Rab Mythic 600 Review – Ultralight 4-Season Down Sleeping Bag
Introduction
The Rab Mythic 600 sits at the very top end of Rab’s sleeping bag range, built for serious cold-weather performance where weight, pack size, and efficiency matter more than anything else. Designed for alpine missions, winter backpacking, and fast-and-light adventures, it combines high-fill-power down, cutting-edge materials, and an aggressively minimalist design.
In this review, we’ll take a detailed look at how the Mythic 600 performs in real-world conditions, how it compares to other 600-fill-weight winter bags, and whether it’s worth the premium price tag for UK wild camping and mountain use.
Specifications and Features
Here’s a breakdown of the key specifications for the Rab Mythic 600
Insulation
900 fill power goose down (RDS certified, fluorocarbon-free hydrophobic treatment)
Down Weight
600g
Outer Fabric
10D Atmos™ ripstop nylon
Inner Fabric
10D recycled nylon
Temperature Rating (EN)
Comfort
approx –5°C
Limit
approx –12°C
Weight
approx 900g (regular)
Pack Size
approx 25 × 20 cm (compressed)
Construction
Offset boxwall baffles with heat-reflective TILT lining
Zip
¾ length YKK zip with insulated baffle
Fit
Slim mummy cut
Extras
Compression sack, lightweight storage sack
The Mythic 600 is designed to squeeze maximum warmth from minimal materials, prioritising efficiency over comfort extras.
Real-World Performance and Impressions
Real-World Performance and Impressions
When it comes to how the Rab Mythic 600 performs in practice, independent reviewers and user feedback paint a clear picture of a bag that delivers excellent warmth-to-weight performance for its class.
Lightweight Warmth in Real Use
Outdoor reviewers highlight the bag’s exceptionally high-quality down and lightweight construction:
“The Mythic 600 weighs 917.6 g on my calibrated scale… this makes the Mythic 600 suitable for people up to a length of 1.85 m… the Pertex has been treated with a water-repellent coating… the material itself feels pleasant on the skin.” — OutdoorGuru review Outdoorguru
This real-world measurement confirms that the bag clocks in close to Rab’s stated weight while still providing substantial insulation.
Effective Loft and Durable Materials
Beyond the numbers, detailed reviews also praise how quickly the bag lofts and retains heat:
“As soon as the bag is pulled from its stuff sack it pops up to a fluffy bundle of warmth… the fabric has actually stood up to everything I’ve thrown at it… I’ve really had no issues with wear and tear.” — PeakMountaineering review Peak Mountaineering
This echoes the sentiment that despite ultra-light fabrics, the construction feels robust and reliable with use.
Performance Across Conditions
Independent testers note that the Mythic 600’s hydrophobic down and thoughtful baffle design consistently deliver warmth:
“The Mythic 600’s chevron baffle design keeps the down in position, optimising core warmth and reducing the amount of fabric needed… highly compressible mummy packs a premium 600 g of 900-fill power goose down to keep you warm.” — UltralightOutdoorGear verdict Ultralight Outdoor Gear
This aligns with many users’ real experience of the bag being compact yet capable, especially on multi-day treks and alpine missions where every gram counts.
Community Feedback
Owners in wild camping forums also share real insights on warmth and performance in UK conditions:
“I’ve got mine now for about 6–8 months & it’s fantastic. Used in every weather & it hasn’t disappointed once.” — Reddit user on UK wild camping experience (about similar Rab bags) Reddit
“I recently used mine a little under 0 °C (with a good rated sleeping pad)… happy to use it down to zero no problem.” — Reddit user on down bag performance (another Rab model) Reddit
While not all posts explicitly name the Mythic 600, the sentiment in UK wild camping communities suggests Rab’s higher-end down bags consistently perform well around freezing point when paired with proper insulation on the ground.
