Sea to Summit Spark -1C Review: Updated and Awesome

By Sam

PUBLISHED Oct 10, 2024

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Photos by Tom Norman

The Spark line of sleeping bags from Sea to Summit has grown to become a favourite amongst weight-conscious bikepackers. We’ve positively reviewed a few of the previous iterations of the bag in both the men’s and women’s styles on the site, and I relied on the Spark III bag for all my rides around the world between 2019-2022. Renowned for their high-quality materials and construction, 850+ ethically sourced down fill, and bikepacking-friendly design, it’s easy to see why these bags feature in many riders’ sleep systems.

However, as good as the bags were, they weren’t without fault. Some people found the mummy-style shape quite restrictive, the uber-lightweight 7D fabric too fragile, and the zipper design draughty and prone to snagging. So, when Sea to Summit announced the updated 2024 version of their Spark series, I quickly pulled the trigger on the Spark 30°F (-1°C) bag for my trip to Peru.

The full Spark lineup consists of four bags, with temperature ratings from 45°F (7°C) and 30°F (-1°C) all the way down to 15°F (-9°C) and 0°F (-18°C) for those winter adventures. For this review, we’ll focus on the Spark 30°F (-1°C), which strikes a balance between warmth and weight that many bikepackers seek.

Specifications at a Glance

• Weight: 1.12 pounds (510 grams)
• Fill: 850+ fill-power goose down
• Shell & Liner: 10D nylon with PFC-free DWR
• Fill Weight: 265g (9.3 oz) in size Regular
• Lower Temp Rating: 30°F (-1°C)
• Style / Fit: Mummy fit with generous wiggle room
• Baffle Design: Hybrid vertical chest, horizontal lower
• Price: $449 USD (£400 GBP/€449.95 EUR)

Three months in The Andes

While lab tests and manufacturer specifications provide a good baseline, nothing beats real-world experience. I put the Spark 30°F (-1°C) through its paces during a gruelling two-month expedition across the Peruvian Andes. Traversing diverse terrain from humid coastlines to frigid 5,000-metre peaks, our nighttime temperatures ranged from +15°C to -12°C for the highest sleeps. However, average nightly temperatures consistently hovered around 0°C to -2°C, which meant I had plenty of opportunities to put the Spark through its paces. 

Note that a good sleeping bag is only as good as the sleep system you pair it with. So, alongside the Spark 30°F (-1°C) bag, I also used the Nemo Extreme Conditions mat with an 8.5 R-value (stay tuned for a full review soon), merino wool base layers, a down puffy jacket and a Mont Zero UL summer sleeping bag 42°F (6°C), which I only added to the Spark when the temperatures dipped below 23°F (-5°C). 

Design + Build

The Sea to Summit Spark 30°F (-1°C) shares many of the same design and build cues as previous generations. However, the updates they made to the 2024 bag really make a big difference to the warmth, comfort, and durability of the overall bag. 

Fill-wise, the Spark continues to use 850+ ethically sourced down, treated with Sea to Summit’s Dry Down coating. This PFC-free coating enhances the down’s ability to loft effectively in moist conditions, repel water, and dry rapidly. Plus, it inhibits bacterial growth, making your bag last longer, which is pretty sweet. 

The outer fabric has been upgraded to a uniform 10-denier Nylon, eliminating the “see-through” appearance found in the previous Spark bags. This ultralight fabric contributes significantly to the bag’s overall low weight and small packed size. And, in my experience, it felt strong and burly enough for most adventures. 

Another significant improvement is the waterproof material used at the hood and foot box. I used a Six Moon Designs Lunar Solo in Peru, which is pretty short, and due to its single skin, suffers from condensation. I regularly woke up with my head or feet touching the inner wall of the tent, so this extra layer of protection was a welcome addition.

 
Construction-wise, the Spark 30°F (-1°C) employs a hybrid baffle construction that optimises warmth where it’s needed most. Vertical chest baffles trap heat efficiently around your core, and horizontal baffles in the lower half of the bag allow for more even down distribution. The top half of the bag features box wall baffles for maximum loft and warmth retention, transitioning to a sewn-through quilted construction in the lower half to reduce weight and bulk. I found the baffle design to work well for me and didn’t experience any cold spots or shifting down in three months of daily use. 

Fit and Zips

One of my main gripes with the previous generation Spark bag was its restrictive mummy design. And while the newer Spark bags retain the same true mummy style, Sea to Summit have completely overhauled the fit of this new bag. Now, the Spark offers more space than many of its competitors through the shoulders and hips—something I especially enjoyed as a side sleeper—but still fits snugly enough to minimise any cold spots. In my testing, the regular-size bag felt comfortable, roomy enough, and not claustrophobic. I particularly liked the generously sized footbox and the technical hood, which really added some warmth and comfort when needed. 

Carrying on with the theme of significant updates, Sea to Summit have also updated the zipper design on the new bags. They now feature a YKK #5 zipper, equipped with a zipper plow to minimise fabric snags. Compared to the old version, this zipper is a dramatic improvement. Sure, it might add a few grams, but in my months of testing, I found it a joy to use every day. The plow feature allows for smooth operation without the annoyance of material getting caught in the zipper track, and the elongated zip pullers allow easy access even in freezing temperatures with big bulky gloves on. 

However, it’s worth noting that the Spark 30°F (-1°C) comes with a half-zip design rather than a full-length zipper. This decision was likely made to save weight, aligning with the ultralight ethos of the bag. While this contributes to the impressive weight savings, it’s a trade-off that some users will want to consider carefully. The half-zip design can make getting in and out of the bag a bit trickier, especially for larger individuals or those who prefer more ventilation options. It also limits your ability to open the bag fully and use it as a quilt on warmer nights. For me, the weight savings justified this design choice, but it’s something to keep in mind if you prefer the versatility of a full-length zipper.

Warmth, Weight, and Packability

On paper, the Sea to Summit Spark 30°F (-1°C) sleeping bag seems to hit the sweet spot between warmth and weight that bikepackers dream of. Tipping the scales at a mere 510 grams for the regular size and compressing to roughly the volume of a 1L Nalgene bottle, it promises ultralight performance without compromising on warmth. However, my previous experience with an earlier Spark model, coupled with Sea to Summit’s historically optimistic temperature ratings, initially tempered my expectations.

I approached the bag figuring its “Lower Limit” ISO rating of 30°F (-1°C) and “Comfort” ISO rating of 39°F (4°C) might translate to comfortable sleep down to freezing at best before I’d need to start layering up or reaching for additional insulation. To my surprise, though, the Spark thoroughly outperformed these conservative estimates. It kept me comfortably warm right down to its lower limit and, astonishingly, even maintained cosiness as temperatures dipped to around 26°F (-3°C). My best guess is that based on customer feedback and previous reviews sharing my same chilly sentiments, Sea to Summit have worked hard to improve the temperature ratings across the range. 

As explained earlier, I decided to use the Spark 30°F (-1°C) as part of my wider sleep system while riding The Peru Great Divide route. With seriously fluctuating altitudes and therefore temperatures, I needed a sleep kit that was versatile and modular. Out of the 60+ nights we slept in Peru, we camped 45 times. During that time, I reached for my extra sleeping bag roughly 10 times, meaning I spent 35 nights in just the Spark bag and my base layers. 

You can learn more about the updated Spark over at SeaToSummit.com.

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