Rab Neutrino and Rab Mythic Sleeping Bags: A Practical Buyer's Guide

Introduction

Rab has spent close to 50 years specialising in down sleeping bags and jackets, and the current line-up shows how much of that experience goes into construction rather than decoration. This year the brand simplified its down bag range, merging several models and renaming them around temperature ratings instead of down weight. Two of the clearest examples are the Neutrino, the all-rounder, and the Mythic, the lightweight specialist. Both use high-grade down, both share a set of construction features, and both are built for people who sleep outside in real conditions. This guide walks through what separates them so you can work out which one fits your own trips.

How Fill Power and Temperature Ratings Work

Fill power describes the quality of the down rather than how warm a specific bag is. Higher fill power means lighter, fluffier clusters that loft more and compress smaller, so a manufacturer needs less down to reach the same warmth. Every Neutrino bag uses 800 fill power down, and every Mythic bag uses 900 fill power down, regardless of the model’s warmth rating. Down only really goes up to around 1000 fill power, so 900 sits close to the top of what is available.

The naming change is the other thing worth understanding. Rab used to label bags by the grams of down inside them, so a Neutrino 400 or a Mythic 400 told you the fill weight and nothing directly about warmth. The current names use the limit of comfort temperature rating instead, which makes it far easier to match a bag to a trip.

Rab publishes three figures for each bag: comfort, limit of comfort, and extreme. The comfort rating is the point where most people sleep well in normal clothing. The limit of comfort is where you may need extra layers and could feel a chill, and it is the figure used in the model names. The extreme rating describes a survival threshold rather than a comfortable night. Both the Neutrino and the Mythic come in 0, -6 and -12 versions, and the Neutrino also goes down to -18.

Rab Neutrino: The Versatile All-Rounder

The Neutrino has long been the classic choice: 800 fill power down, a sensible balance of packability and durability, and a price that stays below the top-end bags. This season Rab combined the previous Neutrino and Neutrino Pro into a single model. The Neutrino -6 covered here is the equivalent of the old Neutrino 400, using 400 g of 800 fill power down.

The down is treated with Grangers proofing, which makes it more water resistant and better able to cope with moisture and humid conditions. If the bag picks up damp inside a tent or after several days packed away, it lofts up and dries more readily than untreated down would.

The shell is Pertex Quantum in two different deniers. The front panel uses a 10 denier fabric where drips are most likely to land, while the areas you lie on and squash use a tougher 20 denier fabric. That 20 denier panel is a yellow ripstop built on a 70 denier Spectre yarn, the same fabric used on Rab’s burly Latok rucksacks.

Loft 3D is Rab’s baffle construction, and it appears on both bags. Offset baffles at the walls are twisted into a helix shape, which is said to create funnel points that stop down migrating and keep coverage even through the night. Longitudinal and latitudinal baffles are combined to make the most of the warmth-to-weight ratio, cutting down on stitching and the cold spots that come with it.

Practical details include a two-way anti-snag zip that you can close in the dark without catching fabric, a small internal zip pocket for a phone or alarm, and a neck baffle that traps warm air around the shoulders. Taken together, that is what makes the Neutrino range the all-rounder: durability, packability and a strong warmth-to-weight ratio in one bag.

Rab Mythic: The Lightweight Specialist

Where the Neutrino is the all-rounder, the Mythic is the specialist. Rab merged the previous Mythic and Mythic Ultra into one model and built it with the brand’s top-specification features. Every bag in the range, whether 0, -6 or -12, uses 900 fill power down.

The down carries the same Grangers Down Defend treatment for better moisture resistance, and Loft 3D construction is used here too. The difference is the shell: 10 denier Pertex Quantum throughout, reinforced with the 70 denier Spectre ripstop yarn across the whole bag, so the very light face fabric still has durability behind it.

