Rab Mythic Ultra 120 review: Ridiculously lightweight, but where’s the warmth?

I put the Rab Mythic Ultra 120 Modular Down Sleeping Bag to the test on a four-day, three-night backpacking adventure along the stunning Northumberland Coast Path - a 100km route stretching from Cresswell in the south to Berwick-upon-Tweed in the north.

Hiking alongside windswept sandy beaches, scrambling over rocky headlands, passing beneath the imposing silhouettes of medieval castles, and winding through postcard-perfect coastal villages, I bivvied each night in exposed spots with overnight temperatures dipping as low as 5C.

My aim? To see whether Rab’s bold claims for this ultralight sleeping bag hold up in the real world.

On paper, the Mythic Ultra 120 looks like a marvel of minimalist design: an exceptionally lightweight and compact down bag that Rab says punches far above its weight.

Combined with Rab’s Ultrasphere 4.5 mat, the full modular sleep system weighs in at under 700g, yet supposedly delivers genuine comfort down to 0C - a headline claim that will turn the head of any gram-counting adventurer looking to shave pack weight without sacrificing warmth.

So, how did it fare? Was it featherlight? Absolutely. Did it pack down to an impressively small size? Without a doubt. Were the clever modular elements and innovative design features as good as they sound? They were spot on.

But did it keep me warm down to Rab’s quoted 0°C sleep limit? For me - nowhere near.

Despite its many strengths, the bag simply didn’t offer enough insulation when the mercury dipped, even at a relatively mild 5C. I found myself layering up and still feeling the chill, night after night. So while the Mythic Ultra 120 earns full marks for weight and design ingenuity, its real-world warmth falls well short of Rab’s claim.

In short? If you’re thinking of taking this bag to 0C, bring a serious dose of scepticism - and maybe a few extra layers.

Temperature Ratings

Sleeping bags are commonly graded by the EN/ISO standard, stating temperature ratings as ‘comfort’ (warm enough for the average female), ‘comfort limit’ (warm enough for the average male), and ‘extreme’ (you won’t die, but it’ll be very uncomfortable).

While this system does has some flaws and certainly isn’t perfect, it’s great for enabling consumers to accurately compare different sleeping bags from different brands in a like-for-like way.

Unfortunately, however, Rab does not state EN/ISO stats for the Mythic Ultra 120. Why? As Rab explains: “Due to its construction, this bag does not meet the criteria for an EN test” – I think because of the total lack of insulation underneath the body (more on that later).

Instead we’re left with Rab’s in-house ‘Rab Sleep Limit’ of 0C. This is not based on lab tests in the same way as the EN/ISO, but instead is based on “many years of experience, supplementary testing, and athlete and test team feedback”, as Rab explains.

The brand adds that “when combined with the Ultrasphere 4.5, [the Mythic Ultra 120] forms a sleep system that weighs under 700g yet allows you to sleep comfortably down to 0C”.

On my first night using the Rab Mythic Ultra 120 along the Northumberland Coast Path, I actually slept fine - until about 4am. That’s when I woke up feeling cold and checked my Garmin watch, which showed the outside temperature had dropped to 5C.

I’d already got a few solid hours of sleep in, so it wasn’t a total disaster by any means - but I definitely didn’t feel comfortable. I was shivery, a little chilly, and certainly not warm and cosy. And that was at 5C, not the quoted 0C.

To be fair, I do feel the cold quite badly - I’m pretty skinny, so I’m naturally more vulnerable to the cold than some. Other campers might be more resilient, but based on my experience, I’d take the 0°C sleep limit with a healthy pinch of salt.

On the following two nights, in similarly cool temperatures, I ended up borrowing my girlfriend’s thick Sea to Summit liner for extra warmth, as I simply wasn’t coping that well with just the Rab bag alone.

Baffle Design, Down Mapping and TILT

The Rab Mythic Ultra 120 also features TILT (Thermo Ionic Lining Technology), an internal heat-reflective layer designed to boost thermal efficiency by reflecting your body’s heat back towards you, without adding bulk or weight.

In theory, it makes a lot of sense and seems to work well in practice - but its effectiveness is almost impossible to measure objectively in the field. Rab is clearly convinced of its value, and given the brand’s strong track record in technical gear, we’re happy to take their word for it.

