Does a sleeping bag liner add warmth?

Some liner makers publish a claim in degrees, and that claim is the manufacturer’s own — no standard governs it, unlike the comfort figure on the bag underneath. A liner does add a layer, and how much it contributes depends on the bag, the mat and the sleeper.

What a liner is for, first

The primary job is not warmth. A liner keeps body oils out of a down bag that cannot be washed often, which is a real service: down loses loft as it takes up oils, and washing a down bag is a job people put off for good reasons.

The added layer is the secondary effect, and it is the one that gets advertised.

Reading the added-degrees claim

Where a maker states that a liner adds a number of degrees, that is a manufacturer’s claim quoted as such. It is not produced by the test behind the bag’s own figures, and there is no standard for it in the way that EN 13537 and ISO 23537 govern sleeping bag ratings when a maker names them.

The practical consequence: do not add the liner’s stated degrees to the bag’s comfort figure. Those are numbers from two different worlds, and the sum has never been measured by anyone.

Fabric decides how much it can contribute

Silk is the lightest and the smallest packed, and it is delicate. Cotton is heavy, cheap and comfortable, and is what hostels ask for. Polyester microfibre and CoolMax dry fast and cost little. Fleece is much warmer and much bulkier, and it is the one that changes a night noticeably.

Weight and packed size scale with that list in the same direction, which is the honest trade rather than a catch.

Fit is what decides whether it works at all

A liner only works if the shape matches the bag. A rectangular liner inside a mummy bag concertinas around the legs; a mummy liner in a rectangular bag pulls the corners in. Take the length and shoulder width from the bag specification, and check whether the liner has a shaped foot box.

A bunched liner is not insulating anything — it is a wad of fabric at your ankles. This is why fit belongs in a warmth answer rather than in a separate one.

The thing people are usually missing

When a bag feels colder than its figures suggest, the mat is the more common cause. A mat’s R-value, measured to ASTM F3340 where the maker states it, describes resistance to heat flow into the ground — and unlike liner claims, R-values do add up when you stack two mats.

Before buying a liner for warmth, it is worth reading the R-value of the mat you already own.

What to compare

Shape and dimensions against your bag, fabric, stated weight and packed size, closure type — a zipped liner is far easier to get out of — washing instructions, and any added warmth the maker claims, read as a claim. Then follow the manufacturer’s instructions for both the liner and the bag.

The manufacturer's own instructions come first, and this page does not stand in for them. Every weight, packed length, hydrostatic head, denier, micron rating, lumen figure and temperature on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A weight is reported as the maker counts it, and is never restated as another kind of weight, because the two are not the same figure. And the statement repeated rather than assumed: a comfort, limit or extreme temperature is a laboratory figure measured to EN 13537 or ISO 23537 on an instrumented manikin, not a promise about you. How cold a night you can spend is answered by the bag's own instructions, by what you sleep on and wear, and by conditions nothing here can know. Nothing on this site is advice about hypothermia.