A Basic Differential Cut Underquilt

[StellaTerra]

Part 1: Plans

The Concept
A differential cut 50” x 38” underquilt with 7 vertical baffles, and a 2”-2.5” loft (something like 20F). A very simple suspension run through channels on the long sides, and cinches at the head and foot. Aiming for less than 14 ounces, really hoping for ~12 for the quilt + suspension.

Me
This is my first MYOG project, and I have no particular skill with sewing. One of my best friends is a professional seamster, though, and has offered to let me use his sewing machine and oversee my work, so I’m not at all worried about screwing it up too badly.

Some math
To reach the cut numbers list below I did the following:

• Inside fabric length = UQ base length (50”) + seam allowances (2 * 0.5”) = 51”
• Inside fabric width = UQ base width (38”) + seam allowances (2 * 0.5”) = 39”
• Outside fabric length = UQ base length (50”) + seam allowances (2 * 0.5”) + channels (2 * 0.5”) + total baffle height (2 * 2”) = 56”
• Outside fabric width = UQ base width (38”) + seam allowances (2 * 0.5”) + channels (2 * 0.5”) + total baffle height (2 * 2”) + magic result of trigonometric calculation from differential calculator spreadsheet (3 15/16” [nearly 4”]) = ~49”
• Baffles length = UQ base length (50”) + seam allowances (2 * 0.5”) = 51”
• Baffles height = UQ base loft height (2”) + seam allowances (2 * 0.25”) = 2.5”
• Suspension length = Length of hammock to end of continuous loop (2 * ~11’) = ~22’
• Cinch cord width = UQ base width (38”) = 38”

Cut/prepare pieces
Outside fabrics - 56” x 49” - Ripstop
Inside fabric - 51” x 39” - Membrane 10
Baffles - 6 * 2.5” x 51” - Noseeum
Suspension - ~22’ - Lash It
Cinch cord - 2 * 38” - shock cord

Shopping List
2 yards of 1.1 oz calendared ripstop nylon ($10.80)
2 yards of 0.66 oz Membrane 10 taffeta nylon ($19.90)
1 yard of 0.5 oz NS50 noseeum mesh ($6.75)
7 ounces of 850 FP goose down ($61.25)
Grey Polyester Thread ($1.99)
25 ft of Lash It 1.75mm ($5.50)
25 ft of 3/32 shock cord ($4.25)
4 Double cord locks ($1.00)
2 Ti Dutch Hooks ($5.00)
Total is somewhere around $130 give or take for shipping/taxes.

Some explanations

• All four sides will have quarter inch channels made of the outer fabric.
  
  • The head and foot are for cinches, and will have cord locks attached on either side, and shock cord running through the channel and both cord locks and tied on either side.
  • The sides will simply have one long (~22’) piece of Lash It that goes down one side and up the other, and then is tied off-center. This way I can add or remove tension by tying the knot looser or tighter. The whole suspension will be attached to the hammock suspension using the Ti Dutch Hooks
• Some of this is based off of this reddit post. I was a little worried about the size, but I contacted that person directly and they assured me that size was good for them on an AT through-hike. In particular I asked about the 38” width, but they said their shoulders were never cold, so I trust that. I plan on using a stuff-sack as pillow, and my backpack under my feet/calves.
• The math just works out so that each baffle takes 1 oz of down. Convenient, with the individually bagged Wilderness Logic 1 oz down bags.
• There are slightly lighter materials for the outside fabric, but I deemed it not worth the fractions of an ounce given the downproof, durable and water resistant choice of plain-old calendared ripstop.
• When I get to sewing on the baffles, I’m not going to worry too much about knowing ahead of time where to place them, I’m just going to measure for the seams and channels, and then evenly divide whatever is left on inside and outside (after baffle height on the outside).
• Saw the trapezoidal approach… I’m not going to worry about that for now.
• Vertical baffles because I’d rather set the down and forget it, than worry about karo step or anything. My feelings on this are very mild.
• I have a plan hopefully might make putting down into the baffles simpler/cleaner. I want to try to cut the 1 oz down bag over, shove it inside the baffle opening first, then hold it in place and try to beat or prod the down out in its compressed state so that I might avoid getting any of it airborn. We’ll see if that actually works out.
• This will be for a couple of hikes in the Sierra this summer, and probably some Oregon/Washington PCT sections next summer.
• Planning on pairing this UQ with an Enlightened Equipment Enigma 20F overquilt, a Dutch Halfwit hammock and a TBD MYOG hex tarp. Aiming for a total sleep system under 3.5 lbs (under, over, hammock, net, tarp, lines and stakes).