The baffle design of the Rab Mythic Ultra 120 is clever and highly specialised, with a real focus on strategic down placement to maximise warmth while minimising weight. The upper part of the bag and the underside of the legs are filled with ultra-high-lofting 900FP down.

But, and this is the crucial (and probably opinion-polarising) part, to save weight there is absolutely no down whatsoever on the underside of the sleeping bag – it is completely un-insulated, just a thin layer of shell fabric with no down fill.

Rab’s logic with this design approach is that any down beneath your body just gets compressed by the weight of your body, thus losing loft and therefore thermal value, making it a futile place to put insulation. Personally I quite like this approach, and paired with Rab’s Ultrasphere 4.5mat (as recommended), I found it worked well - I didn’t rue the lack of down underneath me, mainly because the mat felt adequately insulated from the ground.

The Rab Mythic Ultra 120 bag uses a lightweight stitch-through construction with down placed precisely where it’s needed most. Wide, generously sized baffles allow the down to loft up thickly and plushly, delivering excellent “fluffiness” and warmth. But this comes with a small trade-off - larger baffles can allow the down to migrate and become uneven.

I’ve noticed this slightly in my own bag, possibly from being a bit too firm when stuffing it away, so handling it with care is important. The insulated side baffles are also well thought out, lining up with the sides of your sleeping mat to prevent cold spots - another nice touch in a design clearly focused on efficient warmth.

The other main feature to note is that this sleeping bag is hoodless. This helps saves weight, but may be a decision that polarises opinion. For some it may be a deal-breaker; for others, particularly on milder nights, it won’t be a problem – particularly if you can simply pull on a beanie if your head does get a little cold.

Insulation Type, Fill Power and Fill Weight

The Rab Mythic Ultra 120 is insulated with 120g of premium 900 fill power goose down, treated with a PFC-free Nikwax hydrophobic finish. This is top-tier insulation - 900 fill power is among the highest quality down available on the mainstream market, only surpassed by ultra-specialist options like PHD’s 1,000 fill power bags.

The down used here lofts up beautifully, creating a plush, chunky feel that’s cosy and luxurious for its weight. However, the downside is the limited quantity - 120g is a very low fill weight. For context, the Rab Neutrino 400 (a warmer but heavier bag) contains more than three times that amount, with 400g of down, weighing 775g in total and delivering an EN comfort rating of -1C. The Neutrino 400 costs £420.

Rab’s use of Nikwax’s hydrophobic down does bring added benefit. This treatment helps the down resist moisture, absorb less water, dry out faster and retain its loft better in damp conditions - ideal for condensation-prone bivvying or damp tents.

That said, the real-world difference this makes can be hard to measure objectively, and if you’re regularly camping in wet conditions, a synthetic-filled sleeping bag may still be the more reliable option.

If you love the overall ultralight and minimalist idea of the Rab Mythic Ultra 120, but want an extra touch of warmth, there are two other options available to you. The Mythic Ultra 180---aw24){href='https://www.sportsshoes.com/product/rab1428/rab-mythic-ultra-180-sleeping-bag-(long)---aw24' class='add-referrer-link'} costs £590, weighs 440g in total and has 180g of the same 900 fill power down, so 60g more than the Mythic Ultra 120 – but the EN/ISO comfort rating is a very disappointing 7C.

The Mythic Ultra 360---ss25){href='https://www.sportsshoes.com/product/rab1353/rab-mythic-ultra-360-sleeping-bag-(regular)---ss25' class='add-referrer-link'}, meanwhile, costs £720, weighs 645g and has 360g of the 900 fill power down, delivering an EN/ISO comfort rating of -2C.

Pack Size and Weight

Across the metrics of weight and packability, the Rab Mythic Ultra 120 delivers top-tier performance. Weighing in at just 330g, it’s genuinely featherlight - saving precious grams off your back and earning serious points with ultralight backpackers and minimalist campers alike.

It also packs down incredibly small into the included stuff sack, with a compressed size of just 32 x 16cm, meaning it barely takes up any space in your backpack. These are without doubt the biggest selling points of the bag - its ultralight credentials are exceptional.

