Lightweight Summertime Sleeping Bag
Posted: 09 January 2008 05:08 PM   [ Ignore ]  
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I would appreciate some critical comments on this plan for constructing a lightweight summertime bag (50 degree F) for hiking in the Southern Appalachians.  I currently own a WM Caribou 35-degree bag (sewn-through construction) and Phantom 45 (baffle construction). I am impressed by the weight for each but wanted something for warmer weather. Even the Phantom is a bit warm with temperatures in the 60s and up.

I sleep on my back and side so I wanted to make the width larger to compensate for side sleeping.  My calculation suggests a 50 degree F bag needs 1.2” loft.

PLANS:

Size:  72” (length) x 36” (shoulder width) x 22” (footbox width)
72” zipper down the side
Bottom: 1-layer of momentum 0.9oz taffeta
Top&Sides;: 1-layer of momentum 0.9oz ripstop on top and momentum taffeta on inside of top
Sewn through construction with stitch lines every 6”
Insulation:  ~3.5oz of 800-fill down ONLY on the top (i.e. not bottom)

If I did my calculations correctly the material would weight 5oz, down weighs 3.5oz and zipper at 1.25oz.  Counting the thread and overage I end up with a summertime bag of ~10 oz.  If I can talk AYCE to sell 3.5oz of down (rather than 3.0) then the entire project would cost $88.

Having only sewn the Liberty Pants and Liberty Shell, I would appreciate critical comments and other recommendations before I attempt this project.

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Posted: 09 January 2008 09:33 PM   [ Ignore ]   [ # 1 ]  
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A quick calc based on your dimensions makes me think you determined the amount of down by figuring the volume and dividing by 800 cubic inches/oz.  Common wisdom is that you want to use at least 10% more than that.  Somewhere I read that the better bag makers use 15%-20% more.  I can say from experience that a quilt using 1 inch baffles and NOT overstuffed did not keep me (a relatively warm sleeper) happy at 50*F.

Also, unless you are “twiggy-thin”  (anyone hear old enough to remember Twiggy?), I’m not sure 36” wide insulation will do the trick .... try folding a blanket to the dimensions you have in mind and sleep under it on a cool night to confirm.

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Posted: 09 January 2008 11:25 PM   [ Ignore ]   [ # 2 ]  
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I have a Montbell #7 bag rate at 50F. Even in a 5’10” length it uses 4.5oz of 800+ down (5.0oz for 6’4”), so you may be cutting it a little slim on the fill.

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Posted: 10 January 2008 12:59 AM   [ Ignore ]   [ # 3 ]  
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Thanks for the comments.  The way I calculated the needed down is:

1) Compute Square Inches on Top part of bag as 72 * (22 + ((36-22)/2))  = 2088 sq in.
2) Compute total square inches based on 1.20” loft as 2088 * 1.2 = 2506 sq in.
2) Compute 800-fill down as 2506 / 800 = 3.1 oz
3) Compute 110% as 3.1 * 1.10 = 3.4 oz of down

Does this seem right?

Your experience with 1” baffled quilt at 50 F concerns me and my plan. It also raises another question regarding the shell/liner material.
Heat is kept in the bag by the loft and minimum heated air escaping the bag.  There needs to be some air escaping so body moisture escapes.  I can use a less porous shell/liner to trap more heated air at the expense of trapping in more moisture.  Right?  I have not seen this discussed but this is the concept of the $1 emergency blankets.

If your Montbell bag uses 4.5oz is this amount spread evenly on the top AND bottom?  If so, wouldn’t that work out to be about 2.25oz on top for a 50 degree bag?  (discounting down on the sides)

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Posted: 10 January 2008 01:32 AM   [ Ignore ]   [ # 4 ]  
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if you are making a topbag, you will want to have about 65-70% of the total circumference at the shoulder to be insulated.  I am 6’1, 180 lbs. my 2.5oz xp topbag (formerly 11oz quilt, now with a total of 5 different fabrics on it, quite the frankenbag) has 48” of insulation at the shoulder and is just big enough to not allow drafts.  I think it has 18” wide uninsulated nylon on the bottom at the shoulder, tapering to the about 8” at the ankle where I have insulation for my feet on the bottom.  It now weighs 14 or 15 oz but is much much warmer than the quilt was.  if you are very concerned about staying cool, a quilt is totally the way to go.  Also, the 2.5 XP is comfortable for me to sleep under in the low to mid 60’s down to 40* or less depending on clothes, and packs down to 3 or 4 liters.  You could even go with 1.8 oz primaloft for an even lighter bag without the hassle of down.

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Posted: 10 January 2008 11:10 AM   [ Ignore ]   [ # 5 ]  
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Thanks for the comments.  The way I calculated the needed down is:
...
Does this seem right?

