Speaking of Loft (Sleeping Bags)

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

Tom

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.

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.

http://www.backpackinglight.com/cgi-bin/backpackinglight/bpl_sleeping_bag_position_statement.html

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 *

1.2……………50……………52
1.5……………40……………40
1.8……………30……………28
2.2……………20……………12
2.6……………10……………( 4)
3.0……………. 0…………..(20)
3.5…………..(10)………….(40)
4.0…………..(20)………….(60)

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

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.

>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.

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.

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.

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.

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.

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

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! :)

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.

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.

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.

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.

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"

Agreed!

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.

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.

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.

Some would add evaporation to that list. In one sense not "classic", but for our purposes, important to keep in mind – don't want to sleep too warmly and end up sweat soaked.

I think that is a really good point, PJ. I think a lot of people that "sleep cold" are actually wearing too much clothing to bed and in reality are sleeping too hot.

How come no-one makes continuous fibre synthetic fill with a reinforcing ripstop pattern so that its is strong enough to use as a blanket on its own: it could then be used within a windproof (vapour) bivi with out the overhead of a shell to be quilted to. Also, any rips would be visiable and repairable.

Alternatively, could cut synthetic fill up into chunks and place loose in baffles like down: would this not make it last longer (like down)?

Paul-You are correct. I meant sleeping bag heat transfer. I just updated my post to preceed "heat" with "sleeping bag". In this context, heat is actually generated if any moisture condenses in the sleeping bag… not that we want that.

The variation in a bag's perceived warmth, assuming the same environmental variables, is primarily determined by the individual’s basal metabolic rate. It can be simply calculated (inputs are age, sex, weight, height) and the maximum variance is ~14%. There have been prior forum posts by me explaining this in much more detail. The EN13357 standard, and most other International insulation requirement standards, are based on an average 30 yr old male and female. I created this chart to show why.