The MontBell U.L. Alpine Down Hugger Thermal Sheet is designed to fit inside another sleeping bag to use as a winter liner, but serves equally well as a summer stand-alone bag in warmer weather. I like this simple bag’s features, baffled down construction and the exceptionally durable-for-the-weight ballistic nylon fabric. My only complaints are a slightly trim fit and that the features add weight. You’ll have to decide if the features are worth the extra weight.
- Baffled construction eliminates cold spots
- Full length, two way zipper and drawcord neck edge provide ample venting
- Full length zipper creates a handy foot pocket for hammock use
- Reversible design allows it to be a right or left zip to correspond to the outer bag’s zipper
- Fabric is highly durable for its weight
What’s not so good
- Lack of hood requires a skull cap or balaclava when used as a stand alone bag
- Slightly heavy for the amount of insulation it provides
- Girth is fairly snug in the chest
|2005 U.L. Alpine Down Hugger Thermal Sheet|
|Hoodless down sleeping bag liner or peak-summer (45 °F to 60 °F) ultralight hoodless mummy bag|
|725-fill European goose down, 4.5 oz (128 g) of fill|
Manufacturer Claimed Temperature Rating
|Manufacturer does not specify|
|One size; length 70 in (178 cm), shoulder girth 60 in (152 cm), foot girth 38 in (97 cm), fits a person up to 70 in (178 cm) tall|
|Two-layer loft averages 3 in (7.6 cm)|
|Measured weight 14.4 oz (408 g); manufacturer’s specification 12.5 oz (354 g)|
|15d Ballistic Airlight nylon, 0.9 oz/yd2 (32 g/m2) with standard DWR treatment|
|Box-construction baffling, full length zipper, drawcord neck edge, hang tabs|
The MontBell U.L. Alpine Down Hugger Thermal Sheet is a hoodless, mummy style sleeping bag liner. I found it equally usable as a warm weather stand-alone bag. Compared to other liners, this bag is well outfitted with a full length zipper, baffled down construction, and drawcord neck line.
MontBell uses 725 fill-power down (1 ounce of which will fill a volume of 725 cubic inches). Down is a great material for use as a liner bag. In cold weather, condensation will tend to collect on the inside of the outer most layer of fabric where vaporized moisture hits colder temperatures. When using a synthetic outer bag with the down MontBell Thermal Sheet as a liner, moisture will tend to condense on the outside of the synthetic bag with little reduction in loft of the down liner. When used alone in warmer temperatures, condensation is less of a problem.
The MontBell Thermal Sheet can also be used as a top bag either on the ground or in a hammock. Place the zipper on the bottom, with the foot zipped. The neck drawstring remains adjustable thanks to a Velcro closure at the neck. Using the Thermal Sheet as a top bag allows you to wear insulating layers, such as a synthetic jacket, without compressing the loft of those layers.
The Thermal Sheet is designed with uniform loft. I measured the loft in six places across the bag. The MontBell Thermal Sheet has 3 inches of double layer loft, or 1.5 inches of single layer loft. MontBell does not offer a temperature rating for their Thermal Sheet but the amount of loft compares to other bags rated for 50 °F to 60 °F temperatures.
The U.L. Down Thermal Sheet uses baffled construction. Down feathers will shift unless chambers are created to contain the down into confined spaces. Many bags this thin use sewn through construction, where pockets of down are separated by sewing through the bag. Sewn through baffles use less material and are therefore lighter, but heat is lost through the seams creating cold spots. The box construction baffles in the MontBell Thermal Sheet are only 0.5 inch high, preventing direct heat loss, but still reasonably light. With 1.5 inches of single layer loft, the baffle height suggests this bag is overstuffed, a condition that will prevent down from shifting towards the sides of the Thermal Sheet.
The full length YKK #3 non-separating double slider zipper on the Thermal Sheet allows significant venting options. The bag can be vented from the top down, or the bottom up using the lower slider. MontBell’s logo label would suggest the zipper is on the right side. However, the hoodless, symmetrical shape of this bag allows it to be flipped over, placing the zipper on the left, and consequently the MontBell label on the ground. This allows the Thermal Sheet to be used within either a right-zip or left-zip sleeping bag. A drawcord along the neck edge adjusts for ventilation or draft prevention at the neck and shoulders. Lacking a hood, the Thermal Sheet will prompt the use of a skull cap or balaclava in cooler weather.
The MontBell U.L. Alpine Down Hugger Thermal Sheet is a full-featured sleeping bag liner. It has a full length two-way zipper that closes with Velcro at the top. The top edge adjusts with a drawcord to seal out drafts.
With inside girth measurements of 60"/45"/38", the MontBell Thermal Sheet is sized fairly typically for an ultralight sleeping bag. As such, using the Thermal Sheet as a liner within another ultralight sleeping bag will likely cause some loft compression in either the outer bag or liner. I gained very little warmth when using the Thermal Sheet as a liner inside a Marmot Hydrogen. The similar snug fit of these two bags kept the MontBell liner from lofting fully. When I used the Thermal Sheet with a top bag (homemade, down with 2.5" of loft), I had much better results, lowering the top bag’s temperature rating by 10 °F to 15 °F. Other sleeping bags with an accommodating interior volume should see similar results.
I found the fit to be tight in the chest while moving my arms from the closure cordlock to my sides. This condition only presented itself when closing and opening the neck closure and did not affect comfort while sleeping. For reference, I am 5’7" tall and weigh 155 pounds. For some, the fit may feel confining.
The Ballistic Airlight fabrics used in the construction of the MontBell U.L. Down Thermal Sheet have a high strength to weight ratio. Most of us associate Ballistic nylons with thick, heavy bullet stopping fabrics much more akin to war zones than to ultralight backpacking. It is not the weight of fabric, but the manufacturing process that gives Ballistic nylons their unique characteristics. The nylon fibers are heated and stretched, which aligns the nylon molecules in each fiber. Ballistic nylons are one-and-a-half times more abrasion resistant and have three times more tear strength than heavier nylons (according to MontBell). The fabric weighs 0.9 oz/yd2. MontBell Ballistic Airlight uses hollow core fibers, which are flattened such that they overlay one another like shingles. The resulting fabric is very thin, down proof, and more durable than similar weight fabrics of conventional construction.
Missing from this review (and for all sleeping bag reviews published here, for that matter) will be an assessment of whether or not the sleeping bag performs adequately at temperatures near its manufacturer-reported temperature rating. Click here for the complete Backpacking Light Position Statement on Sleeping Bag Temperature Ratings.
Baffled construction is rare in a bag this thin and eliminated cold spots. We also liked the use of ballistic nylons to create a strong, down-proof fabric at a respectable weight.
Recommendations for Improvement
Many will enjoy the venting opportunities gained from the full length zipper of the U.L. Alpine Down Hugger Thermal Sheet. Others will prefer a lighter weight. My recommendation is to reduce the zipper to a three-quarter length to shave weight.
Lighter materials could be used in the neck drawcord and cord lock, again to shed some weight.
I would prefer a tad more girth in the chest, though not in the shoulder. The shoulder girth should continue a couple of feet down the bag before tapering to the foot girth.
I liked the baffled construction despite the additional weight it adds. However, I do wonder how well this bag would perform with baffled construction only over the chest and feet, with sewn-through construction over the legs.