Questions

• First of all, does all of that check out?
• Will the channels made of ripstop be durable enough?
• Does a simplified, purpose cut, single-piece suspension system make sense?
• Does the cinch system make obvious sense there?
• Any other advice about this before I make the dive and purchase the materials?

[Boston]

1) Thisnis really too much for me to want to critique, but generally: yes. Note: I didn't check any of your math section. There are things I'd do different, but sounds like your mind is made up, and nothing is going to catastrophically fail. Although I'm a bit confused by what you said about sewing the baffles.

2) yes

3) No. Make a suspension out of shock cord, not lash-it. You need stretch.

4) it'll work, but you really only need one cord lock per cinch. For a more elegant solution sew a button hole in the middle of the channel and cinch it from the center.

5) I don't get your point about using heavier ripstop on the outer. The light materials are downproof, durable, and water resistant too. Just use momentum on each side.

Also, 38" width should be fine. All my quilts are 40", and could go a little narrower I think.


Fwiw I think a quilt is a bit ambitious for a first project, but ymmv, hyoh.

[StellaTerra]

1) Thisnis really too much for me to want to critique, but generally: yes. Note: I didn't check any of your math section. There are things I'd do different, but sounds like your mind is made up, and nothing is going to catastrophically fail. Although I'm a bit confused by what you said about sewing the baffles.

??? No, my mind isn't made up at all! This was my first guess, sortof a stab in the dark. I would *definitely* appreciate your opinion. If your reticence is about my rejection of the parallelogram design, I just deemed it too complicated for a first try. The thing about the baffles is just... as for placement I'll cross that bridge when I come to it, rather than calculating ahead, I really want my expert seamster friend to just guide me through that however he thinks best, and my initial impression is just to divide the available space, rather than trying to place baffles at particular angles, or in particular, pre-calculated positions. Is that a bad idea?

3) No. Make a suspension out of shock cord, not lash-it. You need stretch.

Alright, good call. I will make that adjustment.

4) it'll work, but you really only need one cord lock per cinch. For a more elegant solution sew a button hole in the middle of the channel and cinch it from the center.

Huh. That was my first idea, because it matches the way my overquilt is built, but all the UQs I was seeing were done the way described, so I thought I must have it wrong or something, but I prefer it the way you describe!

5) I don't get your point about using heavier ripstop on the outer. The light materials are downproof, durable, and water resistant too. Just use momentum on each side.

Ahh. Fair enough. I suppose it's worth the ~quarter-ounce savings, I just figured greater durability on downward facing side might be a good idea. I've got a dog and I'm worried about a dew claw through the outside when he jumps up to greet me in the morning, but if you think there is no substantial difference in durability, I'll swap the outside over to something lighter.

Fwiw I think a quilt is a bit ambitious for a first project, but ymmv, hyoh.

Oh man... I tried to make the design simple and straightforward. :/

Thank you so much for all the insight!!! <3

[Scoutmaster1056]

I just recently made an underquilt, I would recommend taking a look at the Catsplat's underquilt calculator. All you need to do is enter your information and it will calculate everything for you. I am very happy with how mine turned out. I used 1.0 hyper D on both sides.
image.jpg
I also added draft collars to both ends.

[Boston]

ok, a couple other things I just noticed: your channels are really small. They'll be harder to sew, and work with. Consider at least a half inch. One inch ish is pretty standard.

Also I think you may be under estimating the seam allowance you will need. Talk With your seamster friend about it, and how you plan to construct the quilt.

I highly recommend planning everything out ahead of time. Sketching out what you plan will help you visualize everything, including seam allowance, and maybe find mistakes before hand.

A rectangular quilt is perfectly fine. All mine are currently. If you look near the end of the CatSplat thread I posted an updated calculator that will include seam allowance, and a differential for the length (just adds baffle height to each end).