For storage at home, a much bigger zippered storage sack is provided – it has a premium design, which I really like. Using this cuboid-shaped sack ensures you won’t ruin the down’s loft by storing the sleeping bag compressed for months on end.

Materials and Sustainability

The Rab Mythic Ultra 120 has solid eco credentials, making a commendable effort toward environmental sustainability. It’s entirely PFC-free and has an overall recycled content of 57%.

Breaking that down: the outer shell is 100% recycled, the liner is 96% recycled, the insulation is 0% recycled, the zip is 29% recycled and the trims are 2% recycled. These figures show Rab is making meaningful progress in reducing the environmental footprint of its products.

The bag’s outer shell is made from 10-denier recycled Atmos fabric - a standout in terms of keeping weight to a minimum. It’s ultralight and a key contributor to the bag’s exceptional packability and featherweight status.

However, that low weight comes with a trade-off: the material feels very thin and delicate, meaning it needs to be handled with care. You’ll want to keep it well away from abrasive surfaces like stony ground, twigs or brambles - this isn’t a bag to throw down casually on rough terrain.

Size and Shape

The Rab Mythic Ultra 120 features a very sleek, streamlined profile with a heavily tapered mummy shape. It feels technical, almost alpine in design, and clearly built with weight savings and thermal efficiency in mind. The snug, close-fitting cut helps trap warmth and contributes to the bag’s ultralight performance.

Personally, I like the style - it feels purpose-built and performance-focused -but the close fit may feel too restrictive for some, especially if you like room to stretch, shift or move around at night. It’s a classic case of pros and cons.

The bag is only available in one size - regular - which could be limiting for some users. At 178cm tall with a slim, athletic build, the fit was fine for me. But those who are particularly short, tall or plus-sized might find the fit less accommodating.

The official dimensions are as follows: total length 215cm, max user height 185cm, internal shoulder girth 160cm, hip girth 134cm and foot girth 102cm. Pack size is 32 x 16cm. For most average-sized users, this should work well - but more size options would improve accessibility.

Features

This sleeping bag is all about being ultralight and minimalist, so it’s not packed with bells and whistles – and that’s entirely the point. Many extra features simply aren’t needed when you’re counting grams, and Rab has kept things streamlined while still including a few smart, functional design touches.

There are three main features worth noting. First, the harness-style straps underneath help secure the bag to your mat, in a similar way to Therm-a-Rest’s SynergyLink system. It’s a handy addition that works pretty well - there’s a bit of slippage, and it’s not the most rock-solid hold, but it does the job and adds some welcome stability.

Second, there’s an ultralight hood sleeve that lets you stuff clothing inside to form a pillow. I don’t even use a pillow at home (yes, weirdo!), but it’s an innovative feature that might please campers looking to cut extra weight. Lastly, the diagonal YKK 2-way zip allows for decent venting and flexibility.

That said, the short length of the zip doesn’t make getting in and out particularly easy – in fact, during my test camps I found myself squirming into the bag like some kind of deranged caterpillar. Still, once I was in, it was fine, and overall I liked the design - it’s a fair trade-off for keeping the weight down.

Conclusion: Price and Performance

There’s no denying the Rab Mythic Ultra 120 has some brilliant design touches. It’s impressively ultralight and compact, the 900 fill power down is of exceptionally high quality, and it lofts up beautifully. But Rab’s claimed sleep limit of 0C needs to be taken with a generous pinch of salt.

In my experience, it simply wasn’t warm enough at that temperature, and I’d peg its realistic comfort range more around 5°C to 10°C. That makes this a very niche product. At £390, it’s a serious investment – but one that will never be a suitable winter sleeping bag. And even in the height of summer, temperatures at night can easily dip below 5C in the UK.

In short, this is a specialist piece of gear – best suited to gram-counting obsessives, ultramarathon runners tackling overnight stages or hardened ultralighters who don’t feel the cold. For most general backpackers, the warmth-to-price ratio might not stack up.

Verdict

An incredibly compact lightweight sleeping bag with high quality down and some innovative design features – but it’s very expensive and not as warm as expected.

About the author

James Forrest writes regular features and route guides for Trail and LFTO, and has been one of our main gear testers for the last few years. James is based on the edge of the Lake District so when he isn’t off on his latest crazy adventure or challenge, he’s walking in his local fells.