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Speaking of Loft (Sleeping Bags)
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Ben 2 World
(ben2world) - MLife

Locale: So Cal
Speaking of Loft (Sleeping Bags) on 01/09/2007 12:38:06 MST Print View

Is there a "table" out there that gives a general indication of loft (in inches) required for a given temperature for the "average" sleeper?

I think having the above would be an excellent first step. Let's say that 3 inches of loft is required for an average sleeper at 32F, 4 inches at 20F, and so on and so forth. Knowing that, then all I have to do is to determine whether I am a cold, average, or warm sleeper. And if I know (for example) that I am a cold sleeper who needs an extra inch of lofting, then going forward, I can shop for bags by objectively measuring loft -- adding or subtracting from the standard as appropriate to me -- and thereby free myself from relying on the almost-useless manufacturer warmth ratings.

So does anyone know if this kind of information exists? Funny, this brings us full circle back to the days before manufacturers rated their bags by temp!

Edited by ben2world on 01/09/2007 12:43:00 MST.

Thomas Knighton
(Tomcat1066) - F

Locale: Southwest GA
As a matter of fact.. on 01/09/2007 12:44:03 MST Print View

Actually, I came across such a table here on BPL yesterday. I just wish I had saved it :(


Casey Bowden
(clbowden) - MLife

Locale: Berkeley Hills
Loft vs. Temperature Rating on 01/09/2007 13:00:51 MST Print View

Ben and Tom,

Ray Jardine uses the following formula:

ETR = 100 - (40 x T)

where ETR is the estimate temperature rating in Fahrenheit and T is the loft in inches.

Thomas Knighton
(Tomcat1066) - F

Locale: Southwest GA
Here it is on 01/09/2007 13:15:03 MST Print View

This is BPL's position statement on sleeping bag temperature ratings. While I don't think it's meant to be a solid thing, it's the only type chart I've ever seen.

Edited by Tomcat1066 on 01/09/2007 13:16:03 MST.

Ben 2 World
(ben2world) - MLife

Locale: So Cal
Re: Here it is on 01/09/2007 13:41:26 MST Print View

Thanks, Casey and Thomas!

I copied BPL's "Loft-Temp" table and also computed same using Ray Jardine's formula. The two are very close in warmer temp ranges but diverse alarmingly as temperature drops!

..............Temperature Rating
Loft (in).....BPL.....R Jardine *

2.6...............10...............( 4)
3.0................ 0..............(20)

* Ray Jardine's formula: ETR=100-(40xT)

Edited by ben2world on 01/09/2007 13:47:30 MST.

David Wills
(willspower3) - F
Re: Speaking of Loft (Sleeping Bags) on 01/09/2007 14:02:00 MST Print View

There can be very large differences in loft and warmth provided by synthetic insulation. If calculated by known clo/oz values for insulations, and manufacturers claimed loft and weights, the clo (warmth) per inch of loft would be as follows:
Climashield XP:2.57 clo/in loft
Climashield HL:2.45 clo/in loft
Primaloft Sport:~3.6 clo/in loft (inconsistant weight to loft ratios)
Polarguard 3D:~2.2 clo/in loft
Polarguard Delta: 2.45 clo/ in loft

These were calculated by this formula:
(clo per oz x oz per yard) / claimed loft

All that means is that a primaloft bag will require far less loft (not neccessarily weight) than continuous filament bags (which are all similar) for the same warmth. It comes out to about 68% more clo units (warmth) for the same loft.

I have personally used my bag made of 1.2" primaloft down to 32* before discomfort without anything more than fleece clothes. It's about the same temp I could use my buddies 2 layer (1.7" or so) P3D ray way quilt. I sleep a bit warm.

Hope this helps and doesn't confuse.

Edited by willspower3 on 01/09/2007 14:03:16 MST.

Douglas Frick
(Otter) - MLife

Locale: Wyoming
Re: Re: Here it is on 01/09/2007 15:11:38 MST Print View

>The two are very close in warmer temp ranges but diverse alarmingly as temperature drops!

Ray and Jenny have been using a quilt made with [EDIT] two layers of 0.9" Polarguard 3D (in addition to wearing some insulated clothing) for the last two months in temps typically from 5F to -20F. They reached the South Pole yesterday.