The lining is where the Mythic separates itself. TILT stands for Thermionic Lining Technology, a 10 denier fabric that incorporates a thin layer of metal to help with radiant heat, trapping more of the warmth you generate inside the bag. It is worth noting that the published temperature ratings come from the EN test standard, which uses a heated manikin in a chamber, and the TILT lining is not part of that test. Rab rates the Mythic -12 on the standard while suggesting it may perform slightly warmer in use.

Weight is the other headline. The Mythic -12 weighs 920 g, which is very light for a bag rated that cold. Comparing like for like at -6, the Neutrino weighs 870 g while the Mythic weighs 700 g. In absolute terms that gap looks small, but as a proportion of the weight it is significant, and it adds up across a full kit. To get there, the Mythic uses a shorter, lighter zip rather than the chunkier anti-snag design, since zips lose warmth and add weight. It still runs smoothly.

Shared Construction Details Across Both Bags

Both bags use a sidewall baffle construction, which makes the bag stand a little prouder and allows extra loft for the down. It also makes the shape a bit easier to live with for side sleepers, since there is more room through the middle of the bag.

The outer face fabrics and the liners on both bags are PFAS free. Rab also still fills its sleeping bags in Derbyshire, at Alfreton, which is one of the most skilled parts of making a down bag, and a detail that matters when you are paying for high-grade down.

Fit, Sizing, and Storage

Rab bags are cut generously. A 6’4”, 90 kg user fits inside a regular size, where many other brands would call for a long. If you are comparing a Rab regular against another brand’s regular, you are effectively getting more sleeping bag for the same money and the same packed weight, which is worth factoring into any comparison.

Each bag comes with two storage options: a large zip storage bag for keeping the down lofted at home, and a waterproof dry bag that compresses the sleeping bag small for travel. Down should not be stored compressed for long periods, so the roomier bag is the one to use between trips.

Warmth, Weight, and Packability

The two ranges overlap on temperature ratings but not on weight. At -6, the Neutrino comes in at 870 g and the Mythic at 700 g, and the Mythic -12 still manages 920 g. If you are carrying the bag on your back for long days, that difference is the main reason to look at the Mythic.

The Neutrino answers with a shell designed around durability. The tougher 20 denier ripstop covers the areas that take the most wear, and the full-length two-way anti-snag zip is easier to use in the dark than a short zip. Because the Mythic uses 900 fill power down and the Neutrino uses 800, the Mythic needs less down to reach a given loft, which is where much of its weight advantage comes from. The Neutrino’s 800 fill power down is still a high grade, and the bag remains a strong balance of quality and cost.

Who Each Bag Suits

The Neutrino suits the broadest range of trips. Its combination of durability, packability and warmth makes it a sensible choice if you want one bag that covers three-season camping and occasional colder nights, and the range extends to -18 if you need more warmth.

The Mythic suits anyone counting grams on cold trips, where the 900 fill power down, the TILT lining and the lighter zip make the most sense. The trade-off is a shorter zip and a shell built purely around low weight rather than maximum abrasion resistance.

Buying Advice

Start with the temperature rating that matches the conditions you actually sleep in, and remember that the model name uses the limit of comfort figure, which is the colder end of comfortable. If most of your nights are mild to cool, the 0 or -6 bags will cover them. If you camp on snow or in shoulder-season alpine conditions, the -12 and -18 options are the ones to consider.

Then decide between the two constructions. The Neutrino is the more versatile and more affordable of the two, with a tougher shell and a full-length two-way zip. The Mythic is the specialist, lighter at every rating and built with the TILT lining, at the cost of a shorter zip and a lighter shell.

Sizing is worth checking before you buy. Rab’s regular bags are cut generously, so if you are tall, a regular may still fit where another brand would require a long, and you get more bag for the same packed weight.

Conclusion

Both bags come from the same design philosophy: high-grade down, careful baffle construction, and a shell that balances weight against durability. The Neutrino is the all-rounder for people who want one dependable bag, and the Mythic is the specialist for cold trips where every gram counts. Match the temperature rating to your conditions, decide how much weight you are willing to carry, and either bag will do its job well.