Below are some loosely organized thoughts about sleeping bag specs, weights, and construction. It contains specific reference to the Mont-bell that is my favorite.
Most 3 season campers in the temperate zones (between the tropics of Cancer and Capricorn) at elevations up to 10,000 Feet can safely focus on UL bags in the 18 - 25 oz range. This zone includes for three season UL overnight Sierra backpacking below 10000 feet.
With UL top and bottom layer garments (such as Mont-bell thermawrap, or Columbia Alpino II) plus a slime barrier lightweight poly-pro underwear layer, plus hat, gloves, socks (all worn inside the bag), plus 1.5" or more pad, and perhaps a Thermolite bivvy or TarpTent, you should be good down to at least 28 degrees F. If I wake up cold, I just eat more fat, and do pushups in the bag to re-warm. This happens to me less than one of 30 nights in the backcountry, which is a risk I am more than willing to accept, to achieve the weight savings. If I wanted to be damn sure I was good for lower temps, I would take a pair of down booties, a down hat, and more edible fat.
I use as a baseline what I have tested, and carried for two years now, (as referenced in the [DSOL] gear review). I got this Mont-bell bag at around $130. If they are not on sale right now, similar products are. It is normally possible to find something appropriate for under $150 on sale. If you want to change it out in a year or two, you should have no trouble recovering at least 50% of that expense via sale on CraigsList, or the local newspaper if advertising at the right time of year (late spring or early summer).
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Compare the weights of shell+stitching+zipper+drawcord to fill. For most bags in this range, the fill weight is 10 or 6 oz, yet the rest of the bag weighs 13 oz. (The difference in in these bags can be assumed to be entirely fill weight). This is precisely why, for UL purposes, more layers of shell are to be avoided, because they are heavy, and add *no* warmth. (This is why I am replacing my Thermawrap jacket + vest combo with a single garment - Alpino II jacket.)
The down fill in all these products can be assumed to have identical density. So the weight difference is going to be mainly from shell fabric, and other construction details (zipper length, presence of draft tubes, quantity of baffles and hence stitching weight). For fabric, "Denier" is the key specification. As I understand it, higher denier number basically means more threads per unit area, and hence more weight. Fabric of low denier weighs less, but is more vulnerable to rips and tears. Some manufacturers claim they has an advantage in thread technology, for example Mont-Bell claims their threads are hollow, so that distorts a denier comparison a little. I think learning about fabric technology is very important for choosing gear, precisely because the shells are a huge proportion garment weight!
prolitegear.com is an *excellent* vendor, for accurate specifications, and meaningful product offerings. Bob has proved that published specifications (by manufacturers and vendors) are often inaccurate or incomplete. By weighing items on a repeatable, accurate, high resolution scale, Bob proved this over and over.
With the above as a baseline, here are some quick thoughts on some specific products.
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I carry this:
MontBell U.L.SS.Down Hugger #5
(Nick notes this bag has both a draft tube at side zipper, and adjustable good-fit hood, which acts like a draft collar)
MontBell's Super Stretch(tm) System provides a night's sleep unlike any other. Durable elastic stitching in the baffles of the sleeping bag provide unmatched freedom of movement while simultaneously drawing the insulation closer to your body minimizing dead space to be heated throughout the night.
Temperature Rating: 40 F / 4 C
High quality 800 fill power down
15-denier Ballistic Airlight fiber nylon
POLKATEX(r) DWR treatment
Max User Height: 5ft. 10in.
Inside Shoulder Girth: 53.2" - 70.9"
Inside Knee Girth: 42.8" - 57.1"
Stuffed Size: 4.8" x 9.5"
Size: R/ZIP/ L/ZIP
Color: IKBL(Ink Blue)
Weight: 1 lbs. 2 oz.
Fill Weight: 6 oz.
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this is what I might buy if I had it to do over again:
MontBell U.L.SS.Down Hugger #3
(Nick notes this bag has both a draft tube at side zipper, and a adjustable good-fit hood, which acts like a draft collar)
MontBell's Super Stretch(tm) System provides a night's sleep unlike any other. Durable elastic stitching in the baffles of the sleeping bag provide unmatched freedom of movement while simultaneously drawing the insulation closer to your body minimizing dead space to be heated throughout the night.
Temperature Rating: 30 F / -1 C
High quality 800 fill power down
15-denier Ballistic Airlight fiber nylon
POLKATEX(r) DWR treatment
Max User Height: 5ft. 10in.
Inside Shoulder Girth: 53.2" - 70.9"
Inside Knee Girth: 42.8" - 57.1"
Stuffed Size: 5.4" x 10.7"
Size: R/ZIP/ L/ZIP
Color: BASM(Balsam)
Weight: 1 lbs. 7 oz.
Fill Weight: 10 oz
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Big Agnes Horse Thief 35 Degree 725 Fill Power Sleeping Bag
http://www.gearforadventure.com/Big_Agnes_Horse_Thief_35_Degree_Down_Sleeping_Bag_p/1379.htm
Note that it concentrates all loft above (not underneath) the sleeper, so REQUIREs an appropriate sleeping pad because that is the insulation on the bottom of the bag.
The Big Agnes Horse Thief 35 Degree 725 Fill Power Sleeping Bag is a comfortable sleeping bag that uses the famous Big Agnes System. The system removes the insulation from the bottom of the sleeping bag since when compressed, insulation does a poor job. This lightens the bag significantly. Big Agnes then put a sleeve in it's place for your sleeping pad which provides the insulation. With the pad in the sleeve you can't slide or roll off your sleeping pad ever again! The Horse Thief, like all Big Agnes Sleeping Bags REQUIRE an appropriate sleeping pad because they are the insulation on the bottom of the bag.
