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Sea to Summit Flame Fm IV 15 Sleeping Bag Review

Sea to Summit Flame Fm IV 15 Sleeping Bag Review

The Sea to Summit Flame Fm IV 15 Degree Sleeping Bag is an exceptionally warm lightweight sleeping bag designed for women who care about both low pack weight and comfort. Because I chill easily by nature and sleep cold no matter what season it is, the most important factor for me when choosing a sleeping bag is how efficient the design system is in keeping me warm. The second key factor for me is how heavy the bag is, combined with how well it compresses. Filled with 850+ loft premium goose down, the Flame Fm IV did not disappoint in these areas and I am super pleased with its overall performance.

Specs at a Glance

  • Temperature rating: 15F (comfort range 15-60F degrees)
  • Gender: Women’s
  • Weight: 1 lb 15.4 oz
  • Fill Weight: 1 lb 7 oz
  • Insulation: Ultra-Dry Down 850+ fill power goose
  • RDS-Certified: Yes
  • Compressed Volume: 7.1L
  • Shape: Mummy
  • Zip Options: right side #3 YKK
  • Draft Collar: Yes
  • Draft Tube: Yes
  • Compression sack: included
  • Storage cell: included
  • Shell Fabric: Ultralight 10d Nylon
  • Fits Up To: 5’ 7” Regular size, (5’ 10” long size also available)
  • Dimensions: (Shoulder/Hip/Feet) 59″/56″/35″
  • Visit Sea-to-Summit for complete specs and sizing

A sleeping bag for cold sleepers

I found there to be quite a few highlights and perks with the Flame Fm IV in its performance and construction, especially for a cold sleeper.

The baffles of the Flame Fm IV change direction to keep heat in the places you want it most
The baffles of the Flame Fm IV change direction to keep heat in the places you want it most.

Baffles and Loft

The Flame Fm IV has box baffle construction (the best) to accommodate extra down and to minimize prevent cold air from leaking in around sewn seams. The vertical baffles on the chest help to keep insulation there, a place we can often feel chilled. In using the Flame, I definitely felt warm and toasty in my chest region, with no draft issues either.

What the heck are sleeping bag baffles anyway? Baffles are the down-filled compartments running across the shell of the sleeping bag and they’re needed to keep the insulation from moving around or clumping. When insulation stays in place, baffles effectively reduce cold spots and heat loss.

I found that the Flame IV lofts really well, which means that it doesn’t take long for the fluff to come back after being stuffed. After less than a half-hour, it was fluffy again. The loft is 90% down cluster premium goose down, which also contributes to the warmth of the bag.

Draft Collar, Draft Tube, Zipper, and Hood

The Flame Fm IV has a comfy and poofy draft and neck collar, which is the insulated collar that wraps around your chest and back. It did the job when it came to stopping heat from slipping out of the bag, something I’ve had problems with before. As temperatures dropped through the night, I found myself reaching for the drawcord to tighten it up and mummify myself even more.

To decrease heat loss, the Flame Fm IV has a zipper draft tube (an insulated tube that runs along and behind the zipper to prevent cold drafts from entering and warm air from escaping).

The two-way zipper provides easy access and the opportunity for ventilation if need be. There’s also a handy snap at the top of the zipper which can take pressure off of it, ensuring long-term use. I found the zipper smooth to use with no issues.


Woohoo, the regular size is less than 2 pounds and the long model is just slightly over that! Now that’s a selling point for me as a thru-hiker and one who does backpacking trips; I try to account for as much weight as I can that goes on my back for either a weekend trip or for months at a time.

The Flame Fm IV feels both luxurious and cozy to rest in
The Flame Fm IV feels both luxurious and cozy to rest in.


If you choose to use the Ultra-Sil compression bag it comes with (a nice feature because you don’t have to buy one), you’ll find it packs down small – like Nalgene water bottle size small. The Flame smooshed down really well using this method.

These days I’m into the cloud method of packing where you stuff your sleeping bag into your pack liner (like a compactor bag) and fill it up with your extra clothes, then add your other gear on top of the sleeping bag to compress it further. I like this because a compression sack is this hard, football-type shape that isn’t malleable; when you cloud pack, there’s more usable space.  So whichever style you like, the Flame has a high packability rating.

Shell and Liner

The shell fabric is 10D nylon and inner liner is 7D (D=Denier) which is what helps make the sleeping bag so feather-light. A sweet perk is the Ultra-Dry Down water repellent treatment which can be handy when dealing with condensation or camping in rainy regions.

Beyond this technical stuff, the Flame Fm IV felt good against my body (yes, I actually felt warm enough on a few occasions to have my skin exposed which is rare for me while camping!). The interior had a soft, silky feel, and heck, that’s a nice thing.

The Flame Fm IV has special design features with a woman's body in mind
The Flame Fm IV has special design features with a woman’s body in mind.

