If you can afford it, this pricey winter sleeping bag delivers some truly top-tier performance. Thanks to an 840g fill of 850+ fill power goose down, the Sea to Summit Spark -18C lofts up into an opulently thick, indulgently chunky cocoon of cosy warmth.
You get an official EN/ISO comfort rating of -10C and a comfort limit of -18C, so you’re covered for camping in temperatures well below zero – and all of this clocks in at just 1,255g in a super-compact package. Or, in other words, the Spark -18C delivers a phenomenal warmth-to-weight ratio.
Pros
- Premium design
- Wonderfully minimalist
- 850+ fill power
- Super-lofty
- Incredible warmth-to-weight ratio
- Ultralight
- Excellent packability
Cons
- Very expensive
- Shell and liner materials feel thin and flimsy
| Price: | £625.00/$794.71 |
| Temp rating: | -10C comfort, -18C comfort limit |
| Weight: | 1,255g (regular) |
| Pack size: | 40x25cm |
| Sizes: | Regular, long, women’s |
| Fill: | 840g of 850+ fill power<strong> </strong>Ultra-Dry goose down |
The downsides? The mummy cut may feel a little tight for some, the ultralight 10-denier fabrics feel a tad thin and flimsy, and the £625 price tag seems inflated when compared directly to the Simond Makalu III, for example.
There’s also a slight worry that down migration and separation may be a problem over time. However, for the serious winter camper, this bag is unlikely to disappoint – it’s a bona fide winner. You get first-rate features including ethically-sourced hydrophobic down, a super-warm adjustable hood, a cosy footbox and a ¾-length, two-way zipper with a smooth flow. But ultimately this bag is all about one thing: its unrivalled warmth-to-weight ratio.
Temperature ratings
The Sea to Summit Spark -18C has the following official EN/ISO temperature ratings: a comfort rating of -10C and a comfort limit of -18C (labelled as the ‘lower limit’ in Sea to Summit’s marketing materials). To avoid misleading consumers Sea to Summit does not publish its EN/ISO extreme ratings.
The brand told us the ‘extreme’ rating “isn’t a situation you’d want to a base a buying decision on, so we don’t refer to that”, citing cases of people incorrectly thinking the “the extreme limit is the usable temperature of the bag and getting themselves in trouble”.
As we’ve been reiterating constantly, a more realistic, real-world usable temperature for any sleeping bag is usually closer to its comfort rating, or somewhere between the comfort and comfort limit ratings, depending on how badly you tend to feel the cold.
You certainly won’t necessarily feel cosy and warm at the lower limit. Sea to Summit appears to concur with our stance. The brand told us: “in real terms, if you’re sleeping somewhere between our ‘comfort’ and ‘lower limit’ stated temperatures, you should be fine”.
If you don’t need quite so much warmth as the Spark -18C offers, several other versions are available in the Spark range. These include the Spark 7C, -1C and -9C, all of which have different weights, EN/ISO temperature ratings and prices.
Insulation type, fill power & fill weight
The Spark -18C is filled with 850+ fill power goose down. This is a very high quality grade of down – only a few very, very expensive bags across the entire outdoors market will have a higher 900 or 1000 fill power grade – and ensures the Spark has a superb warmth-to-weight ratio.
In fact, it’s the best warmth-to-weight ratio we’ve seen in our recent round-up of winter sleeping bags, with an impressive -10C comfort rating for just 1,255g (including stuff sack).
The 850+ fill power down fluffs up beautifully, with a thick and plush loft. If you compare this bag side-by-side to a bag with a lower grade of down, such as the OEX Leviathan EV 900, which has 600 fill power down, you can really see the difference in depth, volume and cluster-like plumpness of the down. It’s very noticeable.
The fill weight of the Spark -18C is 840g, which isn’t the biggest amount we’ve ever seen (the Simond Makalu III has a whopping fill weight of 1,100g), but it’s still slightly more than average. Many other bags only have 800g, so 840g is a slight upgrade. Plus, when you combine an 840g fill weight with a premium fill power of 850+, the end result is superb warmth.
All of the down in the Sea to Summit Spark -18C is certified by the Responsible Down Standard, which means it meets important ethical and welfare criteria. It’s also a version of down called Ultra-Dry Down, featuring a hydrophobic treatment for improved water resistance in damp conditions.
While we haven’t been able to objectively analyse how much difference this really makes, it’s a reassuring thought that the Spark’s down will retain its loft and warmth better when wet than regular down.
Pack size & weight
The Sea to Summit Spark range is “engineered to keep you warm at the lightest weight and smallest compressed size”, according to the Australian brand, which is well-known for its lightweight, minimalist and cleverly-designed camping equipment.
