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I am searching for a down sleeping bag and found two alternatives. They are from one manufacturer so data should be comparable.

  1. bag ("A"):

    • T_limit = -13°C
    • weight = 1000g
    • filled with 600g of 850cuin goose down
  2. bag ("B"):

    • T_limit = -17°C
    • weight = 1170g
    • filled with 700g of 850cuin goose down

So far, so good, numbers make sense in my opinion.

But what is very interesting now is the fact, that A is made in trapezoid construction and B is made in V-chamber construction.

Is this already considered in the temperature rating? And should I always prefer V-chamber over trapezoid? If so, why?

  • The temperatures given are consistent with the usually observed values for that type of filling. Cumulus doesn't follow EN13537 for temperature ratings though. Do you know what temperatures you would be encountering? Do you know how you usually compare to given temperature ratings? – njzk2 Jun 18 '16 at 21:09
  • @njzk2 Good guess I was speaking about Cumulus bags. I can't say for sure which temperatures I am encountering. I only own an old synthetic bag (maybe 7 years) which is rated -5°C and I store compressed. I guess it's not insulating that good anymore and it's heavy and big. Still recently we had -13°C in the morning, it wasn't a pleasure sleeping in it but it was OK. So let's assume the coldest nights I will use my new down bag will be -20°C but more regularly -10 to +5°C. – Wills Jun 19 '16 at 9:37
  • If its Cumulus then have you read their explanation page? sleepingbags-cumulus.eu/uk/menu-dolne---uk-ii/construction And also compare the overall measurements, they are close but might be a difference for you – Erik vanDoren Jun 20 '16 at 15:45
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The answer to "it is considered in the temperature rating" can be yes, no and maybe... Really it depends on the manufacturer and how they test it. It also depends where the manufacturer market is: in some part of the world there are some standards and in others there aren't.

One of the ways to find the rating is to test in a controlled environment with a mannequin wired with heat sensors and warmed with an electric current. They base the rating on the measure of electricity required to maintain the mannequin at a constant temperature. On top of that a manufacturer might conduct some field tests to end with a relatively realistic rating. If tested in that way the presence, or not, of cold spots would affect somewhat the results so you could say that the construction is taken into account in the rating.

Other manner of testing could be, completely or in part, based on simple number crunching and in that case it would be difficult to really account for construction unless they do so presuming a "fudge factor" dependent on it (its up to you in this case to decide if this kind of testing is of any value in making a choice between models).

Concerning construction in itself you have various configurations, even the same manufacturer will use different ones depending on the kind (simple and economical vs the more expensive and sophisticated) and usage (summer, winter, extreme etc).
An evolution of the H chamber (the box construction) is the trapezoidal chamber, if you keep going on evolving the trapezoidal along the same line you end with the V. The intent is to reduce the possibility of cold spots along the stitching lines. Even more sophisticated is a brick construction where two layers of boxes alternate (like bricks) but that is complicated and cost more than increasing the fill in a V.

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Comparing ratings and construction of sleeping bags of different manufacturers for the same classes of product can become an useless exercise if the testing methods are not the same. It does, however, become more meaningful when you are comparing products of the same type and from the same manufacturer as it makes sense to presume that they used the same testing methods for both (although a manufacturer could conduct different or less testing for their economical line) Usually one of the things a manufacturer would tell is that in rating their products they assume the use of sleeping pad and tent, obviously the influence of cold spots would be rather different if you were in the open with just the bag directly on the ground.

PS: to clarify V vs trapezoidal chambers after the comments. The baffles (those strips of material that form the chambers) have the function of helping maintaining loft and stopping the down to just pile up wherever it wants (but allowing moisture to move through). In the simplest construction you stop the fill from wandering sewing through (think a quilt) but where the stitches are there’s nothing, no fill and if some fill is there is pinched down by the stitching so no loft. That becomes a cold spot, simply there is no insulation. Now give some room and add a baffle completely vertical (the box construction), you have loft. the fill is kept in place but each baffle is stitched to the top and bottom faces and those stitching lines are vertically aligned. While insulation is much better than before that alignment still represents a cold spot. The solution to this is offsetting the stitching: instead of being one right on top of the other you can build other shaped chambers slanting the baffles, now the stitches are offset which is good but the structure (since the baffles are slanted all in the same direction) doesn’t help the loft much. Now if you build a trapezoid the the baffles are still slanted but in alternate directions: that structure actually helps supporting the loft (think cards castle) and the top stitching is offset to the bottom one. When you get to this trapezoid shape the element that still remains with room for improvement is the amount of offset of the stitching and that happens when you bring the baffles to form a VVVV. Now the bottom lines of stitching are right in between the lines of stitching of the top stitching and you have maximized everything you could in this kind of construction. Obviously the V chambers bring the baffles to a certain degree of slant, while a trapezoid is a bit more vertical, if you have little fill by design then that slant can be so much to compromise the help that the baffle structure gives to maintain the loft unless you start making a lot of narrower chambers, which adds weight and complexity and a lot of stitches (stitches don’t keep you warm). Everything depends on its use and there’s no point to make a sleeping bag structure very complicated when there are more cost effective solutions. These are details that only the manufacturer can quantify and tell you about, you hope they did their R&D on that. At one point things become a compromise to stay viable products. Some manufacturers will go as far as using a mix of chambers in their bags where the more loft and V chambers are on the core and feet and a bit less loft and trapezoidal chambers are for the area in the middle.

Not all the manufactures build their product the same way, with the same size chambers, same materials etc etc. Even if they are all sleeping bags you end trying to compare apples and oranges if you are in front of two different products from two different manufactures. The variables to consider can become a lot and at a certain quality level its a lot of work for nothing. In that case the only hope becomes testing each product to compare results and if one is lucky somebody else has done all the work (in the proper way) and published the results.

You dont have this problem, you are considering two line of sleeping bags from the same manufacturer, and the products are rather similar. Once you understand the construction, and how you will use the product, the other things are money, remaining gear, quality and packability (compresses a lot when packed and returns to proper loft when you unpack it, doesn’t lose loft overtime), is the manufacturer conservative or not in its ratings, and your personal preferences (you can be a "hot person" or a "cold" one, and what kind of sleeping system that bag is gonna be part of.

Lastly just to not make it easy: dawn is good, packs well, keeps you warm and cozy... while its dry! The whole thing changes if the sleeping bag gets wet, so the conditions you use it have their importance.

  • Thx for this good answer. I am living in Europe so I thought there are standardised testing methods. Therefore I assumed it should be possible to compare. – Wills Jun 17 '16 at 15:21
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    @Wills, yes in Europe there is EN 12934 for what concerns the denomination, class and quality of the fill and EN 13537 for what concerns the rating. However the second legislation is not mandatory and manufacturers dont have to follow it if they choose to not do so (theres the issue of them opening the door to "false advertising" but that can be taken care in other ways, and sometimes some dealers import products from different markets). In your case you are comparing two products of the same manufacturer and both rather similar so I guess you dont have to worry too much – Erik vanDoren Jun 17 '16 at 18:39
  • thanks again, very helpful information. I guess you worked in the industry or at least you know it well :) – Wills Jun 17 '16 at 18:53
  • @Wills, no I dont work in the industry. – Erik vanDoren Jun 20 '16 at 11:57
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    I got this answer from the bag manufacturer: "Up to 700g trapezoid shape is more efficient. It is not like: that chamber are better than another one. One of them work better with different amount of down and another one with different." – Wills Jun 20 '16 at 12:25

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