Your arithmetic is right, I misread your original note and was working with 1.5 inch baffles.

Your experience with 1” baffled quilt at 50 F concerns me and my plan.
That experience was with the quilt inadequately stuffed.  It does much better 20% overstuffed


It also raises another question regarding the shell/liner material. Heat is kept in the bag by the loft and minimum heated air escaping the bag.  There needs to be some air escaping so body moisture escapes.  I can use a less porous shell/liner to trap more heated air at the expense of trapping in more moisture.  Right?  I have not seen this discussed but this is the concept of the $1 emergency blankets.

That would make the bag somewhat warmer at the risk of being uncomfortably moist.  I slept warm enough but quite unpleasantly humid a few 50F nights last spring in a Adventure Medical Kits ThermoLite Bivvy Sack.  That would be fine for an unplanned bivouac but I’m not gonna choose that for planned overnights again, YMMV.  But you definitely want the outer shell to be able to pass water vapor well to allow water vapor to leave the bag when you hang it out to dry.

Willspower3 is correct about the insulated part needing to be wider than 36 thru your torso.  Also, are you sure a 72 inch length is adequate?  Even with a boxed foot (as in Jeremy’s quilt instructions here at thru-hiker), I found that the quilt needs to be cut 6 inches longer than my height because:


* we are longer when horizontal than when vertical
* our feet naturally point away from our head when not standing up
* it needs more length to be able to drape over the top of our head

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Posted: 10 January 2008 02:37 PM   [ Ignore ]   [ # 6 ]  
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Thanks willspower3. I had not heard the term ‘topbag’ before your post.  I searched around and found several references to the “Quantum” bags with straps on the bottom.
Yes, I am interested in making a ‘topbag’ but with an outer shell bottom rather than completely open.  My logic is the bottom shell will keep the top from falling off me at night and generally in a position of covering my body better than a truly open ‘topbag’ or quilt.  The bottom shell also keeps little critters out too.

Jim, thanks for the comments on bag sizes and experience with the thermolite bivy sack.  My questions on the shell porosity relate to the different shell and liner materials.  The emergency blankets have 0% porosity and thus, keep moist air (and heat) in.  Other materials will presumably have different ranges of porosity, and thus, will be better (or worse) on trapping heated air (and the moisture).  I’ve never seen such ratings on the Momentum ripstop or taffeta material.  Have you?  Is my logic right?

I’m 5’11” and always sleep with a closed-cell foam pad (e.g. Ridge Rest).  I never sleep with a hood over me or around my head unless its very cold (e.g. < 45) or the bugs are REALLY bad and I’m in a shelter.  I had estimated the 72” length would be adequate for covering my head if the bugs were bad (won’t worry about the cold since this bag was meant to be a summer bag only).  But your comments have made me remeasure my other bags to see what will work.

Willspower3, I do like the XP option (should be easier to work with than the down.)  But I don’t understand the clo versus loft relationship. I calculate a 1.2” loft for a 50 F bag.  AYCE lists the climashield XP as:

”  2.5 oz/sq yd: @.6 ” loft. clo=.82/oz. Total CLO : 2.5 oz * 0.82 clo/oz = 2.1 (approx temp rating 40 degrees)  “

Is the .6” loft for XP equivalent to the down loft?  If so, how can a 1.5” down loft requirement for a 40 F bag be equal to a .6” XP loft?  What am I missing here?

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Posted: 10 January 2008 05:59 PM   [ Ignore ]   [ # 7 ]  
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I dug back into the gear archive and pulled out one of the old moonstone (@2.0 oz basis weight) liteloft liners from the early 90’s so that I could relate better to the dimensions you’re suggesting.  Folks will remember these Moonstone liners from the excellent 800 zone series.  So many people bought these liners in 800 fill down on closeout from REI-outlet in 2001 and made various incarnations of quilts with them.  The width at the shoulder is 30”, even smaller than the 36” you suggest.

Back in July ‘94 I was at a hostel stayover in Delaware Water Gap and hand-sewed a zipper and ripstop to the bottom of the the liner in the picture.  I sent my bag ahead a couple of resupplies.

Was it adequate: no.  I was glad to get my bag back- it was cold even in the summer.  I move around too much in sleep and semi-sleep to make that narrow of a bag work. If I were to go down that route again (I’m not- I use a 14 oz full mummy bag even in summer), I would change the dimensions and figure out a way to fix the thing to the ground so that it stays in a fixed geometry.

The liner-to-quilt folks from the 800 series liners came to the same conclusion.  I don’t know of anyone who managed to come up with anything workable without significant modifications, but some of the end products were pretty interesting.  The internet has a long memory- you could probably come up with the original discussions with the right combinations of keywords.

