Sleeping Bag Terms Explained — Temperature Ratings, Fill Power & More

Sleeping Bag Terms Explained — Temperature Ratings, Fill Power & More

Sleeping bag specs look simple until you're standing in a shop trying to figure out whether 650 fill power is enough for a Routeburn Track hut in April. Here's a clear breakdown of every term that matters — and what it means for tramping in New Zealand.

Temperature Ratings — Comfort, Lower Limit, and Extreme

Modern sleeping bags are rated to the EN/ISO 23537 standard, which gives you three numbers instead of a single vague "rated to X°C" claim.

• Comfort rating: The temperature at which a standard adult woman can sleep comfortably in a relaxed position. This is the conservative figure — the one to target if you run cold or are unsure.
• Lower limit: The temperature at which a standard adult man can sleep curled up without waking from cold. This is often what manufacturers lead with in marketing.
• Extreme: The survival threshold — the temperature at which the bag provides enough insulation to prevent hypothermia for six hours. Not a comfortable sleep temperature. Not a target.

The practical rule for NZ tramping: use the comfort rating as your benchmark, especially if you're tramping in shoulder season, at altitude, or if you know you sleep cold. NZ alpine huts in winter can drop well below 0°C overnight.

Browse sleeping bags at Dwights — look for EN/ISO 23537 ratings on any bag you're seriously considering. If it doesn't list all three numbers, treat the spec with scepticism.

Fill Power — What the Number Actually Means

Fill power measures the loft quality of down — specifically, how many cubic inches one ounce of down occupies. Higher fill power means each gram of down traps more air, which means more warmth for less weight.

• 600–650 fill power: Budget to mid-range down. Heavier for equivalent warmth. Still works well.
• 700–750 fill power: Good quality. A solid balance of warmth, weight, and price for most trampers.
• 800–900+ fill power: Premium. Noticeably lighter and more compressible. Worthwhile for multi-day trips where pack weight matters.

Fill power doesn't tell you how warm a bag is — that depends on how much down is used (fill weight), not just the quality of the down. A 900fp bag with minimal fill can be colder than a 600fp bag stuffed with more material. You need both numbers to compare accurately.

Down vs Synthetic Fill — The NZ Tradeoff

This is one of the more contested decisions in sleeping bag selection, and NZ conditions make it particularly relevant.

Down: Superior warmth-to-weight ratio, compresses smaller, and lasts longer when cared for properly. The significant downside: down loses most of its insulating ability when wet. Wet down clumps together, collapses, and stops trapping air. In NZ's notoriously damp conditions, this is a genuine risk.

Hydrophobic down: Down treated to resist moisture absorption. It's not waterproof, but it retains warmth significantly better when damp compared to untreated down. Worth paying extra for NZ conditions.

Synthetic: Retains warmth when wet — the big advantage. Dries faster. Usually heavier and bulkier for equivalent warmth. Less packable. Better for wet environments, budget-constrained buyers, or anyone carrying their bag in conditions where waterproofing can't be guaranteed.

For serious NZ tramping — particularly on the West Coast, Fiordland, or in winter — hydrophobic down or synthetic is worth the consideration. A wet down bag is a cold night.

Loft — Why It's the Real Warmth Indicator

Loft refers to how much a sleeping bag puffs up when fully expanded. It's a direct measure of insulating ability — insulation works by trapping air, and a higher loft means more air is trapped between you and the cold.

When a down bag gets damp, loft collapses. When a bag is old and the down is compressed, loft decreases. When you store a sleeping bag compressed for extended periods, loft degrades over time.

Always store your sleeping bag loosely — in a large mesh or cotton storage sack, not stuffed in the compression sack. This preserves loft over the bag's lifetime.

Mummy vs Semi-Rectangular vs Rectangular

Shape affects warmth, comfort, and packability.

