How many degrees does a sleeping bag liner actually add?

A liner adds about 1 degree C if it is silk or thin synthetic, and about 3 if it is a thick thermal or fleece liner. Both follow from the ISO 23537 rating table, which converts thermal resistance into degrees at roughly 1.3 degrees C of comfort per 0.04 square metre kelvin per watt. Claims of 8 to 14 degrees would need a layer at least 6.6 to 11.5 mm thick, even if it insulated as well as still air.

Why the numbers on liner packaging disagree#

The figures in circulation run from 5 degrees F to 25. REI's buying guide says a liner adds anywhere from 5 to 15 degrees F of extra warmth, "depending on the liner material", and lists fleece at up to 12 degrees F and insulated liners at "up to a claimed" 25 degrees F. It cites no test, and other pages repeat its range.

The maker figures are no steadier. In a January 2025 capture, REI's product page for the Sea to Summit Thermolite Reactor Extreme lists "Up to 25 degrees" F and "Up to 14 degrees" C. Sea to Summit's own advice page now gives the Reactor Extreme up to 6 degrees C, the Reactor up to 4 and the Reactor Fleece up to 8, under the heading of reasonable estimates. The same page says the company has attempted to quantify liner warmth in degrees, "even considered measuring the insulation value of a sleeping bag with and without a liner", and concluded that "there are simply too many factors".

None of these figures comes with a test method, and ISO 23537 rates sleeping bags, not liners. That leaves two questions this page can answer: how much insulation a liner of a given thickness can physically add, and what the bag standard says that insulation is worth in degrees.

What the ISO 23537 manikin is already wearing#

The detail that settles most of the argument is in clause 5.1.2.1 of ISO 23537-1:2022. The heated manikin does not go into the bag bare. It is dressed in a two-piece suit, a long-sleeved top and trousers, with a material thermal resistance of 0.040 to 0.060 m2K/W measured to ISO 11092, plus knee-length socks of the same range. It lies on a foam mattress of 0.85 m2K/W.

Clause 3.6 defines the bag's thermal resistance as including "the effects of the shell fabrics and filling materials, air volume in the cavity inside the sleeping bag, boundary air layer on the outer face of the sleeping bag, mattress underneath the sleeping bag and garments worn by the sleeping bag user". The rating describes a system, as covered in sleeping bag temperature ratings.

A thin liner is the same kind of object as the test suit. Both are one layer of fabric next to the body. So the suit's specification is a natural yardstick: a liner whose fabric matches it adds 0.040 to 0.060 m2K/W to the system.

The rating already assumes you are dressed. If you sleep in shorts and a T-shirt, you are colder than the manikin before any liner goes in. A thin liner then does little more than replace the long sleeves and trousers the rating counted. That is much of why one person calls a liner transformative and another feels nothing.

The heat balance: turning resistance into degrees#

ISO 23537 converts the measured resistance into temperatures with a physiological model, and it publishes the result as Table 1, which labs may use with linear interpolation. Here are rows from it.

Each step of 0.040 m2K/W moves comfort by 1.2 or 1.3 degrees C, limit by 1.4 to 1.6 and extreme by 1.9 to 2.2, from a summer bag to a winter one. Between 0.86 and 1.06, where most three season bags sit, it is 5.6 plus 0.7 = 6.3 degrees across 0.20, so 31.5 degrees C per m2K/W. Limit is 7.4 across 0.20, or 37, and extreme is 10.2 across 0.20, or 51.

Now add a liner. We model it as one more layer in series with everything else, so its resistance adds straight onto R c. Take a bag at 1.020 m2K/W, which the table rates at plus 0.6 degrees C comfort and minus 5.2 limit, and add a liner matching the middle of the test suit's range, 0.050:

• New resistance: 1.020 + 0.050 = 1.070 m2K/W.
• Comfort, interpolating between 1.060 (minus 0.7) and 1.100 (minus 1.9): minus 0.7 minus a quarter of 1.2 = minus 1.0 degrees C.
• Gain: 0.6 minus (minus 1.0) = 1.6 degrees C of comfort, and about 1.9 on the limit figure.