You got some really excellent advice from everybody in this thread.  I think the best chance for you to get what you want would be to build in a way to change the design after use feedback.  Note that willspower3 mentions the ‘frankenstein’ quilt made from adding bits to get closer to what he wanted.  Also, with so little fill there’s not much weight advantage to using down.  As mentioned earlier, the lowest basis weight insulations would probably be about the same weight, be a lot easier to work with, and allow you flexibility to tweak after you’re done.  And to answer your question, no- loft is not equivalent between synthetics and down.  There’s so much that has been discussed RE this topic- search on what CLO is or look for the posts with the highest ‘view’ counts to learn more.

By starting with a size so narrow it’s likely to be inadequate and then adding bits until you reach adequate, you’ll probably end up with a pattern at the lowest weight.  But if you take a work-in-progress bag like you’re planning, even in the summer, take your regular bag to fall back on.

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Posted: 12 January 2008 10:05 PM   [ Ignore ]   [ # 8 ]  
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Thanks for the critical comments and the historic Moonstone liner revisit.  I have about been talked out of this effort.  The insulated width requirement issue is a real killer.  After taking the advice and trying some different pieces in a sleeping position I came to the conclusion that at least 42” would have to be covered by down at the shoulders.  I might still end up with a 11-12 ounce 50 F mummy bag by leaving the down out of a ‘long triangular’ shaped piece at the bottom of the bag.  This, of course, would assume that I still use down as insulation and that a sewn through construction would be adequate. 

I did find several posts on another forum to help understand the clo-temperature relationship and better interpret those listed here (I seem to always obtain 0 results from any search on this web site).  I also found a bag (i.e. the SleepLight bag) with essentially the same construction concept at http://www.gossamergear.com/cgi-bin/gossamergear/sleeplight.html

Has anyone had positive experience with a sewn-through construction in a mild temperature bag (e.g. > 45 degrees)?

Finally, what would be the estimated size of a compressed 40-50 degree F bag using either Primaloft or XP?  (for comparison my Phantom 45 is 6” x 10”.  I use a small pack—the Conduit.)

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Posted: 12 January 2008 11:40 PM   [ Ignore ]   [ # 9 ]  
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i use a sewn-through bag in the summer (Lafuma Warm Lite 600 40deg, got it cheap, it’s ok) and i find it warm most of the time, just right sometimes and haven’t been cold yet (within the realistic limits of a 40 deg bag) (i cut out the zipper & use it as a quilt)

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Posted: 13 January 2008 12:34 PM   [ Ignore ]   [ # 10 ]  
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CLO insulation comparisons: 3D, Delta, Primaloft, XP Combat


How much insulation do I need for a 40* quilt


5 oz XP for 20 degrees


Sewn through works fine for something 40 degrees and up.  If you *want* to baffle and can pull it off, I see no reason why you shouldn’t, but don’t let it get in the way of the most important factor: your pattern.

willspower3’s post RE his 40-50 degree XP quilt (one layer 2.5 oz) gives a stuffed volume of 3 liters.  Primaloft, esp PL1, compresses smaller but is very hard to work with unless prequilted for large panel items like sleeping gear.

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Posted: 04 August 2009 12:25 AM   [ Ignore ]   [ # 11 ]  
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Thanks for all the good suggestions from more experienced than I.  I have decided for this first go to abandon the top-bag concept and make a ‘quilt’ with a size of 72” length x 48” top x 36” bottom.  I will be filling with 4.2oz down for about 1” loft.  This should be equivalent to about a 55 degree comfort range.  Seem right?

Questions:

1) If I use noseeum for baffles will I need to sear the edges to prevent the baffles from unraveling after a wash or use?

2) If I use a sewn through method instead, will this result in a ‘shorter’ length quilt (as the down will expand the top and bottom of material)?

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Posted: 04 August 2009 01:29 AM   [ Ignore ]   [ # 12 ]  
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1)  No need to sear nanoseeum

2)  This is so little fill you may have trouble with standard construction methods.  You are using less fill than is used in even the extra small Whitney jacket.  I suspect you would be better off boxing or using karo-step rather than using lateral baffles, or perhaps even going with a thin synthetic.  Down is more attractive as the amount of fill increases.  With such a small amount of fill the shell material makes up a greater percentage of the weight than the fill itself.  You could use 2.5 basis weight XP in this case for only about an ounce more than the down weight, plus you would save weight by not needing the baffle material. 

Usually down items shrink in length by 10 or 15% depending on the degree of stuffing and the spacing and type of baffling, but again you’re using so little fill I don’t know.  You could determine this pretty easily using your own parameters prior to finalizing the pattern.

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