Fill weight and temp rating: Western Mountaineering vs. Montbell UL SS

Without looking at the specific measurements I would guess that a more fair comparison between the WM 20* bag and the montbell 25* bag would be to compare to WM ultralite to the SS#2 rather than the alpinlite. The alpinlite is cut larger than the "normally" sized ultralite.
The Western Mountaineering Ultralite is rated to 20*, weighs 26 oz, 14 oz of which is fill weight. this gives a closer comparison to the SS#2 although even with these numbers the montbell bag still has the lowest total weight to fill ratio.
Without owning either (but having seen them and comparing specs online) I can come up with three plausible reasons for these discrepencies. 1)western mountaineering bags are typically conservatively rated so the 20*bag might be a little warmer (or the montbell could be a little colder) although a bag's temp rating is relatively subjective. 2) I believe the montbell shell fabric is slightly lighter than the material used in the WM bags although I'm not 100% positive on this. 3) as others have claimed before-since the baffles stretch and conform to your body dead air space is eliminated so superstretch bags may be warmer as a result (that being said they also require a little more material to make to allow for the stretch so this may be moot).

In the end I think I would chalk it up to slight variations in determining the bag's comfort rating.

I can't speak definitively on this, but I do have a Montbell SS #2 (older style). I know the company openly says they rely on the stretching of the bag to effectively increase loft, making a warmer bag for less down. I've found the temperature rating to be pretty spot on.

Good point Ryan. I was comparing the Montbell UL SS to the Alpinlite because that bag has a wider cut than the Ultralite, but maybe the more a comparison of the #2 and the Ultralite would be more valid. Although, the ability to wear puffy jackets in the bag is important to me, so that's why I made the comparison that I did.

By the way, that'd be impressive if the Montbell has a lighter fabric than the WM bags as their fabric is really, really thin.

The answer is the stretch/baffle system in the MB bags.
I own 3 MB bags (and others) and appreciate them primarily for that system. The fill to total weight ratio is a little lower because the bags use more yards of fabric. This is necessary to allow the bag to stretch (at the shoulders to a circumference of 75" for a long model for example).

MB bags are amazingly hot because the inner baffles pull the inner layer of the bag in gently against your body while the outer layer of the bag remains at full size. Less dead air to heat, less pumping of dead air out of the bag as you move, etc.. Other companies have to pack their bags with more down to get this loft, thus resulting in their higher down/weight ratios.
I can push any of my MB bags down 10'C by sleeping in my Thermawrap layer, which I would carry anyway. My lightest MB bag is a #7 zipperless at 16.1 oz I took down to 0'C.

If you do not want or need the room to stretch out when sleeping, or dont want the option of sitting up in your bag with your legs crossed in your tent, you can get a lighter bag with the same temp rating; WM and FF are both outstanding mummy bags.

David, regarding your research for a new bag, you could consider assigning numerical values to all your candidates. I used cost/fill weight ratio and down density. When graphed in numerical order, a 'sweet spot' can be identified, where the two lines cross, one particular bag is most warm, light, and relatively cheap. Other bags are either too expensive, heavy, or cold.

Hi Brett. Thanks for sharing your thoughts. I know you have a lot of experience with Montbell equipment and that you think highly of them judging from your previous posts.

And that graph is just awesome. A great way to break it down.

I ordered a Montbell bag online the other night. We'll see how it works. I may send it back. The first thing for me to test is length. The bag specs state that it'll work for users up to 6'4", but it may be too short for me given that I'm a little over 6'3". (Actually I kind of hope that it is too short as that'll make my decision easy).

I decided that I needed to just try it so that I could be sure I was making the best choice. If it seems under par, I'll ship it back and go with the Western Mountaineering bag.

Thanks again.

David,
If length is your only concern, you'll need to work on another excuse to send it back. I'm 6'1" and find it has plenty of extra room. You should be ok at 6'3" if their cut/fit is consistent.

Well, if it's not too short you'd better believe I'm going to be graphing thins out like Brett in order to make a final decision. ;)

Given that the fill weight of the Montbell bags is lower than that of the WM bags it really is too bad that the overall weight of the Montbell bags is higher! That is really what keeps me from buying them.

Unless you're hooked on the Montbell stretch feature I'd go for W.M. bags any day. Their qulity and light weight far offset any higher price they charge.

I love my W.M.Megalite bag for 3 season backpacking. Opened completely as a quilt in warm temps or zipped up and cinched down for 20 F temps I just love it for a good night's sleep.

Eric

I received the Montbell UL SS #2 in the mail today and…

…I'm very, very underwhelmed.

Firstly, I don't think this bag would really fit someone who is 6'4". It's too short for me and I'm 6'3".

That aside, the bag's down fill is *clearly* inferior to the Western Mountaineering bags. The Montbell bag's fill is somehow both thin (i.e. there isn't enough of it) and chunky (i.e. it doesn't spread smoothly within its bafles). It lofted up so much slower than the Western Mountaineering bags that I've been comparing it to that I was really taken aback. I'm surprised that so many posters here rate these bags so highly. Arbitrary temperature ratings aside, I don't think there is any way this bag will be warmer than the Megalite – the loft just isn't there. I don't think that the super stretch design will make up for this either, or that it any any way compensates for the fill inadequacies.

This is a surprising let down. The bag is definitely being returned.

