Underquilt Temperature Ratings Explained

By topquilt.com · Updated Oct 21, 2026 · 7 min read

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Underquilt temperature ratings look like precise numbers, but they're estimates. Two underquilts both labeled 20°F can feel very different on the same windy ridge. This guide explains how makers arrive at underquilt temperature ratings, gives loft-to-temperature rules of thumb you can use to compare models, shows how wind, top covers and gaps shift the real-world number, and helps you match an underquilt to your top quilt. Our hammock temperature guide has a full insulation table by temperature range.

How makers rate underquilts

Sleeping bags sold by many large brands are tested to the ISO 23537 standard (formerly EN 13537) with a heated mannequin on a pad. There is no equivalent test for hammock underquilts. The mannequin test assumes a ground setup and a bag, so it doesn't translate to an insulator hung under a hammock.

Instead, underquilt makers set ratings in one of a few ways:

• Loft targets: the maker decides how many inches of loft a rating needs, then fills baffles to hit it. This is the most common method among cottage makers.
• Fill weight tables: for down, a given weight of a given fill power across a given area; for synthetic, a stated insulation weight in oz/yd².
• Field experience and owner feedback: refining ratings over years of customers reporting how warm a product is.
• Marketing: some budget products state optimistic ratings without loft or fill details.

Some makers describe their ratings as "comfort" (a typical sleeper is comfortable), others as closer to a "limit" (you'll survive but not enjoy it). Unless the maker says which, assume the number is for an average sleeper wearing light base layers, in still air, with a well-adjusted hang. This is similar to the situation with top quilts, covered in our top quilt temperature ratings guide.

Loft-to-temperature rules of thumb

The table below collects commonly cited community rules of thumb. They are rules of thumb, not tested figures, and makers' own targets vary by half an inch or more. Use them to sanity-check ratings, not to predict your exact comfort.

A few things to know when using loft numbers:

• Loft varies across the quilt. Baffled down underquilts loft highest at the baffle center and less at the seams. Makers usually quote the target or average.
• Down shifts. If baffles run across the width, down can migrate to the sides as the quilt hangs. A little overfill compensates. See fill weight and overfill.
• Synthetic loft isn't equivalent. Synthetic insulation is a bit less warm per inch than down in many comparisons, so synthetic makers rate by insulation weight instead. As a loose guide, single-layer Climashield Apex around 5 oz/yd² is commonly rated in the 35-40°F range for underquilts, and about 7.5-10 oz/yd² total for 15-25°F. Details in Climashield Apex insulation.

Worked example: checking a 20°F rating

Say you're comparing two full-length underquilts labeled 20°F:

• Underquilt A: about 12 oz of 850 FP down, stated 2.25 in loft, roughly 45 x 78 in.
• Underquilt B: "20°F," no fill weight or loft listed, weight 30 oz including a "polyester insulation."

A is consistent with the rule-of-thumb table: 2.25 inches lands in the 20°F zone. You can also roughly verify the fill: 850 FP down lofts about 850 cubic inches per ounce in a lab cylinder, so 12 oz is roughly 10,000 cubic inches. Spread over about 3,500 square inches of quilt, that's close to 3 inches of theoretical loft; real baffles, shell resistance and overfill eat into that, so a stated 2.25 inches is plausible.

For B, you have no way to check. In practice, many budget synthetic underquilts stated at 20°F perform more like 35-45°F. Plan accordingly or buy from makers who publish specs. Our best budget underquilts page flags this where relevant.

What shifts the real-world number

Wind

Wind is the biggest factor that's specific to underquilts. An underquilt hangs in open air, and moving air strips heat from the outside of the loft and pushes cold air into any gap. On a breezy night, a 20°F underquilt can feel like a 35°F one. Pitching your tarp low, choosing a sheltered site and using an underquilt protector all cut this loss.

Gaps

An underquilt that doesn't touch the hammock everywhere leaves cold pockets. The usual culprits are a suspension that's too tight, an underquilt hung too low, or a hammock pitched with too little sag. Gaps cost more than any difference between two ratings. See how to hang an underquilt and cold butt syndrome.

Top covers

A top cover is a light fabric shell over the top of the hammock. It traps a layer of still, slightly warmer air around you and blocks drafts. Owners widely report about 10-15°F of extra comfort in cold or windy conditions; the gain is smaller in still air. Combined with a protector, it effectively encloses your whole hammock in a windproof shell. Covered in top covers and hammock socks. Some are sold on Amazon, for example the hammock top covers (check price on Amazon) from mainstream hammock brands.

Humidity and condensation

Damp loft is thinner loft. On multi-night trips in humid air, a down underquilt can gradually lose a few degrees. Air it out in the sun when you can. See down vs synthetic underquilts.

You

Metabolism, fatigue, what you ate, sleep clothing and whether you went to bed already chilled all move the needle. Cold sleepers should subtract about 10°F from any rating; warm sleepers can sometimes add 5-10°F.

Real-world adjustments, summarized

Matching your underquilt to your top quilt

Your hammock sleep system is only as warm as its weaker half. If your underquilt is rated 40°F and your top quilt 20°F, you'll be cold underneath at 30°F no matter how toasty your chest feels. The common guidance:

• Match the ratings, or go 5-10°F warmer underneath. Underquilts are more exposed to wind and gaps, and a cold back is harder to fix than a cold front (you can add a puffy on top, but not easily underneath).
• Cold sleepers: lean warmer underneath, e.g. a 10°F underquilt with a 20°F top quilt.
• Warm sleepers in still forests: matched ratings are usually fine.

For more on how the two halves work together, read top quilt vs underquilt and the hammock top quilt guide.

How ratings line up between makers

Because there's no common test, the same number means slightly different things from one brand to another. Broadly, owners report a few patterns, which are useful when comparing:

• Established cottage makers generally publish fill weight and loft, and long-time users tend to describe their ratings as realistic for an average sleeper in still air.
• Mass-market and budget underquilts often state a rating without fill details. Owner feedback frequently describes these as warmer on the label than in the hammock.
• Synthetic underquilts from makers who state the Apex weight are easy to compare with each other; the insulation weight is the real spec.
• Overfill options from many makers add an ounce or two of down for a few extra degrees of margin and to counter down shift. For cold sleepers, overfill is often a better buy than stepping up a full rating class.

Whatever the brand, treat a rating as accurate only for the conditions it assumes: a sheltered site, a well-tuned hang, light clothing and a typical metabolism.

Extending an underquilt's range

If you own an underquilt and want it to go colder:

1. Add an underquilt protector (+5-10°F in wind).
2. Add a thin foam pad inside the hammock (+5-10°F).
3. Add a top cover (+10°F or so in wind).
4. Stack a second, lighter underquilt; some makers sell overstuff options or "add-on" underquilts designed for layering.
5. Wear insulated pants and a puffy to bed.

Beyond that, it's time for a dedicated winter underquilt. See best winter underquilts and our Warbonnet Yeti review or Hammock Gear Incubator review for how makers describe their ratings.

Bottom line

Treat underquilt ratings as a starting point. Compare loft and fill weight, buy from makers who publish them, pick a rating at or a little below your coldest expected night, and spend a few ounces on wind protection. A well-hung, sheltered 20°F underquilt will often beat a poorly hung 10°F one on a breezy ridge. Shortlists are on our best underquilts page.