Sea to Summit Spark -18°C Review: The Best Winter Sleeping Bag?
We own over 10 of these sleeping bags. We purchased the first ourselves, were gifted two by Sea to Summit, and purchased the rest for our guided snow camping trips at a discounted rate.
Introduction
When searching for a winter sleeping bag in Australian alpine conditions, the conversation is typically a comparison between the Mont Spindrift XT 850 and the Western Mountaineering Antelope, long considered the gold standard options. However, the Sea to Summit Spark -18°C (-0°F) stands as a worthy competitor and is arguably the best winter sleeping bag on the market. This is primarily due to its superior warmth-to-weight ratio and a suite of well-executed features – not to mention the price point.
Testing
We’ve spent ~30 nights snow camping in Jagungal Wilderness using these bags. The coldest night was -15°C.
Additionally, the bags have undergone over 100 nights of client usage on our guided snow camping trips in the Snowy Mountains. While this cumulative use isn’t enough to definitively assess long-term durability, our experience with thousands of nights with bags using similar lightweight fabrics gives us confidence that durability will not be an issue.
Direct Comparison: S2S Spark vs. WM Antelope vs Mont Spindrift XT
To see how the numbers stack up, let’s compare the specifications of the Sea to Summit Spark -18°C, the Western Mountaineering Antelope, and the Mont Spindrift XT 850.
| Specification | Sea to Summit Spark -18°C (Regular) | Western Mountaineering Antelope (180cm) | Mont Spindrift XT 850 (Standard) |
|---|---|---|---|
| Temp Rating | -10°C (comfort) or -18°C (lower limit) | -15°C | -19°C |
| Price (RRP) | $999.99 | $1,499.95 | $1,119.95 |
| Total Weight | 1113g | 1105g | 1430g |
| Fill Power | 850+ Loft Ultra-Dry Down | 850+ Power Goose Down | 800+ RDS Premium White Down |
| Fill Weight | 840g | 735g | 850g |
| Down volume/overall warmth | 25.2L | 22.0L | 24.0L |
| Down ratio | 76% | 67% | 59% |
| Fabric | 10D nylon shell | 20D microfiber | 12D Hydronaute XT™ (waterproof) |
| Max user height | 185cm | 180cm | 185cm |
| Circumference: shoulder/hip/foot | 155/135/102cm | 157/135/99cm | 157/133/115cm | Full length zipper? | Yes (#5) | Yes (#5) | Yes (size not specified, likely #5) |
The two key metrics for any down item are the down volume and down ratio. The down volume describes theoretically how much warm air can be trapped by the down, which is what keeps you warm. The down ratio gives you a measure of which item is the most weight efficient, which is important for big alpine adventures. Whilst these measures are flawed and far from perfect, it’s an excellent first order approximation when assessing sleeping bag specifications.
Perhaps surprisingly, the Sea to Summit Spark -18°C bag has both the highest down volume and down ratio, making it the warmest, and most weight efficient option. At 76%, the down ratio is one of the highest values we’ve ever seen for a sleeping bag.
In this comparison it’s worth noting the slight difference in dimensions for each bag, where they are all roughly the same – except the Mont Spindrift XT 850 has a much larger footbox. Our preference is for a smaller footbox, as we find less dead air to heat keeps our feet warmer.
The most significant difference between the bags lies in the fabrics used. The Spark’s 10D shell is lighter, contributing to better breathability and faster drying times compared to the Antelope’s more traditional 20D microfiber and the Spindrift’s waterproof shell. Whilst a waterproof shell might seem like a great idea for winter, it almost always results in a sleeping bag that gets wet more quickly, and is significantly harder to dry out. The problem is that your body is constantly emitting moisture whilst sleeping, and the waterproof layer means the vapour gets stuck in your sleeping bag and turns to water, instead of being dispersed into the tent and (ideally) out your vents.
Although the waterproof fabric on the Mont bag does has a small amount of breathability, it is insignificant when considering the amount of moisture your body generates – and is indeed the problem in general with rain jackets and why we like pit zips so much for venting.
Without going into too much detail, the only way to truly make the most of the waterproof fabric would be to carry a set of vapour barrier liner (VBL) clothing to prevent any moisture from your body entering the sleeping bag. This is essentially sleeping in plastic and not very comfortable. We carried VBL clothes for our winter traverse of the AAWT and never felt the need to use it as we found it wasn’t cold enough in Australian temperatures to justify the discomfort.
In our opinion, the Sea to Summit Spark -18°C strikes the perfect balance by using a highly breathable and lightweight 10D fabric for the main body, whilst using a waterproof fabric for the hood and footbox where condensation from brushing up against the tent is extremely likely in small winter alpine tents.
Pros & Cons
Pros
- Exceptional warmth-to-weight ratio: With a total weight of just 1113g, the bag offers an incredible warmth-to-weight ratio with an excellent down ratio of 76%. On all of our guided snow camping trips, no one has reported being cold in Australian alpine conditions.
- Weather resistance: The footbox and hood feature a waterproof fabric, protecting the down from tent condensation.
- Lightweight fabric: The use of 10D fabrics makes the bag highly breathable. This allows it to manage internal moisture effectively and dry quickly if it does get damp.
- Space for layering: There is space inside the bag for you to comfortably layer a down jacket and down pants. We find this bag strikes the balance between having space for layers, whilst being narrow enough to be thermally efficient.
- Full-length zipper: We have found the zipper performs reliably without snagging.
- Cost-effective: The Spark -18°C is more affordable than its direct competitors, and is often on sale for around $800.
Cons
- Zipper side: There is no option to have the zipper on either the left or right side of the bag. This is particularly useful for couples who want to zip two bags together to create a two-person sleeping system. Sea to Summit does, however, offer dedicated two-person bags.
- Compression sacks: The included storage sack is neither waterproof nor ultralight. Furthermore, the sack has compression straps, a design we are generally not a fan of as it more easily allows for the user to damage the down through excessive compression.
- No small size: If you’re 165cm or shorter, you are probably better off getting a smaller sleeping bag so there isn’t excessive dead air to heat up.
Discussion
The key advantage of the Sea to Summit Spark -18°C is its excellent performance-to-weight ratio. Weighing only 1113g, it delivers reliable warmth for Australian winter backcountry conditions. The design uses a waterproof shell on the hood and footbox to protect against condensation, while the main body’s breathable 10D fabric minimises the buildup of moisture.
The bag’s few drawbacks are practical rather than performance-related. Not being able to zip sleeping bags together will be a limitation for some couples. Additionally, the included stuff sack is heavy and encourages excessive compression, which will damage the down over time. We’d also love to see a 165cm option.
Conclusion
The Sea to Summit Spark -18°C is an outstanding piece of equipment that successfully challenges the top contenders in the winter sleeping bag category. Its combination of low weight, excellent warmth, quality features, and a more accessible price point makes it a truly compelling option. For those seeking a high-performance sleeping bag for winter adventures, the Spark -18°C should be at the very top of the list.