Glad to read all this. We were considering the Montbell SS bag only because we could get the desired temperature rating with the desired shoulder girth. That option is now off the table. The real downer is that Western Mountaineering doesn't do custom work any more. All they'll do is over-fill. I have owned a WM bag for about 10 years and can attest to the very high quality of their line. I personally prefer the more robust Microfiber series to the Extremelite series because of the added water resistance and wind block capability.

If like me, you want to change the girth on a bag, Feathered Friends in Seattle ( http://www.featheredfriends.com ) will do full custom work for you. For example, they can change girth on any of their many bags in one inch increments or lengths in 6 inch increments but the $150+ cost is(IMO)prohibitive. Optional removeable down collars are also available for $50.

Nunatak ( http://www.nunatakusa.com ) will also custom-build a down bag for you, although their primary focus is their ARC quilts and garments.

Sorry to hear that, but I must agree with you David.
I did love the bag at first..till I actually used it on the trail.

At one time I attempted to sell my barely used bag here, it failed me on the JMT, but I thought well maybe I am a cold sleeper and I need a 0* bag. I'll sell this one and simply solve my problem.

I received a tip in e-mail shortly after that to check to see if my bag was under filled. It was way under-filled and a couple of spots have almost no down in them at all.!!!!

Hope you get better customer service than I did. I have been trying to get this corrected since April 2007.
Three letters, and I am ready to burn the bag and send the ashes to Japan for proper burial!!!
But…I will miss the wonderful creamy puff and the amazing stretch…but I need warmth!

Sorry Brett not trying to knock this product … perhaps David and I just got bad ones, but I am very disappointed.
I think I will buy only from REI from now on.

REI looks like they are carrying Sierra Design's Stretch bags (which appear to be a copy of the Mont Bell bags). Has anyone seen or tried them ? Are they a good copy ?

Thanks.

I'm finally posting some photos today before I ship the Montbell bag back. I think they get the point across. The difference in loft between the WM Megalite and UL SS #2 is pronounced:

David

Like the old saying goes: A Pictures Worth A Thousand Words…

You cant go wrong with Western Mountianeering

Thanks for posting the pictures

Will Rietveld discussed this "under filling" problem here: http://tinyurl.com/3kgc76

Last year I made a trip to the Boulder, CO store and found that a Montbell bag I was considering had the same problem.

In the end I went with a WM UltraLite. The WM is vastly superior in my mind.

Montbell's quality control for their bags and other gear has pretty much turned me away from their products in general.

David,

I am sure that you will be pleased with your WM bag. It is an excellent product as is the MB SS. Others considering MB SS bags will be well served if they attempt to understand some aspects of thermal physics, as it relates to down. For the same down fill power, aerial density, not loft, determines down’s insulation value. The MB elastic compresses the down resulting in higher aerial density, with commensurate lower loft. Although the aerial density to insulation value curve is hockey stick shaped, it is linear in the narrow range of density compression resulting from the elastic in the MB SS bags. I have tested the rating on the MB SS #2 and it appears to be accurate.

Richard,

Interesting info. You're more familiar with this sort of thing at a technical level that goes beyond me by a fair bit, so I trust what you're saying.

Still, even so, if I'm dropping a significant chunk of cash on a bag, I want it to loft well given that, without getting into technical modeling, I *know* it'll work.

That said, if I had the extra money, I'd be interested in testing out the Montbell bag's rating. Unfortunately I don't, so I find myself being conservative in my evaluation.

None the less, good to know.

Dave

I would just like to add that I have a SS#3 and I LOVE LOVE LOVE it!!!! I've slept it in down to 30 (-8) and been toasty. That's with some clothing on… but still… it's a great bag. I love it (did I mention that?). And this is the older version with the lesser quality down. I also wouldn't give up the stretching feature for anything!!! It makes a big difference to me to be able to have a warm bag with little dead air on the one hand while also being able to move around and not feel like I'm entombed like a mummy. And yes… just looking at the bag hanging up… it doesn't seem to have much down it in… but in use… it's very warm. To me anyway. I sleep warm. I never thought about the fact that it has more fabric because of the stretch feature… making it seem less lofty… but that makes sense.

I'm confused. Wouldn't higher density with less loft be just like a bag that hasn't fluffed up yet ? Doesn't letting the down bag fluff up make it warmer ?

Please enlighten us non-physics folks.

Thanks.

John,

The answer to your question is "No". You can increase the aerial density of 800 fill down about 2.5x before it is becomes less thermally efficient than fully lofted down. A compression sack will easily compress the down much more than this; so, it is important to let it loft up.

This same phenomenon applies to an earlier thread dealing with the diamond quilting on a MB women's down jacket resulting in less loft than the rectangular quilting in the same man's jacket. Again, it doesn't make any difference. MB, like WM, makes extremely well engineered and high quality products.

I'd guess the WM bags are easier to shake and fluff, and reposition the down where you want it.

Richard – Your comments on down compression not impacting the thermal efficiency until it is compressed 2.5x has really changed my thinking about clothing layers. I've always used very loose shells to minimize compression when used over insulation. What about synthetic insulation such as primaloft? Can it also be used under a snug windshell without limited compression compromising the insulation value? Thanks Richard, your posts on insulation are a tremendous help.
Ron

I also am interested that you think you can compress fully lofted down 2.5 times without loss of insulation value. Can you supply references for this?
I imagine fully lofted down only occurs in a open tub with no enclosure at all so this situation is not practical, but still 2.5 times is an awful lot. What ratio can you compress practically enclosed down? Do you have a reference for that?