Summary of Real-World Insights
Across independent reviews and user feedback:
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The down lofts quickly and the Pertex fabric feels strong yet light in real conditions. Outdoorguru
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The baffle construction and hydrophobic treatment work together to retain warmth and resist moisture. Ultralight Outdoor Gear
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Many users report being comfortable down towards freezing or below when supported with a quality sleeping pad. Reddit
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Durability and build quality hold up after extended use, even with ultralight materials. Peak Mountaineering
Strengths & Advantages
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Strengths & Advantages
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Outstanding Warmth-to-Weight Ratio — The 900FP down and slim design deliver very good warmth for the weight carried, ideal for cold weather trips where pack weight matters.
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Compact Packability — Packs down impressively small, making it a favorite among backpackers and bikepackers.
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High-End Materials & Construction — Premium fabrics and down quality contribute to both warmth and durability.
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Heat-Reflective Lining — Improves thermal efficiency without extra weight.
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Versatile Use Cases — Works well from late autumn through winter and even into shoulder seasons when temperatures dip.
Weaknesses & Limitations
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Weaknesses & Limitations
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Slim Fit May Feel Restrictive — The narrow mummy profile maximises thermal efficiency but can feel tight compared to roomier bags.
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Temperature Claims Can Be Optimistic — In practice, comfort depends on pad insulation, body temperature, and weather conditions, with some users finding the lower end of claims on the edge.
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Moisture Sensitivity (Typical of Down) — Hydrophobic down withstands light dampness better than untreated down, but significant moisture can still degrade performance.
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Premium Price Tag — The Mythic 600 sits at a higher price point than many winter bags, so it must be weighed against performance gains.
Performance Scenarios & Use Cases
Performance Scenarios & Use Cases
Here’s how the Mythic 600 typically performs across a range of conditions:
• Winter Camping in the UK / Scottish Highlands: Performs admirably down to just below freezing with a good pad and layering system.
• Alpine / High Altitude Trips: A strong performer thanks to low weight and high down quality.
• Deep Winter / Snow Adventures: Capable, but may benefit from a warmer liner or pad combo in extreme cold.
• Ultralight Backpacking & Bikepacking: Excellent choice due to its compact packing size and low weight.
Tips for best performance:
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Pair with a high R-value sleeping mat
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Use a liner for extra warmth
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Keep it dry; ventilate intelligently to manage condensation
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Store uncompressed to preserve loft
Overall Verdict
The Rab Mythic 600 is a top-tier ultralight, 4-season sleeping bag that successfully balances weight, warmth, and packability. For backpackers, wild campers, and alpine users who prioritise performance per gram, it’s a compelling choice.
It is by no means perfect — the snug profile and premium price put it in a specialist category — but for its intended audience, it delivers real value and performance that lives up to its reputation.
Ultimately, if you want a high-performing cold-season bag without carrying cumbersome bulk, the Mythic 600 should be on your shortlist.
If you're not too concerned about being ultra light weight and counting every gram then check out our review of the Alpkit PipeDream 600 another excellent down bag thats got similar specs to the Mythic but is much easier on the old bank balance.
For another offering from Rab, check out our Rab Neutrino 600 Sleeping Bag Review
FAQ
Q1: What temperature can the Rab Mythic 600 handle?
A: There’s no universal EN rating published directly by Rab, but based on user experience and down quality it’s comfortable to around –5°C, with capability toward –10°C+ with good insulation underneath.
Q2: Is it suitable for beginners?
A: The slim cut and technical focus make the Mythic 600 best for users who already have experience with cold weather camping; casual or warm-season campers might prefer something roomier.
Q3: How does it compare to other 600-fill bags?
A: The high fill power and heat-reflective lining give it an edge in warmth per weight compared to many standard 600-fill sleeping bags.
Q4: Is the heat-reflective lining effective?
A: Yes — Rab’s lining helps retain more body heat without adding weight, improving thermal efficiency in cold conditions.
Q5: Does it work in deep winter?
A: With the right setup (good mat, liner, base layers) it performs very well. For extreme cold (–15°C or colder), more specialised gear may be preferable.
Q6: How should I store it?
A: Store the bag uncompressed in a large loft sack to maintain loft and preserve down quality over time.
Q7: Is it worth the money?
A: For experienced, weight-focused campers who need dependable cold-weather performance, yes — the Mythic 600’s premium materials and performance justify its price. For comfort-oriented or casual users, more affordable bags may fit better.