My Ray-Way quilt made with [EDIT] three layers of 0.75" Polarguard 3D is good to about -10F, when supplemented with insulated clothing (Patagonia expedition-weight capilene or R.5 shirt, MicroPuff pull-over, and BMW Cocoon pants).

EDIT: Sorry, David, my mistake. My quilt has _three_ layers, not two. That gives it a RJ +10F rating. Further, I stand corrected on RJ's quilt--it has _two_ layers. I was remembering the 3-layer quilt they made for the Antarctica trip, but they didn't take it.

Edited by Otter on 01/09/2007 19:31:37 MST.

David Wills
(willspower3) - F
Re: Re: Re: Here it is on 01/09/2007 16:57:13 MST Print View

His site says 2 layers of .9" 3D, while he says it is comfortable because the inside of his tent is above freezing due to greenhouse effects of night time sun.

you must have a very high metabolism to take a 2 layer quilt claimed by jardine to be 40* (as conservative as it is), down to -10. I used a 3 layer quilt with VB clothes, 200 weight fleece jackets, down vest, 200 fleece pants, and wind pants down to snowy, windy, 10* in a shelter, but found that to be about the comfort limit.

First Last
(snusmumriken) - F

Locale: SF Bay Area
Re: Speaking of Loft (Sleeping Bags) on 01/09/2007 19:58:06 MST Print View

If you look at the specs for Western Mountaineering bags their loft to temperature ratings in the Extremelight series stacks up as follows:

2.5 inches - 45 degrees
3 inches - 40 degrees
3.5 inches - 35 degrees
4 inches - 30 degrees
5 inches - 20 degrees
6 inches - 10 degrees
7 inches - 0 degrees

This looks different from both Jardin's method and BL because here we're talking the full loft of the bag, not just the loft above you.

As many people on this board seem to agree that Western Mountaineering makes some of the best bags around, their loft vs temperature rating should be fairly accurate.

Edited by snusmumriken on 01/09/2007 20:01:17 MST.

James Pitts
(jjpitts) - F

Locale: Midwest US
Re: Re: Speaking of Loft (Sleeping Bags) on 01/09/2007 20:27:56 MST Print View

Design plays a big role as well.

I have a 30 degree bag that has 4" of loft (I have measured it) but it has a sewn-through design.

I have a 30 degree bag that is fully baffled and I swear I could sleep down to 20 in it.

(dealtoyo) - F

Locale: Mt Hood
Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 02:11:42 MST Print View

IMO the method of loft vs temp rating used by Western Mountaineering is more for marketing reasons rather than actual function. My Nunatak Arc Alpinist has 2.5" of loft and only covers the top of me. By WM's math my bag is only a +45 degree bag, simply not true.

Since down is compressed by your body while laying on it, the only thing that will keep you warm under your body will be the R-value of your sleep pad. That's the reason BPL and Ray Jardine only look at the loft that covers the top of your body. Which would place my bag at about +20 degrees.

John S.
(jshann) - F
Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 07:45:48 MST Print View

You simply cut the WM lofts in half for an approximate of what they are posting above.

James Pitts
(jjpitts) - F

Locale: Midwest US
Re: Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 10:15:59 MST Print View

That's one way to look at the compression of down under a bag. The other way is this: The down is only compressed in places where your body is near or contacts the ground. In other places (all the nooks and crannies) it is fluffy and insulating. So in a sense a sleeping bag is a quilt with a "perfect fit". The amount of down that gets compressed at any one time in a bag is actually fairly minimal given how perfect the fit of the insulation is.

Now that ought to draw some heat! :)

David Wills
(willspower3) - F
Re: Re: Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 13:46:20 MST Print View

I think minimal may be optomistic, but for really cold weather , 20* and less, a topbag with one layer of insulation on the bottom would be pretty efficient unless you have the benefit of a DAM. Good observation.

Mitchell Keil
(mitchellkeil) - F

Locale: Deep in the OC
Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 14:36:02 MST Print View

Haven't we forgotten the cut of the bag and its impact on how cold or warm a sleeper feels in the bag? As James pointed out above, stitch through vs. baffled makes a difference, well so does the cut of the bag and how much volume the sleeper has to heat under that loft. Its no wonder that BPL has delayed the much anticipated article on this subject. There are so many variables that coming up with the nice simple table Benjamin wants may well be impossible though desireable. We can argue till we are all blue (from the cold) in the face and still we will not arrive at any agreement on whether 2.5 inches of top loft will keep one warm at 30 or 25 or 20 or 15 degrees.