Women’s-Specific Design

What are the factors at play when designing a women’s sleeping bag and why would you want to choose one (or not) as a woman? The bottom line is that typically many of us women sleep colder than men and a woman’s body frame is also different than a man’s. The Flame Fm IV is designed with these things in mind.

How Does the Flame Fm IV Keep a Woman Warmer?

This sleeping bag is narrower in the shoulders and wider from the knee to the hip area, to accommodate a woman’s shape. The wider mummy shape and style allows you to sleep in whatever position you want without compressing all the down insulation so you don’t get those evil cold spots.

There’s nothing worse than staying in one position in your sleeping bag because you dread finding a cold spot. Sea to Summit says they made the Flame with ‘expedition sizing’ to create room to move and for additional layered clothing when really cold. This is vitally important for me since I often have extra layers on when sleeping and I don’t like to feel constricted.

All wrapped up, snug as a bug in a rug, in the comfy hood of the Flame Fm IV
All wrapped up, snug as a bug in a rug, in the comfy hood of the Flame Fm IV.

One of the features I liked the most is how the Flame is shorter in overall length, which minimizes dead space and augments thermal efficiency. I can’t stand it when my feet have no sense of where the toe box is and I have to stuff a ton of extra (usually dirty) clothes at the bottom of the bag to stay warmer. The Flame Fm IV felt snug and tight-knit, but in a good way, to maximize heat retention. The trimmer girth in the shoulders works the same way.

Length and Sizing

One point to make is that I’m about 5’5”, so the regular size works great for me, being suited for women up to 5’7”. There is a long size for women up to 5’10”, but if you’re taller than that, this wouldn’t be the best choice for you. Also, be sure to look at the specs at a glance above to consider your own amazing and individual body shape, especially in the shoulders and hips. to see if the sizing of this sleeping bag would work for you.

Comparable Women’s Sleeping Bags

Make / ModelWeightDown Fill PowerLengths
REI Magma 152 lb 4 oz8505'6", 6'
Sea to Summit Attitiude At1 252 lbs 6.1 oz7505'7", 6'
NEMO Riff 302 lbs 3 oz8005"6", 6'
NEMO Disco 152 lbs 15 oz6505'6", 6'
Feathered Friends Egret UL 201 lb 9.6 oz9505'3", 5'9"
Marmot Phase 201 lb 13 oz8505' 6"
Marmot Xenon 152 lb 6 oz8005'6"
Marmot Angel Fire 252 lbs 7.5 oz6505'6", 5'10"
Kelty Galactic 302 lbs 9 oz5505'8"
Sierra Designs Backcountry Bed 202 lbs 7 oz7005'8"


Sea to Summit’s Flame Fm IV 15 Degree Sleeping Bag is ideal for any woman who values a lightweight bag that doesn’t cheat on warmth and comfort. It’s a fantastic option for cold sleepers, with 850+ premium goose down, a poofy draft collar and hood, and female-specific sizing.  There aren’t many good sleeping bags designed for cold female sleepers, but I think the Flame Fm IV is a solid choice, and I can’t wait to use it on my last section of the PCT this summer.

Sea to Summit Flame Fm IV 15 Sleeping Bag


Designed specifically for Women

Weighing less than 2 lbs, the Sea to Summit Flame Fm IV sleeping bag is an 850 fill power waterproof down women's sleeping bag body-mapped to provide the ultimate in warmth and comfort.

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Disclosure: Sea-to-Summit provided the author with a sleeping bag for this review.

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About the author

Heather Daya Rideout has been a life-long outdoorswoman. Her pursuits and passion for hiking and camping have taken her around the world for many long-distance trips; such as backpacking in Nepal, India, South America, Morocco, Europe, and North America. Heather has hiked the Appalachian Trail, the Pacific Crest Trail, and a route of 1,500 miles combining several Camino routes through Spain and Portugal. On any given day she would rather be outdoors than anything else and her lifestyle is a direct reflection of that deep love affair with nature. Heather currently lives in Idaho and she’s having a wondrous time experiencing the beauty it offers. You can read some of her other writing at


  1. Eric Blumensaadt

    Thanks to this review this is the first 3 season down bag I’ve considered to replace my older 30 F. Western Mountaineeing Megalite that I had factory overstuffed to 20 F. I’m assuming there is a men’s version!
    This StS bag is very well designed with TOP quality down and innovative quilting.

    I like the fact that StS down is DWR treated. I now ONLY buy down garments that are DWR treated despite naysayers opinions on its durability.

  2. Eric:

    The men’s version is called the Spark.

    Backpacking Light gave the women’s version a glowing review and the men’s version a bad review, even though they are very similar.

    They criticised the Spark’s air-trapping fabric (makes it hard to stuff into its sack) and especially the zipper, which is prone to snagging.

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