The Horse Thief was named after a local mountain near Big Agnes Headquarters in Colorado.
The Big Agnes Horse Thief uses a 20" wide Mummy style sleeping pad.
Features
Design Features of the Big Agnes Horse Thief 35 degree sleeping bag:
Integrated pad sleeve. Never roll off your pad again
No-draft yoke seals around neck to keep cold air from sneaking in. Not available on our hoodless bags
60" YKK #8 zipper. Mate together any of our left and right zip bags with 60" zipper
Mummy shape decreases weight & packed size
Cotton storage sack & nylon stuff sack included
Built in pillow pocket holds a fleece or our Air Core Pillow
No-draft wedge insulates the connection between the bag and pad. Not available on our hoodless bags
No-draft zipper tube insulates along the length of the zipper. Not available on our hoodless bags
Shell fabric: WRM breathable, water resistant and wind proof nylon microfiber rip-stop
Interior lining: WRM breathable, water resistant and wind proof nylon microfiber rip-stop
LEFT OR RIGHT ZIPPER? When you are in the bag, on your back, the right zip will be on your right side, left zip on your left side.
Regular Horse Thief Sleeping Bag:
Weight - 1Lb. 8oz.
Fill Weight - 9.5oz.
Shoulder Girth - 67.5"
Hip Girth - 64"
Pad Length - 20"x72"
Long Horse Thief Sleeping Bag:
Weight - 1Lb. 11oz.
Fill Weight - 11.5oz.
Shoulder Girth - 72.5"
Hip Girth - 69"
Pad Length - 78"
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Lafuma Pro 650 Sleeping Bag:
This is the most similar of the 4 choices presented, to the UL bag that I use.
While it presumably does not have the Mont-Bell Super Stretch technology, nor the hollow shell threads, it is roughly the same weight.
Specs below say "no draft collar", but it must have some kind of way to prevent drafts from escaping past the head (drawcords at hood, etc). All these bags must have some sort of draft tube at side zipper, even if not explicitly specified.
http://www.backcountry.com/store/LAF0068/Lafuma-Pro-650-Sleeping-Bag-30-Degree-Down.html
Material: [Shell] water-repellent 20D polyamide; [Lining] water-repellent 20D polyamide
Insulation: 750-fill down
Shape: Mummy
Draft Collar: No
Max User Height: 6ft (183cm)
Stuff Size: 11 x 5in (28 x 13cm)
Stuff or Storage Sack: Stuff sack
Degree: 30F (-1C)
Weight: 1lb 7oz (650g)
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Lafuma Warm'n Light 800 Sleeping Bag:
The bag specs claim: [Shell] water-repellent 40D mini-ripstop nylon; [Lining] water-repellent 40D Tactel nylon
Specs below say "no draft collar", but it must have some kind of way to prevent drafts from escaping past the head (drawcords at hood, etc). All these bags must have some sort of draft tube at side zipper, even if not explicitly specified.
http://www.backcountry.com/store/LAF0070/Lafuma-Warmn-Light-800-Sleeping-Bag-30-Degree-Down.html
Material: [Shell] water-repellent 40D mini-ripstop nylon; [Lining] water-repellent 40D Tactel nylon
Insulation: 650-fill down
Shape: Mummy
Draft Collar: No
Max User Height: 6ft (183cm)
Stuff Size: 11 x 5in (28 x 12cm)
Stuff or Storage Sack: Stuff sack
Degree: 20F (-6C)
Weight: 1lb 12oz (800g)
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GoLite Ultra 20 Quilt:
Interesting (and perhaps challenging) for many of the same reasons as I detailed below
http://www.prolitegear.com/golite_ultra_20.html
A premium 800-fill goose down, 20ยบ sleep system that offers superior warmth, unmatched comfort and versatility for the ultra-lite purist. Waterproof Pertex(r) Endurance Arid Zones(tm) at the head and foot of the bag protect against condensation in critical areas. The comforter-style top can be attached directly to a sleep pad at variable girths for ideal temperature regulation.
Specifications
Weight: Short 1lb 1.7 oz, Reg 1lb 3.3 oz, Long 1lb 5.5 oz(Includes stuff sack)
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Rab Quantum Top Bag AR:
This *is* an interesting concept, *if* your body will allow you to get to sleep with only a single layer of Quantum fabric on the base. To be sure, smashing down the fill under the body seems a waste of transported weight. However, most humans do not lie flat against the pad while sleeping; where there are gaps, the bag's fill expands to, well... "fill" them. (on a related note, this is why I recommend the Z-Lite pad - divots for the fill to expand into). Bob and I have owned bags that permitted fill to be moved circumferentially in a baffle tube from under the body to on top. I felt this was an important design advantage.
http://www.prolitegear.com/rab_quantum_top_bag_ar.html
The Quantum Top Bag AR has 200g of down on top and round the foot plug and only a single layer of Quantum fabric on the base. It has a sleeping mat sleeve in the base which will fit a 3/4 or full length mat. The sleeping bag is designed to be used on adventure races, mountain marathons and minimalist backpacking trips in conjunction with an insulating pad that is best cut to the shape of the bag (or for the hard core use bubble wrap!)
Specifications Weight: 1 lb 5oz (Including Stuff Sack)
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