(dealtoyo) - F

Locale: Mt Hood
Re: Re: Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 14:52:50 MST Print View

No heat James, just more about my technique.

Instead of packing the weight of down for the underside of my body, I would rather carry a slightly thicker pad to boost the R-value of my sleep system. If you are a restless sleeper I'm sure the down in the bottom of you sleeping bag will serve you well when you're tossing and turning. This is not a concern for me, I'm a back sleeper.

I also bring an insulated jacket with me on every trip. If it is cold enough, I will wear it to bed. The jacket will fill most of the nooks and crannies, and doesn't add to the weight of my pack.

Back to the question at hand. Ben, there appears to be no answer for you in the near future. There are too many views and even more types of sleeping bags and methods of manufacturing. The other problem is how you camp(tent, tarp, hammock, open air) and what type of ground insulation you use(or more importantly the R-value). All are factors in how warm you sleep in any given bag. Until there is a standard in how all of us camp, their can't be a standard for how we veiw the warmth of a sleeping bag. The only standard is trial and error.

James Pitts
(jjpitts) - F

Locale: Midwest US
Re: Re: Re: Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 15:25:25 MST Print View

Sleeping posture plays a role here as well. A side sleeper compresses a lot less insulation beneath them than a back or stomach sleeper. So someone sleeping on their side would be getting more use out of their down and less from their sleeping pad than someone sleeping on their back or front.

I agree with you comments, Duane, and I think the core of the problem is that we are trying to align an objective measure (a "rating") to a subjective evaluation ("comfort"). This is probably a marketing persons dream and a consumers nightmare.

Edited by jjpitts on 01/10/2007 15:26:03 MST.

Mitchell Keil
(mitchellkeil) - F

Locale: Deep in the OC
Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 16:52:33 MST Print View

James wrote, "I think the core of the problem is that we are trying to align an objective measure (a "rating") to a subjective evaluation ("comfort"). This is probably a marketing persons dream and a consumers nightmare"


Perhaps this may be the place for another Thread, but I am wondering about the testing and reporting thereupon that is done in reviews of bags. It has always been something I have scratched my head over. When a bag is tested by a reviewer (even here), what are the actual conditions under which the bag is tested. What I mean: Does the reviewer sleeep naked or clothed and what is worn? Does the reviewer sleep in a tent? If so, what kind? What kind of pad is used by the reviewer and its R value? How often does a reviewer wake up during the night because of the bag's warmth or lack thereof? I know that ratings in Europe use a stainless steel dummy for testing purposes, but I have few friends mad of stainless steel. So, a bit more explanation of the actual conditions of the reviewers' test would be appreciated. Maybe then we could begin to sort out the hype from the truth about ratings.

Edited by mitchellkeil on 01/10/2007 16:54:03 MST.

Jason Brinkman
(jbrinkmanboi) - MLife

Locale: Idaho
Re: Re: Speaking of Loft (Sleeping Bags) on 01/10/2007 23:14:05 MST Print View

A few points:

1. Most all sleeping bag manufacturers list full bag loft (2 layer), even though the bottom layer is compressed beneath you. The BPL and Jardine table/formula is for top layer (1 layer) loft, which is the effective loft of a bag. The BPL table states that it is derived from a few top tier manufacturers, and in my experience is quite accurate for fully baffled down bags (not sewn thru).

2. As previously pointed out with the "clo" readings, the table/formula will vary for synthetics. I have theorized that this must have to do with insulation density (size of air voids).

3. With all insulations, the shell fabric air and vapor permeabilities should also affect felt warmth.

Edited by jbrinkmanboi on 01/10/2007 23:37:49 MST.

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Re: Re: Speaking of Loft (Sleeping Bags) on 01/11/2007 00:31:47 MST Print View

Relative to point 2:

Sleeping bag heat is lost through three transfer mechanisms.
-Natural convection heat loss is negligible for all insulation types.
-Conduction heat loss is inversely proportional to the thickness for all insulation types.
-Radiation heat loss is inversely proportional to the majority fiber sizes down to about 2 microns (not voids). This is the reason why down and Primaloft are significantly warmer, for a given loft, than Polarguard.

If available, the EN 13357 rating is the most accurate way to determine a bag's relative warmth.

Edited by richard295 on 01/11/2007 09:49:33 MST.