Synthetic sleeping bags offer better value for money, easier maintenance and improved insulation when wet. For beginner and price-conscious campers, they fit the bill. The Thermarest Saros 0F is one such option.
It’s an excellent money-saving choice, offering a superb -10C comfort rating and -18C comfort limit rating for only £225, weighing in at a just-about-manageable 2,408g – a decent balance for campers more concerned with price than the weight of their backpacks.
The negatives are quite obvious. It’s very bulky when packed away and the hefty weight will be a deal-breaker for some. But others will only see the positives. The gentle mummy-shaped bag is filled with Thermarest’s EraLoft 100% polyester hollow fibre insulation, which feels thick and warm – there’s a whopping 1,820g of it – and fluffs up pretty impressively despite being synthetic.
The fit is roomier and more relaxed than many others, with Thermarest’s WARM (With Additional Room for Multiple positions) cut delivering enhanced comfort for side sleepers and anyone who shifts around at night.
Pros
- Affordable price
- Top-notch features
- Synthetic insulation
- Excellent bag-to-mat integration
- Easy to maintain
- Improved insulation when wet
Cons
- Heavy
- Bulky
- Fit might be too roomy for some
| Price | £225 / $284.61 |
| Temp rating | -10C comfort, -18C comfort limit |
| Weight | 2,408g/ 5.3lb (regular) |
| Pack size | 52x30cm |
| Sizes | Small, regular, long |
| Fill | 1,820g/4lb of EraLoft synthetic insulation |
Other features include ‘SynergyLink’ connectors which strap your bag to your mat for an integrated sleeping system, a super-cosy ‘Toe-asis’ footbox, and heat-trapping draught collars and baffles. A compression and stuff sack are provided too.
Temperature Ratings
The Thermarest Saros OF has the following lab test temperature ratings: comfort -10C and comfort limit -18C (an extreme rating is not stated). These are superb stats for just £225 – it’s excellent value for money.
As with almost all sleeping bags, we’d suggest using these temperature ratings as a guide only – you will probably feel quite cold at the ‘comfort limit’ temperature and you might not be totally comfortable at the ‘comfort’ temperature.
Our approach would certainly be more cautious. We’d never camp in this sleeping bag anywhere near -18C, but we’d feel confident in it down to -5C perhaps. You might be better in the cold and, thus, could push things closer to double-digit negative temperatures, but the main lesson overall is to take the official stats with a pinch of salt.
The downside with the Saros 0F’s excellent warmth is that this comes with a hefty weight. The bag weighs 2,408g, which is very heavy – that’s 1,153g heavier than the Sea to Summit Spark -18C, for example.
This is usually the case with synthetic sleeping bags. They offer superb warmth for the price, but they’re heavy – not exactly ideal for long-distance treks or tough mountain missions. For many though the extra weight won’t matter when the price is that good.
Thermarest are one of, if not the, best manufacturer of sleeping bags out there. The US brand has five main sleeping bag ranges, most of which are available in three temperature ratings of 32F, 20F and 0F (Fahrenheit).
The Vesper range is an ultralight backpacking quilt, there are three premium down sleeping bag families (Hyperion, Parsec and Questar), and then the affordable Saros range utilises Thermarest’s EraLoft synthetic insulation. If price is a big priority for you, but you want the reassurance of a market-leading brand like Thermarest, the Saros range is a great choice.
Insulation Type, Fill Power & Fill Weight
There is no goose or duck down in this sleeping bag. Instead the Saros 0F is stuffed with a 100% polyester fill, insulation Thermarest calls EraLoft. The structure of the polyester’s hollow fibres is designed to trap an insulating layer of warmth within the fibres, thus providing decent levels of insulation and loft.
As Thermarest explains: “Advanced EraLoft polyester fibres are spun with a hollow channel at their core. This design traps more heat than solid fibres and sheds weight. They’re also highly compressible, repel water and dry fast while superior temperature regulation delivers comfort across a wide range of temperatures.”
It would be wrong to suggest, however, that this synthetic insulation fluffs up as opulently as down. The insulation is not as thick or voluminous. This is because the synthetic fill is more like a sheet of flat, low-profile, wadded insulation, rather than a plumper, loftier, cluster-like fill in baffles. The Saros feels more like a high-tog winter duvet you’d have on your bed at home, than a fluffy down cocoon.
