Sleeping Bag Terms Explained

Sleeping-bag specifications are useful only when the terms are clear and comparable. This glossary explains common labels; use the separate selection guide to choose a bag and the ratings explainer for the full ISO framework.

Temperature terms

Comfort

The lower end of the comfort range for the standard female reference sleeper in the ISO test. It is a conservative comparison point for many people who sleep cold.

Lower Limit

A lower comparison point associated with the standard male reference sleeper. It is not a universal comfort guarantee.

Extreme

A survival threshold with expected severe cold stress. It should not be used as a target for choosing a bag.

ISO 23537

The international test and labelling framework used for certain adult leisure sleeping bags. Check the exact product's claimed test basis.

Insulation terms

Fill power

A measure of down loft per unit weight under a stated test method. Higher fill power can support efficient warmth-to-weight design, but it does not state the total amount of down or the bag's temperature rating.

Fill weight

The amount of insulation inside the bag. It cannot be compared without construction, fill type, size and tested performance.

Down

Natural insulation valued for loft and compressibility. Moisture management, sourcing, treatment and care vary by product.

Synthetic insulation

Manufactured fibres arranged to trap air. Constructions differ in warmth, bulk, durability and moisture behaviour.

Construction terms

Baffle

An internal chamber that controls insulation position. Baffle shape and height affect how fill is distributed.

Draft tube or collar

Insulated sections intended to reduce heat movement around a zip or neck opening.

Footbox

The shaped section around the feet. Fit and insulation affect comfort; the name alone does not indicate warmth.

Mummy, tapered and rectangular

Shape descriptions that trade internal room against unused air space, packed size and intended use.

Shell and lining

The outer and inner fabrics. Check durability, feel, breathability, water resistance and care rather than assuming a fabric name determines performance.

System terms

R-value

A measure of a sleeping mat's resistance to heat transfer. It belongs to the ground-insulation part of the system, not the bag rating.

Sleeping-bag liner

A removable inner layer used for hygiene, feel or product-specific warmth. Do not assume a universal temperature increase.

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