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MontBell U.L. Spiral Down Hugger Thermal Sheet

Montbell UL Spiral Down Hugger Thermal Sheet
Montbell UL Spiral Down Hugger Thermal Sheet

Earlier this year, I bought a Montbell UL. Spiral Down Hugger Thermal Sheet (down sleeping bag) and I’m really pleased with its versatility for summer and autumn camping.

Rated at 50 degrees (F), I’ve used the Montbell comfortably to 40 degrees with down booties and long underwear (which I always wear when I sleep). Contrary to its name, it’s not a sheet and it doesn’t use the spiral system that Montbell’s heavier down bags use. It’s just a down-filled rectangular sleeping bag, without a hood, that scrunches up incredibly small in a stuff sack.

I originally bought the Montbell to use during the summer when my Western Mountaineering Ultralite 20 is simply too hot. I’ve owned other warmer weather bags in the past and not been wowed by them, but what attracted me to this particular sleeping bag was its 800 fill down, the fact that it could be used as a quilt as well as a sleeping bag, and of course the fact that it weighs 13.6 oz, well under 1lb.

So far, I’ve just used the Thermal Sheet in its sleeping bag configuration, but it does have a wrap-around zipper so that you can open it up and use it as a quilt, for one person, or even two good friends. In quilt-mode, I’d keep the bottom zipped up to my calves to create a foot box like you find on other quilts, like the Golite Ultralite Quilt.

In addition to the YKK zipper, there are two loops sewn into the bottom hem of the bag and two plastic hooks attached to the top hem. I presume these are for attaching to a portaledge, but they’re certainly not strong enough to substitute for a proper harness. There’s also a drawstring and cord lock at the top of the bag, for cinching it around the tops of your shoulders to better retain warmth.

Size Comparison: Western Mountaineering Ultralight 20 on left. Montbell UL Spiral Down Hugger Thermal Sheet on right
Size Comparison: Western Mountaineering Ultralight 20 on left. Montbell UL Spiral Down Hugger Thermal Sheet on right

I’ve read about older versions of this bag that did not have baffles and allowed you to move the down fill around. That’s not the case with this latest model. The baffles are sewn diagonally across the bag to keep the 800 fill goose down evenly distributed.

A Winter Sleeping Bag Liner

In addition to using the Montbell Thermal Sheet in the summer, I plan on testing it as a liner inside my 20 degree Western Mountaineering bag to boost its warmth and fill a hole in my down sleeping bag lineup, which is the absence of a 0 degree bag. This worked well without compressing the loft on the Western Mountaineering bag when I tried it at home, and as you read this post, I’ll be testing it out in the White Mountains on a late autumn overnight trip (oops, I came down with a bad cold and the trip got postponed.)

Western Mountaineering Ultralight 20 with Montbell Thermal Sheet Liner
Western Mountaineering Ultralight 20 with Montbell Thermal Sheet Liner

The irony of this setup is that I’ll be using a Western Mountaineering sleeping bag with a Montbell thermal sheet liner, something I can’t see either manufacturer endorsing due to their marketplace rivalry.

If it works, which I expect, then I can use the two bags together and get a functional 42.6 oz, 0-degree down sleep system, without spending the money for yet another sleeping bag. When you consider that a 0 degree Western Mountaineering Kodiak weighs 44 oz, it becomes apparent just what a good deal this really is: I’ll get the equivalent of three sleeping bags for the price of two.

Of course, it has to work, but regardless, the Montbell Down Hugger Thermal Sheet has been a resounding success as a summer-weight sleeping bag for me.

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  1. I’ve been planning a test of a similar configuration myself (also in the Whites) but haven’t seen the temps drop enough to test it. I plan to combine a Marmot Helium (15F) with a Mountain Hardware Phantom 45 (45F, obviously, but closer to 35F in my ~2 years of use). I figure the combo plus my normal winter sleeping attire (mid-weight wool on my legs/torso, heavy socks or down booties on my feet, and a wool hat+neck gaiter to regulate exposure on my face) should be more than adequate down to -5F, and possibly safe down to -10F (I’ll test that on a car camping night with a warmer bag in close reach!).

    I’ve also toyed with adding a down jacket to the mix, but want to test the two bag performance on their own first. Regardless, I like the concept of using your existing gear to see what (reasonable) combinations you can make to avoid adding yet one more item to the gear closet.

    That said, in testing at my home, I did find the double bag/double zipper a bit claustrophobic feeling when I wanted to get out. Definitely something I’m willing to put up with if the combination performs. Total weight of the two bags (no stuff sacks) is 1501 grams, or 53 oz. Definitely heavier than your combo, but probably a good 10-15F warmer as well.

    Looking forward to your results!

  2. Nesting sleeping bags does help extend the range of what you have.

    I was surprised to see that there is little or no weight penalty since more weight and bulk is the price you typically pay for this technique.

    As I have written before..I use a 15 deg F down mummy cut [MARMOT HELIUM] inside a 15 deg F synthetic, semi-mummy cut (BA ENCAMPMENT) with a light wool base layer with great results. Have been toasty down to 0 deg F. Haven't found the lower limit yet.

    The key is the fit between the two bags. The inside bag needs to have room to loft. I find the inside bag also expands "in" against my body virtually eliminating any dead air space.

    I expect that with multiple layers of sleeping bags, top quits and pads I am going to be able to get down to -20 deg F. I will need a pulk to bring all the gear but I am going to be able to without buying a $ 600 bag that is highly specialized.

  3. You're right about the key – which is that the outer bag can't after the loft or the inner and vice versa. But while the WM Kodiak weighs more, it also has larger shoulder girth dimensions, so it's not really a fair comparison on weight alone.

    Obviously, your repetition is helping Tommy. It's all beginning to sink in…:-)

  4. Sorry to hear about the cold–get well soon! I'm looking forward to hearing how your new system works!

  5. I always read all good reviews at Sectionhiker. This time I really miss one important detail.

    Max User Height: 5 ft 10 in

    This exclude us over 6 ft wich means no reason to read any further.. anyway, nice for shorter hikers!

  6. Let me measure my bag. This bag does not have a hood, so the extra 2 inches will not make a difference, what matters is where your neck is, unless of course, your taller. I'll post the measured spec later today.

  7. Hi Philip – my REI Polar 15 degree ripped last weekend on a trip so am now considering a new bag…what's your experience on temp ratings with this thermal sheet? I have a zero degree bag for winter – and am looking for something more for the fall – I sleep warm. Clearly I can layer – I have a patagonia nano puff which I have with me for fall hiking plus base layers…

    another alternative is the Mountain Gear ultralamina – a synthetic bag – weighs just under 3lbs ($165 on sale). Thanks

  8. I used it on my trip last week, opened up like a quilt. In general, it's good to 50 degrees but not lower. I sleep cold. It also doesn't have a hood – purely rectangular. For autumn, I'll definitely switch back to a 20 degree bag, but that's New England.

  9. So how did using the two bags together workout? Western Mountaineering and Montbell.

  10. Never got around to it, but it is still on my to-do list. The problem is that our weather is often subzero or above 20, but never right about zero. At least, on the nights that I have permission to sleep outdoors!

  11. Thanks for a great post. Did you ever manage to combine the two bags? It seems like a very attractive option, at a premium.

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