With the Saros 0F, you get a whopping fill weight of 1,820g of the EraLoft insulation. That is humongous – most down winter bags have about 900g, for example, so there’s double the weight of insulation in this bag. This is understandable, however, because synthetic insulation is naturally less warm than down, so you need more of it to achieve the same temperature ratings.
Unlike down, synthetic insulation does not have a fill power rating. Instead synthetic sleeping bags are commonly graded in GSM (grams per square metre). The higher the figure, the thicker, heavier and warmer the insulation will be. Unfortunately, however, Thermarest do not state the GSM figures for the Saros 0F – but, anecdotally at least, the insulation feels very thick and substantial.
Pack Size & Weight
The Thermarest Saros 0F does not perform particularly positively across these two metrics. It is heavier and bulkier than many other sleeping bags, particularly down-insulated products, and therefore will take up more space in and weigh down your backpack.
It weighs 2,408g in total, including the stuff sack (126g) and two SynergyLink connectors. For some campers the high weight and size will be a deal breaker; for others it won’t matter, particularly those who prioritise price over gram savings.
The Thermarest Saros 0F compresses down to a packed size of 52cm x 30cm, when packed inside the included stuff sack. This is a larger packed size than most winter sleeping bags and it does feel quite bulky and large. It may well fill up half of your backpack. But, if you’re using a large 65L backpacking pack or similar, it shouldn’t be a major issue.
For storage at home, a fabric storage sack (55cm x 35cm) is provided. Using this sack ensures you won’t excessively crush and squash the insulation.
Materials & Sustainability
The shell of this sleeping bag liner is made from a 20-denier polyester fabric with a DWR (durable water repellent) coating, which ensures the exterior has an added bit of protection and durability in damp conditions. The liner is a 20-denier polyester taffeta, while the insulation is 100% polyester. None of the materials are recycled. Consequently the sustainability credentials of this sleeping bag are not particularly strong.
Size & Shape
The Thermarest Saros 0F has a gentle mummy shape, with a slight taper but nothing too aggressive. Consequently it’s not a particularly restrictive or tight mummy cut, and this sleeping bag has a more relaxed feel, rather than a technical alpine one.
Consequently you get some extra roominess in the Saros OF. Thermarest calls this cut its WARM (With Additional Room for Multiple positions) fit – an approach designed to allow for various natural sleep positions while maintaining thermal efficiency. This is the most relaxed of its three mummy shapes, with the ultralight and performance fits providing a tighter, more alpine cut.
The WARM approach has pros and cons. The risk is that you might get a few cold spots or ‘dead space’ inside the bag, and some campers may find themselves wishing for a snugger fit. However many others campers will probably find the fit comfier, particularly side sleepers or anyone who has a tendency to shift around at night.
The regular size of the Saros 0F fits anyone up to 183cm tall, according to Thermarest. Its other official dimensions are as follows: 117cm girth in the footbox, 155cm girth at the hips and 160cm girth in the shoulders.
The Saros 0F is also available in a small size (fits anyone up to 168cm tall) and a large size (fits anyone up to 198cm tall).
Features
This excellent sleeping bag is loaded with all of the comfort features Themarest bags are known for. The best of these features is Thermarest’s clever SynergyLink system. These removable, ultralight and super-stretchy straps connect to your sleeping bag via little fabric hooks before looping over your sleeping mat.
The end result is that your sleeping bag is strapped in place on your mat, ensuring you don’t slide off it overnight. This provides a more integrated sleeping system and improves overall thermal efficiency. Each sleeping bag comes with two SynergyLink straps.
Other features include a heat-trapping draught collar at the neck, a full-length zipper draught tube, snag-free main zipper, cinchable hood and external zippered pocket, as well as a super-cosy, ergonomic footbox (Therm-a-rest’s wonderfully named ‘Toe-asis’, which is designed to “rapidly warm cold feet in a pocket of warm insulation”).
In terms of baffles, you get a horizontal box baffled construction with the Saros 0F, with mesh walls used to maximise loft and minimise cold spots. Zoned insulation – with more insulation placed where you need it most – further helps to improve the warmth-to-weight ratio. 60% of the insulation is placed on the top and sides (where you need it most) and 40% on the underside.
Verdict
The Thermarest Saros OF sleeping bag is a well-priced synthetic offering with excellent temperature ratings – but it’s very bulky and heavy.
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About the author
James Forrest writes regular features and route guides for Trail 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.