My hesitance on quilt as first project is because it's a lot of sewing, and you basically have no skill at this point. Having a mentor will help, but it helps to have practiced sewing long seams and rolled hems before hand.

[Double]

I hang my underquilt from the corners and use lineloc 3's to adjust it. Ran out of 1/8 shock cord after 2 of the corners so used 2.2 lashit for the other two. It works but I have a fear I'm going to rip it every time I enter or exit the hammock.

[StellaTerra]

ok, a couple other things I just noticed: your channels are really small. They'll be harder to sew, and work with. Consider at least a half inch. One inch ish is pretty standard.

Alright, half-inch channels sound good to me.

Also I think you may be under estimating the seam allowance you will need. Talk With your seamster friend about it, and how you plan to construct the quilt.

I highly recommend planning everything out ahead of time. Sketching out what you plan will help you visualize everything, including seam allowance, and maybe find mistakes before hand.

A rectangular quilt is perfectly fine. All mine are currently. If you look near the end of the CatSplat thread I posted an updated calculator that will include seam allowance, and a differential for the length (just adds baffle height to each end).

Alright. Yeah, I was using your updated calculator (transferred to Google sheets, as I don't have Excel) to get a rough idea, then I went through and validated everything it was saying by hand, except for the trigonometry. I'll trust you and CatSplat on that one. If you think more than half-an-inch of seam allowance is a good idea, I will definitely take it up with my friend, and make sure whatever we come up with is reasonable.

Thanks so much again, Boston! <3

I hang my underquilt from the corners and use lineloc 3's to adjust it. Ran out of 1/8 shock cord after 2 of the corners so used 2.2 lashit for the other two. It works but I have a fear I'm going to rip it every time I enter or exit the hammock.

Interesting... thanks for the insight!

I just recently made an underquilt, I would recommend taking a look at the Catsplat's underquilt calculator. All you need to do is enter your information and it will calculate everything for you. I am very happy with how mine turned out. I used 1.0 hyper D on both sides.
image.jpg
I also added draft collars to both ends.

Yup! Totally making use of the calculator. Thanks for highlighting it! And thanks for the picture! I'll have to learn about draft collars.

[Boston]

Thanks so much again, Boston! <3

No problem, good luck with your project!

[Scoutmaster1056]

Yup! Totally making use of the calculator. Thanks for highlighting it! And thanks for the picture! I'll have to learn about draft collars.

Draft collars are just a tube filled with down attached to each end, they fold in helping to seal up each end under your hammock.

image.jpg

[StellaTerra]

Part 2: Marking, Cutting & Sewing

Adjustments

• I talked this project through a bunch with my seamster friend, and got some great advice here, as well. I made the following adjustments to the plan before starting:
• Revisited the math and changed the seam allowance slightly (Baffles - 6 * 3” x 51”)
• Performed some trigonometry and realized my baffles weren’t quite high enough. (Outside fabrics - 57” x 51”)
• Scrapped the Lash It entirely, in favor of shock cord cinches.
• Thread was free with purchase from RSbtR.
• Went from dual cord locks on the head and foot cinches to a single center one on each.
• Swapped Dutch Hooks for quilt hangers. Just didn’t know they existed, but obviously purpose made.
• Went ahead and marked the baffle locations on both sides just to make sure. This ended up being a *great* move.
• I’m no longer allowed to refer to it as a quilt. My seamster friend (and wikipedia) have made it clear that technically quilting has to do with making a stitch through three fabrics, which would be horribly cold. My friend insists I call it a down comforter. (Yes, this is in jest, I know it’s referred to as a quilt by everyone in this community.)