• Mummy: Tapers from shoulders to feet, matching body contours. Less dead air space means more efficient warmth. Lighter and more packable. The right choice for most tramping scenarios.
• Semi-rectangular: Tapered but with more room through the torso and hips. Warmer than a rectangular, more comfortable than a mummy for people who find the tight taper restrictive. Good compromise for Great Walk–style hut tramping where weight isn't critical.
• Rectangular: The classic flat shape. Roomy, can often zip together with another bag. Heaviest and bulkiest. Fine for car camping. Not ideal for carrying on your back.

Draft Collar and Draft Tube

These are two insulating features that deal with heat loss at specific points.

• Draft collar: An insulated baffle around the neck/shoulder area, separate from the main zip. It seals the gap between your body and the bag opening, stopping warm air escaping and cold air entering at the top. Important for cold-rated bags.
• Draft tube: An insulated tube that runs along the inside of the zipper. Without it, the zip is a cold strip running the length of the bag. Essential on any bag being used below 0°C.

Budget bags often omit one or both. If you're tramping in alpine conditions, check that both are present — their absence can make a significant difference to effective warmth.

Shell Fabric — Ripstop, Pertex, and Nylon

The outer shell of a sleeping bag affects durability, weight, and moisture resistance.

• Ripstop nylon: A woven grid pattern that stops small tears from spreading. Light, durable, commonly used. The standard for quality bags.
• Pertex: A branded high-performance nylon. Tight weave provides some inherent moisture resistance, good breathability, and very low weight. Found on premium bags — worth looking for.
• Standard nylon: Fine for general use. Heavier and less moisture-resistant than ripstop or Pertex, but perfectly adequate for hut-based tramping.

Shell fabric matters most when you're in damp conditions — a tighter, more moisture-resistant shell gives down more time before it starts absorbing ambient humidity.

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Frequently Asked Questions

What temperature rating do I need for tramping in New Zealand?

It depends on where and when you're going. For summer Great Walks, a bag with a comfort rating around +5°C to +8°C is usually adequate for hut-based trips. For alpine areas, shoulder season, or winter tramping, you'll want a comfort rating of -5°C or lower. Always use the comfort rating, not the lower limit, as your target.

What is fill power and does a higher number always mean warmer?

Fill power measures how efficiently down traps air — higher numbers mean lighter, more compressible insulation. But fill power alone doesn't determine warmth. A bag with 900fp down but minimal fill weight can be colder than a heavier bag with 600fp down. Check both fill power and fill weight (grams of down) to compare bags properly.

Is down or synthetic better for NZ tramping?

Down offers better warmth-to-weight and packability, but loses insulating ability when wet. Synthetic retains warmth when damp and dries faster. For wet NZ conditions, hydrophobic (water-resistant) down is a good middle ground — it performs closer to synthetic when damp but retains down's weight and packability advantages.

What's the difference between comfort and lower limit temperature ratings?

Comfort is the temperature at which an average adult woman can sleep comfortably in a relaxed position. Lower limit is the temperature at which an average adult man can sleep curled up without waking from cold. For most people, targeting the comfort rating gives a realistic and safe warmth expectation — especially for NZ conditions.

Do I need a draft collar on my sleeping bag?

For cold-weather tramping, yes. A draft collar seals the gap at your neck and shoulders, preventing warm air from escaping out the top of the bag. Combined with a draft tube along the zip, it can make several degrees of effective difference. Less critical for summer hut trips where temperatures stay mild overnight.

How should I store my sleeping bag to maintain its loft?

Always store loosely in a large mesh or cotton sack — not compressed in the stuff sack. Compression over time permanently reduces loft by damaging the down clusters. Only compress the bag for transport. When you get home, let it air out fully before storing.

What does loft mean on a sleeping bag?

Loft is how much the sleeping bag puffs up when fully expanded. It's the real indicator of insulating ability — insulation works by trapping air, so a higher loft means more warmth. Wet down collapses loft dramatically, which is why moisture protection matters so much for down bags.