes, covers without trapping heat, and on the cool side of midnight is warm enough that you are not reaching for a jacket. The dust cover sleep sack is the same idea with a beeswaxed cotton outer for dew and ground contact, cut to the Rewilder’s taper with a drawcord at the head.

Hostels, huts and hot tents

Alpine huts issue blankets and expect you to bring a sheet; hostels and pilgrim refuges often require one. A liner is that sheet. Bunk rooms are warm and airless, and a wool liner sleeps cooler than a fleece one and smells better on day five. In a hot tent the liner is the bed until the stove dies, and then you pull the bag up over it.

Sizing: 28 in or 33 in

The merino sleep sack comes in two girths, 28 in and 33 in. Merino jersey stretches, so the fit is forgiving, but the wrong choice still shows: too narrow and the liner pulls under your hips when you roll; too wide and it bunches at the foot and leaves a cold gap along your back.

• 28 in: slim to average build, side and back sleepers who like a close fit, and any mummy or tapered bag. Pairs with a Medium (56 in girth) Rewilder.
• 33 in: broad shoulders, restless sleepers, anyone who wears a jacket to bed, and rectangular bags. Pairs with a Wide (66 in) Rewilder or an Aurora Borealis.

If you are between the two and will mostly use the liner inside a bag, go narrower; the bag holds it in place. If you will mostly sleep in it alone, go wider.

The liner built into Lucky Sheep bags: merino or WeatherWool

The Rewilder and the Aurora Borealis are built with a wool lining sewn in against the US wool batting, so the liner question is answered at the point of ordering. Two options:

• Merino jersey. The same soft knit as the sleep sack. Stretchy, warm to the touch, the closest thing to a base layer you can sleep in. This is the default and the right choice for most people.
• WeatherWool® worsted. A woven American wool cloth rather than a knit. Smoother, denser and tougher against abrasion, with less stretch and a drier, cooler hand. Choose it if you sleep in boots-off work clothes, share a bag with a dog, or simply prefer a woven sheet to a knit one.

Either way the lining is wool against wool batting, so the whole bag manages moisture the same way from skin to shell. A Lucky Sheep bag does not need a separate liner for warmth; a merino sack is still worth carrying for hygiene on long trips and for the nights you sleep in it alone.

Care

The merino sleep sack is machine washable, which is most of the point of owning one. A few rules keep it soft for years.

• On the trail: turn it inside out and hang it in the sun for an hour at lunch. Wool releases odour as it dries; a week’s trip usually needs no wash.
• At home: machine wash cool on a wool cycle with a wool detergent. No bleach, no softener, no enzyme detergents (they digest the protein in wool).
• Drying: line dry or lay flat. A hot dryer felts and shrinks merino jersey.
• Storage: clean, dry, folded loosely. Moths go for soiled wool, so wash before a long lay-up.
• Repairs: jersey snags rather than tears; a few stitches of wool yarn closes a pull. Workmanship is guaranteed for life.

The merino sleep sack is $200 to 235, ships in 2 to 5 days from Burnsville, North Carolina, and pairs with any bag you already own. If you are not sure which bag or liner suits the trips you actually take, the sleep-system finder asks five questions and tells you.

Reviewed August 2026. Warmth figures for silk and synthetic liners are makers’ claims, not test results; the 10°F figure for the merino liner is a field estimate, not an ISO 23537-1 rating.