The Bob bag

Lets get this out of the way: I won’t make you one of these.  Working with these fabrics and with stretchy Climashield is not something I find fun.  This design is straightforward and quick to make, so create your own ugly.

Ever since my first Wilderness Classic nearly a decade ago I’ve been turning this idea over; what is the lightest and fastest way to get a bit of sleep in the midst of a fast wilderness trip?  Curling up around a fire would seem to be the easiest answer, and has the advantage of self-selecting for only the most vitally needed sleep (read; you get cold and wake up).  The problems are the questionable quality of rest, and the potentially considerable time put into making a fire under unideal conditions.  Adding a tarp or bivy sort of addresses the second issue, but not the first.  In the last decade truly UL sleep items have become common enough that most peoples answer to this question has been to just bring a standard backpacking kit, or at least a light bag, tarp, and minimalist pad.  These systems can be in the 2 pound range, but usually come in between 3 and 4 all told (stakes, etc).  Not much weight, but not a tiny amount either.

The functional intermediary between these has long seemed to be a light synthetic bag come insulated bivy sack.  Enough insulation to maintain ~4 hours of warmth around freezing, and a waterproof/breathable shell with minimal seams, that sort of thing that would allow you to flop under a half ideal spruce and stay protected enough in the just the bag.  Synthetic insulation, as sub 10 oz down fills tend to be overly sensitive to moisture accumulation.  This winter a friend bugged me enough that I finally overcome my reluctance and made two such bags.  In the next few months we’ll truly find out how they perform in the field. 

I used 10D WPB for the shell, .66 oz/yard taffeta for the liner, and 3.6 oz (120 grams/meter) Apex.  The former is the obvious choice, being essentially alone at that weight.  The taffeta has a nice feel and is calendered, with synthetic insulation I reckoned that eeking out every little bit of warmth with low CFM fabrics all around was a good call, with no functional downside.  I went with safety orange for use in signalling aircraft.  I certainly could have used lighter insulation, but past experienced suggested 120 g/m was the lightest that would still be useable in all but the most specialized situations.  I made the neck cinch out of 30D ripstop, as anything lighter doesn’t let the cord run so smoothly, and in time abrades along the opening.

Using the (raw cut) dimensions in the above photo, finished weight was just over 16 ounces.  The fit is narrow, on purpose, but long enough to mostly go over the head of someone a 6 feet tall.

The main design challenge was avoiding any exposed seams in the top of the shell, as I really didn’t want to get into sealing anything.  To fix the top of the insulation to the bag without doing this, I stitched the liner, shell, and cinch tunnel together (left photo) and then folded the shell out of the way, slid the insulation in, and sewed through the interior seam, insulation, and liner fabric (right photo).  Apex is stretchy enough that you can be imprecise here with no problem.

After this, stitch around the side and bottom edges, then put the footbox together.

The footbox is a point down triangle.  The photo show it inside out (left) and then right side out (right) in both cases with the top of the bag facing up.  What you can’t see well is that the top of the footbox is longest, making the two seams run backwards, with the footbox overhanging them.  My expectation is that anything short of serious, sustained rain will not wet this out.

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The ~1 pound weight it what I wanted out of this.  It approaches down bags of comparable warmth, and should exceed them in damp conditions over a few days.  Packed size is another matter.  Squeezing air out of the bag is not a simple thing, and without tons of compression it wants to stay as a roughly basketball-sized lump.  It will go smaller, but in the game of ounces the pack space this demands is less than ideal.

Field report to follow this summer.

10 thoughts on “The Bob bag

  1. Cool project Dave… might have to give it a go. Thanks for sharing.

    I have a bivy in that 10D stuff that’s been great and has held up surprisingly well. one-sided cuben tape sticks to the inside really well. Yours is lighter than my bivy+bag, or a tarp+bag like you said, but I’m philosophically into the discomfort of the bivy 🙂

  2. Nicely done 🙂 I definitely agree with your choice of 3.6 oz, I have a 2.5 oz quilt and it’s definitely on the cool side- too cool for almost anything in Montana.

    I would think that the bag would also be nice to stick in your day hunting pack as an emergency shelter.

  3. Dave,
    I’m glad i could talk you into this and thanks for putting one together for me. I started thinking about this after reading a story that the cavalry in the late 1800’s found that giving a soldier just enough bedding to take the chill off got them moving and subsequently cover more ground. Comfort in the BMO is a motivation killer in my eyes. I plan to use this as an essential BMO tool, an over bag for winter, and as Mike mentioned a hunting day pack bag for glassing, emergency, or for remote satellite camps while bow hunting. It packs into my 30 degree down quilt dry bag easliy. I will follow up with you on my thoughts..

  4. Dave I think I recall you tried making a quilt/insulated poncho once. How did that go?

    I’m thinking a wearable waterproof quilt would be nice for hunting. You could use it to stay warm glassing, and use it if you had to make an unplanned bivy.

    1. I still have and use it. I thought about going that route with this one, but decided to seal it and thus prioritize absolute warmth. You’d also want heavier fabrics for something that would be routinely used in poncho mode.

  5. Jan Nikolajsen April.16.2020 — 23:04

    Impressive weight. I have made similar insulated bivy bags with 3.6 and even lighter fabrics and they are 3-4 ounces heavier

  6. How breathable is the shell material when backed with insulation like that? Assuming this years AKWC happens I was going to bring a tarp and the lightest weight synthetic sleeping bag I can find, as the lightweight bivvys that I have tried were pretty wet inside and not very rain proof…

    1. We’ll find out! I’m not expecting miracles out of light PU coatings in either direction of moisture management. The theory, especially regards breathability, is that the temp and moisture gradients should be aggressive enough that performance will be at least acceptable. Warmish nights (~40F) with high humidity is probably worse case scenario.

  7. Follow up on the bags performance. I used the bag on a 3 day trip in the Bob a few weeks ago. With only slight dew to test the waterproof capabilities it was flawless in that condition. It never felt clammy and the material was really quite breathable and it cut the wind extremely well.
    more later…

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