Progress

IMAG0036.jpg
I cut open an Amazon shipping box, then cut it to the length of the outside fabric (57”). This was useful because it had two good useable flat sides at right angles. Then I drew a mark at the width length (51”). I placed it on the weighted fabric on the table and drew one side, then rotated the fabric on the table, and used the mark as the stop point to draw the the next, then back to the full length, and then the notch again to confirm and finish. There were many false starts before I landed on this approach. There are a lot of straight lines from other attempts with other techniques. I strongly recommend the use of templates.
IMAG0037.jpg
I then cut the template down to the length of the length (51”) and marked the width on the side (39”) just the same as with the outside piece. Then marked in the same manner with the sharpie.
IMAG0039.jpg
Cut both pieces out with normal fabric shears. The shell weight without stitches or cinches is 2.4 oz. The entire thing might be less than 10 oz!
IMAG0040.jpg
As an adjustment to the challenges I was facing, I decided to go ahead and mark the baffle seams on both sides of the shell. It was helpful here to place marks on a template for all six baffle seams, so I could just move it along, making marks every few inches, and then come through and connect them later. Again, templates are so good! If anyone is curious, for my dimensions (rounded) I made marks at:
outside shell

• 9 13/16
• 16 1/16
• 22 6/16
• 28 10/16
• 34 15/16
• 41 3/16

inside shell

• 5 15/16
• 11 6/16
• 16 13/16
• 22 3/16
• 27 10/16
• 33 1/16

IMAG0041.jpgIMAG0042.jpg
Baffles on the inside shell. Notice the untrimmed baffles. Also notice the ominous curling. I still have no real idea how long these things are, because they stretched while I sewed them.
Completed baffles.
IMAG0043.jpg
Oops! Two bad things happened at once. First, I started sewing the outside shell to the baffle starting at the edge of the fabric. That isn’t supposed to happen, as there is both a channel, and a seam allowance to consider. The second is that the sewing machine badly compressed the outside shell fabric, and stretched the baffle, so what was 57” of outside shell sewed completely to something that was less than 51” of inside shell.
IMAG0044.jpg
I screwed up. I was crestfallen.
IMAG0045.jpg
But my friend suggested getting an X-Acto blade cutting the thing free. Here you see the mess of thread that I cut out of that one seam.
IMAG0047.jpg
The fabric held to the light. I was really worried about those holes letting down through. I don’t think it’s something to actually worry about, though.
IMAG0048.jpg
I return to the scene of my earlier mistake, now wised up to use pins. I start on either end, with a mark where the baffle should begin and end on the outside shell. Then I pulled it taught with some help, and placed the middle pin, then split the difference again on either side, etc. until the whole thing was pinned up.
IMAG0051.jpg
I did each baffle this way, and you can see now how the shells were supposed to be aligned.
IMAG0052.jpg
Then I sewed up a side and tried out one of the channels, just to see how it would work. My meandering seams aren’t great, but so far, so good.
IMAG0055.jpg
The corners were giving me some trouble. I couldn’t figure out how they should fit.
IMAG0060.jpg
This is what I came to. Here you can see the differential cut in action. It looks so good!

Learnings
Measuring and marking such large stretches on fabric is *very* difficult.
Just keeping this stuff in one place is a massive challenge. It moves like liquid! Weights are your friend: use them heavily and frequently.
Chalk does not stick to any of these kinds of fabrics. I just sucked it up and used a sharpie.
Sharpie goes right through this fabric and will mark your table. Douche.
Do *not* use 0.5 oz Nanoseeum mesh on your first project, it rolls badly, and provides almost no weight savings over 0.67 oz Nanoseeum, which I’ve heard lays flat naturally. I was able to work around the problem with liberal use of pins, which is probably a good idea regardless, but it was quite a challenge keeping the baffles in the right place.
My seamster friend thought for sure I’d need to use a backing on every seam, but that would have been ridiculously time consuming. I was just very very careful and moved slowly, especially on the first few stitches.
Pins are also your friends. Use them heavily and frequently, and the seams will be less challenging.
Templates are your third friends, not that we’re ranking you. If you can draw it and cut it on cardboard or opened up grocery bags, you will have a much better time doing so on fabric.
Holy cow does this stuff take forever! But it’s so so so rewarding!
Differential cut leads to some interesting challenges in three dimensional space. The head and foot need the “gift wrap” treatment on the ends to lay flat, but man does it look cool!

What’s left?

• Add a reinforcing stitch to all three sewn sides, at the recommendation of my friend.
• Filling it up and closing the top.
• Cutting the channels and installing cinches
• Working out the suspension system.
• Trying it out for a few nights in the back yard and adjusting.
• Finalized complete instructions to make the thing.
• Trip report and review from the PCT near Lake Tahoe (expect a necro in early July).