Across the suit's range, 0.040 to 0.060, the gain is 1.3 to 1.9 degrees C of comfort. That is the honest size of "one more base layer's worth" of fabric.

What each claim would need: the reverse calculation#

Run the slope backwards and every claim becomes a resistance, and every resistance a minimum thickness. Still air conducts at 0.026 watts per metre kelvin, and a textile is a structure for holding air still, with fibres that conduct better than the air they hold. So no fabric layer of thickness t can beat t divided by 0.026. The table uses comfort figures; the last two columns use our existing figures of 0.20 to 0.28 RSI per centimetre of lofted fill and 35 g of 650 fill power down per degree, from quilts against sleeping bags.

The rule worth remembering: 0.001 m divided by 0.026 is 0.038 m2K/W per millimetre, and 0.038 times 31.5 is about 1.2 degrees C of comfort per millimetre of liner, at most.

Read the bottom row against the ISO table itself. The resistance a 14 degree liner would need, 0.44 m2K/W, is 88 percent of the 0.500 at the top of Table 1, a resistance that includes the suit, the mattress, the bag and its boundary air. REI's January 2025 page put that claim on a liner weighing 14 ounces, about 397 g. The comparison column says a bag would need about 490 g of 650 fill power down, lofted, to gain the same.

So which figure is right?#

Our existing figures hold: about 1 degree C for silk or a thin synthetic liner, about 3 for a fleece or thermal liner. The 1 degree figure corresponds to 0.032 m2K/W, a single fabric layer a little below the ISO test suit's own specification. The 3 degree figure corresponds to 0.095, roughly two test suits, and needs at least 2.5 mm of fabric that holds air as well as possible. That is a thick liner, not a thin sheet, which is consistent with the 300 to 500 g our adjustment table gives fleece or thermal liners, and the 3 degrees the sleep system calculator applies for one.

Sea to Summit's current figures are not impossible in principle, but they are ceilings of a different kind. Its 4 degree Reactor figure needs at least 3.3 mm of still-air equivalent, its 6 degree figure 5.0 mm and its 8 degree fleece figure 6.6 mm, all as comfort figures. If those "up to" numbers are read on the limit scale instead, they shrink only to 0.108, 0.162 and 0.216 m2K/W, and still need 2.8 to 5.6 mm. The page names neither the scale nor the bag assumed.

Why the big numbers keep circulating comes down to four things:

1. "Up to" with no scale named. The same added resistance is worth about 1.6 times as many degrees on the extreme scale as on the comfort scale (51 against 31.5 per m2K/W).
2. Underdressed sleepers. Against a sleeper in shorts, a liner replaces the test suit and the change feels large, though the bag never got warmer than its label.
3. Old figures outliving new ones. A retailer's product page can keep a maker's earlier claim long after the maker revises it.
4. Draughts and fit. In an oversized bag a liner fills dead air, which a static manikin in a closed bag never shows. That gain is real, but it is fit being fixed.

Where this model stops applying#

• It is a model. It assumes the liner adds its resistance in series over the whole body. A liner that bunches, twists or stays off the shoulders covers less of you and adds less.
• It does not credit stilled air. A liner divides the air gap between you and the bag lining. If that suppresses some air movement in the gap, the real gain could exceed the fabric's own resistance. We have found no measurement that isolates it, so we do not add a figure for it.
• It is dry heat only. ISO 23537 measures dry heat loss. A vapour barrier liner works on evaporation instead, which is why we give it about 4 degrees C, separately from fabric liners.
• A thick liner can cost loft. In a snug mummy bag, a 6 mm fleece takes up girth, and fill pressed flat by it loses resistance. The gain can then be smaller than the table says.
• Table 1 stops at 1.420 m2K/W. For bags rated below about minus 12 degrees C comfort the slope is extrapolated.

