Two underquilts can carry the same temperature number and still feel different in use. The number may describe a field estimate, a manufacturer’s target or a test of some materials. Without a stated method, it should not be treated as an exact threshold.
What the standards do – and do not – cover
ISO 23537-1:2022 specifies thermal testing and labelling requirements for adult sleeping bags. Its published scope says sleeping bags; it does not establish a standardized comfort-temperature procedure for an underquilt hanging beneath a hammock. The familiar sleeping-bag terms should therefore not be transferred automatically to an underquilt.
ISO 11092:2026 can measure the thermal and water-vapour resistance of textile materials and multilayer assemblies under steady-state conditions. ISO explicitly says those test conditions are not intended to represent specific comfort situations and that the standard gives no performance specification for physiological comfort. A material measurement can be useful, but it does not by itself produce an underquilt comfort temperature.
It is therefore too broad to say that an underquilt “cannot be tested”. Materials and assemblies can be measured in useful ways. What is missing is a generally adopted underquilt equivalent to the sleeping-bag rating method.
What Momo Jord’s temperatures are
Momo Jord publishes approximate guidance built from our own use and customer experience. It is not an ISO certificate, a survival value or a promise that every person will feel comfortable at that exact temperature.
- UQ300: around +5 °C
- UQ400: around 0 °C
- UQ500: around −2 °C
- UQ800: around −10 °C
These are the current values used by the underquilt calculator. They describe the underside only. You still need a sleeping bag or topquilt suitable for the conditions above you.
Why fill power and fill weight are not a thermometer
Down works by creating loft that holds relatively still air. Fill power describes the volume a specified mass of down reaches under a defined test; fill weight tells you how much down is used. Both matter, but multiplying them does not yield a reliable comfort temperature.
Warmth also depends on baffle depth and layout, how evenly the fill is distributed, the size and coverage of the quilt, shell construction, moisture, wind and – crucially – whether the quilt fits the hammock without gaps or compression. Precise universal conversions such as “one centimetre of loft equals a certain number of degrees” hide those variables. We do not use them as a customer-facing guarantee.
Fit can matter as much as the label
An underquilt should hang close enough to follow the occupied hammock. A visible air channel at an end or along a side allows moving air to replace warmed air. On the other hand, excessively tight suspension can flatten part of the insulation. Adjust the main suspension first, then the end closures, while someone lies in the hammock. Change one thing at a time.
A sleeping pad is another way to provide bottom insulation because it is designed to resist compression. It can be a practical choice, especially when the same pad is also used on the ground. The comfort and stability of a pad inside a gathered-end hammock depend on its width, shape and the hammock.
Variables the label cannot know
- Actual overnight low: use the expected minimum at the campsite, not a daytime value.
- Wind and exposure: moving air beneath the hammock can increase heat loss, especially if the underquilt has gaps.
- Personal response: people do not produce or retain heat identically, and fatigue, food and health can change a night.
- Top insulation and clothing: these affect the complete sleep system but do not change the physical specification of the underquilt.
- Setup: coverage, adjustment and moisture management affect the result.
How to choose responsibly
- Find the forecast minimum and assess how exposed the site will be.
- Use a published underquilt figure as guidance, not a cliff edge.
- Choose more margin if you sleep cold, the forecast is uncertain, the trip is remote or the setup is unfamiliar.
- Match the sleeping bag or topquilt to the same conditions.
- Test the complete system in controlled conditions before relying on it far from shelter.
The Momo Jord calculator helps structure that choice, while the complete underquilt guide covers setup and care. Neither can guarantee comfort or safety.
Primary references
- ISO 23537-1:2022 – scope and requirements for adult sleeping bags.
- ISO 11092:2026 – measurement of textile thermal and water-vapour resistance and its stated comfort limitation.
- Momo Jord Underquilt and calculator – current product data and field-based guidance.
Standards, product data and calculator checked on 12 August 2026.
