Sleeping bags and fill weight

Yes, if they use the same fill power of down.

Agree with Richard. And at the risk of stating the obvious, the two should be similar in size (length, girth) as well.

That combination provides an 11F sleeping system.

11 ounces of 800 fill is the norm for an accurate 30F rated bag. Some vendors are conservative in that they provide up to 13 ounces for this 30F rating. The extra down will compensate for wind and a thin pad but are not required for the scenario you stated.

The clothing will add 1.90 clo to your sleeping system. This will yield a total clo value of 7.78 which is typically an 11F system. These temperature rating assume you are an average 30 year old male in addition to the other variables which you explicitly specified.

I would be very cautious about theoretically sound but practically untested scenarios in (sleeping) warmth. As many have mentionned before, a lot of factors (hydration, food, pad, tent/tarp bag, age, gender etc.) play a role in determining personal comfort. I have used a 30F bag (WM) in addition to a micro-puff pullover, baselayer and rain gear with a full length sleeping pad and a thin foam pad and was JUST OK at 30F in a tent and after eating a warm meal. It really all depends.
:-)
Sven

I am all for standard measurements as a starting point. Given that bags — even mummy bags — come in a myriad of different sizes and shapes and construction methods and liner material and shell material — taking a mathematical formula — such as the number of ounces of down — is inadequate.

I see myself as a pretty average sleeper — neither sleeping overly hot or cold. My MontBell No. 3 bag (rated 32F) has only 9.5 oz of 725 fp down — nothing near 11 oz of 800fp! And yet, over the last four years, I have slept oh-so-comfortably in this bag down to 30F or thereabouts. MontBell claims that the somewhat elastic nature of the bag adds to its efficiency – and a straight mathematical equation as above does not take into account bag construction.

As I mentioned in another post, I believe that a good standard measurement should contain two components:

1. A standard, objective loft measurement
2. A standard, objective mechanical (eg mannequin) test

Loft affects warmth in a very direct way. This is simple to meausre (once defined into a standard) and anyone can verify the numbers.

A mannequin will never reflect our individual physiology and conditions — but so long as the same mannequin test is applied to all bags, then we will have a good starting point with which to compare across different brands and bags. In other words, a standard 30F bag may or may not keep us warm at 30F — but it will still be wonderful as a starting point to know that a Western Mountaineering 30F bag will provide the same warmth as a Slumberjack 30F!!! Right now, without any standards, such a comparison is just a fantasy.

Hi David:

I should clarify that my disagreement above isn't with you or anyone else in particular — but with the general concept of using one mathematical criterion for determining bag warmth.

Be it Montbell's method or the EU's method, I am happy with either — but just hoping and wishing and pining for one standard (whatever it is) to be applied uniformly.

Yes, we are in the same wavelength — just want to clarify what I am disagreeing with. :)

Hi Guys-

I wanted to chime with a couple of notes.

I agree with Ben that a loft measurement and some standard warmth rating would go far to supplying a baseline warmth assessment of a bag. As far as BPL ratings go, we have some additional considerations:

1) We need a system that is uniform and easy for individual reviewers to apply to the bags, even bags of widely different construction — hence our single-layer loft policy where we measure and publish the loft on the top of the bag or quilt.

2) While I’d love to have the capability to do heated dummy tests on all the bags we review, the truth is that we just don’t have the resources to have the needed equipment in-house, and labs like those at KSU are too expensive to use for all the bags we test. (FYI, We do have a project in the queue with KSU, but this is a special case…) ;-)

3) Environmental, physiological, and bag construction variables will inevitably cause enough variability to swamp any high-precision dummy/warmth test — hence again, our imperfect-but-best-we-can-do-with-what-we-have single-layer loft policy.

I do appreciate everyone’s input and advice in this area and hope and expect that eventually we will be able to migrate to a more rigorous warmth rating system.

BTW, if you haven’t seen it, our rating policy can be viewed here.

Cheers,

-Mike