Does it achieve this goal? The answer is a resounding yes. In terms of weight and packability, the Spark -18C is a 10 out of 10. For a winter sleeping, it’s all very impressive. On our scales the Spark -18C (regular) weighs just 1,255g, including its stuff sack (103g). This is about as light as it gets for a four-season, winter sleeping bag, and a -10C comfort rating for 1,255g is superb – a revelation in terms of warmth-to-weight ratios.
Once packed away and compressed, the stuff sack is sized about 40cm x 25cm by our measurements, which is nicely compact and will fit into your winter backpack fine. However, if you really want to, you can be even more forceful with the stuff sack’s compression straps and reduce the pack size down even further. The 20L stuff sack is made from Sea to Summit’s ultra-sil fabric.
For safe uncompressed storage at home, the Spark -18C comes with a well-designed “storage cell” (92g), which is cuboid shaped.
Materials & sustainability
The Sea to Summit Spark -18C is made from a few different materials. The shell is a lightweight 10-denier 100% nylon, and the hood and footbox sections of this shell (coloured yellow) are slightly more weather-resistant than the rest of the bag. The liner is again a 10-denier 100% nylon. The insulation fill is goose down, with a minimum 90% down and 10% feathers.
Our only concern with the Spark -18C’s materials is how ultralight they are. They feel thin and flimsy, and there’s a slight worry that you might accidentally rip or damage them. But, fingers crossed, if you’re careful and considerate, you shouldn’t have any issues.
None of the Spark -18C’s materials are recycled, however there are a few positive environmental and ethical considerations here. No harmful PFC chemicals are used in the manufacturing process, while the down is RDS certified.
Overall, therefore, the sustainability of this product is ok, but far from exceptional. Several other sleeping bags in Sea to Summit’s range feature recycled materials, but the brand has not done so with the Spark family of sleeping bags in order to ensure a better balance of performance and weight.
Size & shape
The Sea to Summit Spark -18C is available in regular and long versions. According to the brand’s official size guides, the regular Spark -18C has the following dimensions: max user height 185cm, shoulder girth 155cm, hip girth 135cm and foot girth 102cm.
The long version is for taller and bigger individuals. It is suitable for anyone up to 198cm tall, and adds 10cm in girth to the shoulder, hip and foot areas of the sleeping bag.
The Spark -18C has a mummy fit, with a quite pronounced taper towards the foot end. Sea to Summit describes this as a “fitted cut with a generous footbox and technical hood” designed to “maximise thermal efficiency and minimise bulk and weight”.
In our view, it’s an alpine-style fit that’s relatively tight. As with all mummy-shaped bags, this may polarise opinion. Some may find it a tad too restrictive and limiting; others will see it as wonderfully snug and cosy.
Sea to Summit, however, is more confident that the fit will suit most users. It claims: “ultralight does not mean ultra-tight: the Spark mummy sleeping bags are snug but still have adequate wiggle room for the performance-oriented, backcountry user”.
Features
To keep weight down, this is quite a minimalist, no-frills sleeping bag, so there aren’t excessive bells and whistles when it comes to features. But you still get everything you need, really. Down one side is a ¾-length zipper with two sliders and a “zip-plow”. It flows very smoothly, has a useful toggle, rarely snags and can be used two ways, meaning you can vent from the base if required.
The top of the zipper has a little flap of fabric known as a “zipper garage”, as well as press stud button to completely seal out the cold. Behind the zipper is a thick, tube-shaped down baffle to ensure the zip isn’t a weak spot for heat loss. The all-round zipper design is premium end and high quality.
You also get an excellent hood, with shed loads of insulation. It is adjustable via a drawcord and fits very nicely. The footbox is similarly impressive, with a touch of extra room and high levels of cosiness. Internally there are no features at all and you don’t get any pockets.
If you need extra warmth, the external shell has four little fabric loops – two at the hips, two at the footbox, with one on each side in both cases. These are part of Sea to Summit’s QuiltLock system, which enables seamless syncing of your sleeping bag with an additional sleeping quilt. This can add significant warmth to your sleep system if you’re planning on camping in very low temperatures.
The baffle design of the Spark -18C is particularly impressive too, with different shapes and structures used across different parts of the body. Across the chest you get vertical baffles, while the lower half and base have horizontal baffles. All of these feature boxwall construction techniques. There is a slight niggle however – with such fluffy down combined with such lightweight shell and liner fabrics, there’s a chance that down migration and uneven distribution might be a problem. Although we haven’t noticed this ourselves during our limited tests to date.
Verdict
The Sea to Summit Spark -18C Sleeping Bag is an ultralight sleeping bag with a superb warmth-to-weight ratio courtesy of 850+ fill power down – but it's very pricey.
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.