The Sea-to-Summit Spark IV is a 5-degree (F) down ultralight sleeping bag that can be used for cold weather and winter and backpacking. It is insulated with 850 fill power goose down and comes with cold-weather features including a full-length snag-free zipper, a down-filled draft tube, and a draft collar. At 31 oz, it’s also incredibly lightweight for a bag with its temperature rating, although it trades off weight for a much more confining fit. We prefer to have a little bit more space inside a cold-weather sleeping bag to accommodate extra clothing layers and store electronics and water inside to prevent them from freezing at night when temperatures drop close to zero.
Specs at a Glance
- EN Temperature Rating Lower Limit: 5F
- EN Temperature Rating Comfort (Women’s): 18F
- Gender: Unisex
- Weight: 1 lb 15 oz
- Fill weight: 21.9 oz
- Draft Collar: Yes
- Draft Tube: Yes
- Zipper: Left-side #3 YKK, two-way
- Baffles: Box
- Insulation: Ultra-Dry Down 850+ Loft
- RDS-Certified: Yes
- Storage sack: included
- Compression sack: included
- Exterior Shell Fabric: 10D Nylon; Interior Shell: 7D Nylon
- Length: 6 feet, (6′ 4″ long size also available)
- Dimensions: (Shoulder/Hip Feet) 61″/53″/37″
- Recommended stuff sack size: 10L
The Sea-to-Summit Spark IV is a cold-weather sleeping bag insulated with 850 fill power down and rated to 5 degrees Fahrenheit. Weighing 31 oz, it’s has a classic mummy shape with a shallow mummy hood on top. The down is distributed on both the front and back of the bag, with vertical baffles over the chest and horizontal ones from the knees down to keep the down from migrating and creating cold spots.
The Spark IV comes with all of the normal features you’d expect on a quality cold weather sleeping bag including a full-down-filled draft tube behind the full-length zippers, a down-filled draft collar that wraps around the front of your chest, and around the back of your shoulders, and neck. The fabric along the length of the zipper is reinforced with a stiffener to prevent snags, which can lead to tears in the fabric, and is a feature you really only find on premium sleeping bags.
A draft tube is designed to block drafts from entering a sleeping bag through the zipper and seal in your body heat. While it does its job on the Spark IV, we were surprised by the size and width of the draft tube which is one of the narrowest we’ve ever seen on a cold-weather bag.
The same can be said for the bag’s draft collar, which we also found small. In this case, we found it to be too shallow to form a tight seal around our neck and prevent heat from escaping. We addressed this by wearing a down sweater to help fill the gap.
The shoulder girth of the Spark IV is 61″ which is on the smaller end of the spectrum for a bag of this temperature rating. But that measurement is somewhat misleading as the top of the bag narrows considerably between the shoulders and hood. It’s so narrow and tight that it becomes very difficult to adjust the draft collar and hood controls one-handedly to get a snug and comfortable fit. The combination draft collar and hood control also proved difficult to manipulate with one hand in the dark.
Once we got settled, the bag kept us warm throughout the night but it did take a while to “heat the bag up” inside. Still getting in and out for bathroom breaks at night was quite a production, since the collar and hood need to be released and readjusted every time you go outside. Even something as simple as zipping the bag is difficult because it tapers so much between the shoulders and the hood, putting considerable tension on the zipper, which is reinforced with a snap at the top.
We didn’t use the Spark IV in warmer weather because we were mainly interested in testing the bag’s cold-weather performance. The sleeping bag does have a two-way zipper, however, so you can vent the footbox or the upper part of the bag if you get too warm.
Comparable Cold-Weather Sleeping Bags
At 1 lb 15 oz, the 5-degree Sea-to-Spark IV is considerably lighter weight than the following 0-degree bags. While these bags are only rated for 5 degrees colder, the weight difference is largely due to the fact they have larger dimensions than the Spark IV, have larger sized draft tubes, draft collars, and hoods, or use heavier weight fabrics
|Make / Model||Weight||Fill Weight||Fill Power||Price|
|Big Agnes Starfire UL 0||2 lbs 14 oz||31 oz||850||$580|
|Feathered Friends Ibis EX||3 lbs 1 oz||28.5 oz||900||$635|
|Feathered Friends Snowbunting EX 0||2 lbs 13 oz||25.3 oz||900||$599|
|Marmot Lithium 0||2 lbs 9.5 oz||27.8 oz||800||$499|
|NEMO Sonic 0||2 lbs 11 oz||23.6 oz||850||$479|
|Sierra Designs UL Nitro 0||2 lbs 8 oz||26 oz||800||$379|
|The North Face Inferno 0||2 lbs 14 oz||29.3 oz||800||$519|
|Western Mountaineering Kodiak MF 0||2 lbs 12 oz||30 oz||850||$675|
|Big Agnes Blackburn UL 0||2 lbs 12 oz||23.5 oz||850||$500|
The Sea-to-Summit Spark IV is an ultralight mummy-style sleeping bag rated at 5 degrees Fahrenheit. While it only weighs 31 oz, which is extraordinarily lightweight for a bag with its temperature rating, it is important to recognize that the weight savings comes with a significant reduction in internal space, ease of use, and comfort. While the Spark IV will probably work well for back sleepers with a smaller stature across the shoulders, we’d recommend that you try some larger dimensioned colder-weather sleeping bags, so you can make the weight/comfort-tradeoffs that align with your preferences and objectives. If you have to spend 12 hours in a sleeping bag on a cold night, some extra comfort is often worth its weight in gold.
Disclosure: The author received a sample bag from Sea-to-Summit for this review.Editor's note: Help support this site by making your next gear purchase through one of the affiliate links above. Click a link, buy what you need, and some sellers may contribute a small portion of the purchase price to support SectionHiker's unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.