Check this yourself: the thickness test#

You need a ruler with millimetres, a sheet of stiff card and the liner.

1. Lay the liner flat and fold one panel into a stack of ten single layers. Avoid seams and hems.
2. Rest the card on top to flatten wrinkles without pressing. Do not use a book, which compresses fleece.
3. Measure the stack height in millimetres and divide by ten. That is the single-layer thickness.
4. Multiply by 1.2. The result is the most comfort, in degrees C, that the fabric can add. The real figure is lower, because fibres conduct better than still air.

A liner claiming 8 degrees C needs a single layer at least 6.6 mm thick.

Then test it on yourself: two nights at a similar low, same bag, pad and clothing, one with the liner and one without, changing one variable per night as in why you sleep cold.

Common mistakes with liners#

Using a liner to close a big gap. At 1 to 3 degrees, a liner trims a margin. It does not turn a plus 5 bag into a minus 5 bag. Buy the right comfort rating, as set out in how to choose a sleeping bag.

Sleeping in underwear and crediting the liner. The rating assumes long sleeves, trousers and socks. Put them on first, then judge what the liner adds.

Buying a liner for a cold back. Heat lost through the pad is a conduction problem. The sleeping pad R-value guide is where that is fixed.

Ignoring the other reason to own one. A liner keeps body oils out of the fill and is easier to wash than a bag, which means fewer of the full washes covered in washing down gear.

Frequently asked questions#

How many degrees does a silk sleeping bag liner add?#

About 1 degree C of comfort, which corresponds to 0.032 m2K/W of added resistance, since the ISO 23537 rating table converts resistance at about 31.5 degrees C per m2K/W. A silk liner whose fabric matched the ISO test suit, 0.040 to 0.060 m2K/W, would add 1.3 to 1.9 degrees. A maker figure of up to 10 degrees F, about 5.6 C, would need a layer at least 4.6 mm thick.

How much warmth does a fleece sleeping bag liner add?#

About 3 degrees C for a thick fleece or thermal liner. To add 3 degrees on the comfort scale a liner needs about 0.095 m2K/W, which takes at least 2.5 mm of fabric holding air as still as possible. Sea to Summit now estimates its fleece liner at up to 8 degrees C, which would need at least 6.6 mm. Measure the single-layer thickness before relying on the larger figure.

Is a liner better than wearing extra clothes in a sleeping bag?#

They do the same job. A long-sleeved base layer and a thin liner are both one layer of fabric next to the skin, and the ISO test dresses its manikin in exactly such a suit. Per millimetre of fabric the two are worth about the same, at most 1.2 degrees C. Clothing also works outside the bag; a liner is easier to wash and keeps oils out of the fill.

Why do some people say a liner made them much warmer?#

Usually because they slept underdressed. Bag ratings assume the sleeper wears a long top, trousers and socks, so anyone in shorts is colder than the label before a liner goes in. A liner restores what the rating already counted, and that feels like a large gain.

Can a sleeping bag liner add 15 or 25 degrees F?#

Only if it is several millimetres thick. Fifteen degrees F is about 8.3 degrees C and needs at least 6.9 mm of fabric on the comfort scale. Twenty-five degrees F is about 14 C and needs 0.44 m2K/W, close to the resistance of an entire ISO-tested summer sleep system. REI listed that 25 degree figure for a liner in 2025; the maker now estimates up to 6 degrees C for it.

Does a liner change a sleeping bag's ISO temperature rating?#

Not officially. ISO 23537 rates the bag as tested, with the manikin in a base layer suit and socks on a 0.85 m2K/W foam mattress. A liner changes the system, and the standard's own table lets you estimate the effect: about 1.3 degrees C of comfort for each 0.04 m2K/W the liner adds.