Ultralight karo-step hammock down underquilt with some innovations (?) Video and Pictures!

[Kevin Duvernay]

Hey guys, just finished my latest project and I'm so proud of it that I just have to show it off. Its a
hammock under quilt for 3-season backpacking.

I think there are some design details that many of you may be interested to see.

The specs:

– 13.2 oz finished weight, without stuff sack. (said 13.4 in video, I was wrong)
-60'' x 45'' (pretty big, lets say 4/5 for my 6' frame ;) )
-7oz of 950 fp water resistant down – provides ~3 inches of loft which I'm thinking will get me
down below freezing, maybe even down to 20 with an appropriate top quilt.

Materials:
-outer shell is calendared 1.0oz hyperD from RSBR
-inner shell is 0.66oz Membrane, also from RSBR
-baffling is noseeum mesh, 0.75-1.0 inch thick
-thread is blaze orange mara 50
-suspension consists of 1/8 inch shock cord and 1.25mm z-line from z-packs.

Interesting design aspects:
-complex differential cut with the karo-step, see video for explanation. Will elaborate here if people
have questions. This design was such a PITA to calculate and to sew, but I think totally worth it.
-the z-line and shock cord combo suspension seems to be really quite nice. The z-line is ultralight
and has the perfect amount of friction. See video.

Video:

https://www.youtube.com/watch?v=lhVgUGx0xAM&feature=youtu.be

Pictures:

[todd]

That's a whole lotta puffiness!

I want a Karo UQ – if down shifts due to gravity (& it will) it will go where it's needed most.

Very nice job.

[JCH]

Damned sweet looking quilt. Kudos.

[Kyle Baker]

Congrats again Kevin. That really is an awesome quilt.

[Tom Peterson]

Looks like it a made by a professional!

[Rich K.]

Most excellent, wow! Sounds and looks like a TON of work, nice job. One question though, why the short baffles? Is there a reason other than weight savings? Given the loft I would think you could have doubled to 1.5" or more. Great colors too.

[James Marco]

I agree, Most Excellent!

[Kevin Duvernay]

Rich,

The more quilts I make, the more I realize I like having the baffles at about an inch or two less than the my final desired loft, rather than at the full loft. This gives me 'overstuff' without having to add more down. It just works better for me. Nothing hugely advantageous but if anything its just becoming a personal preference. I think I could convince myself it does mean less down shifting and a more efficient/lighter overall design.

For example, lets say you want a quilt with 3 inches of loft. Traditionally people would say cut three inch baffles and ad 20-30% over stuff. This would actually give you 4-5 inches of loft depending of the quality of down. Rather that approach, why not just just do 1.5 inch baffles with zero overstuff… it would give you the 3 inches of loft and provide the desirable characteristics of overstuffing. Make sense?

[Jordo _99]

Kevin,

First off, I'm impressed with the quilt. It looks very professional.

I don't agree with your comments on baffle height though.

---

Before I start, I'm just going to touch on how down insulation works…as that's important to why I have the opinions I do.

Down acts as insulation by trapping air (this is actually how most insulation works). It's efficiency is caused by preventing air circulation within the shells (cold air near the outer shell can't cool the warm air near your body as well if down is there to prevent "miniature warm/cold fronts" from circulating). The air is actually what's keeping you warm…so adding more down will further inhibit circulation better but it doesn't really impact warmth much.

---

"For example, lets say you want a quilt with 3 inches of loft. Traditionally people would say cut three inch baffles and ad 20-30% over stuff. This would actually give you 4-5 inches of loft depending of the quality of down."

Honestly, this isn't very realistic. Down will compress on itself before it makes fabric expand like this. Regardless, it's going to cost a lot more and the money doesn't go very far in terms of adding warmth…so this option is really just adding 15-25% to your budget, adding 15-25% to the weight of the quilt and providing 5-10% more warmth.

---

"Rather that approach, why not just just do 1.5 inch baffles with zero overstuff… it would give you the 3 inches of loft and provide the desirable characteristics of overstuffing. Make sense?"

---

That doesn't actually make sense…I think the real solution is to use a differential cut (which I believe is fairly common around here). It's almost the same method as what you have done but 1.5" baffles and 3" loft seems too aggressive, especially for an under-quilt (which is much more susceptible to drafts)

Going off some estimates on loft height and temp ratings (these are fairly established estimates):
1.5" = 33F
2.0" = 22F
2.5" = 11F
3.0" = 0F

The problem is that a quilt is only as warm as it's coldest spots (or rathers, slightly warmer than it's coldest spots). The parts of the quilt with a 1.5" baffle are only rated to 33F. The areas that have "full loft" are rated to 0F. That's a massive difference and would almost certainly create cold spots if used at 20F even though you have enough down for a 5-10F quilt.

The Karo step baffles help with this by having less sewing/baffle overall as well as evenly spacing them out…but it's still there.

If you haven't yet…check out the UQ calculator here over at hammockforums and play around with different baffle/loft combinations.

https://www.hammockforums.net/forum/showthread.php/53701-DIY-Differential-Cut-UQ-Calculator

My preferred specs for quilt construction are as follows (this is not talking about karo though):

Baffle = 1/2-1" less than loft height…inner shell is 4.5" wide for baffles and outer shell is 5" wide for baffles.

[Kevin Duvernay]

Jordo,

Thanks for your comments and what you say makes sense. This quilt was intended to be good to freezing (`30F) and I can attest that it works down to that temp. According to what you're saying, you think its good to 33. Sounds like we can agree the design is effective?

I will add this, I'm not sure you're fully articulating why people over stuff. It's not to add loft and warmth, it's primarily to account for degradation of down over time and to minimize down shifting and cold spots. What I'm suggesting (and saying works well, from my experience) is rather adding more down to get these benefits of over stuffing, a good alternative is to reduce the width of the baffling material while keeping the same amount of down. This will provide 90% of the benefits you get from over stuffing while eliminating the increase in cost and weight you mention. I do agree it may reduce the temp rating a few degrees, but to that I'd just say put on a layer. Make sense now?

Also, If you re-watch the video you'll see the quilt does indeed use a pretty significant/complex differential cut in all four directions. I certainly do agree that a differential cut is very important in underquilt design.

Cheers,
Kevin

[Jordo _99]

I use 10% over-stuffing to offset lost performance that is caused by down losing it's FP rating over time as well.

I follow your logic that taking a quilt that would use 2.5" baffles with 3" loft and reduce the baffles to 1.5" but keep the same amount of down…if 1.5" baffles require 10% less down, then your quilt is suddenly overstuffed by 10%…I really do understand that.

Where I think the disconnect is…I'm taking it a step further and comparing the warmth afterwards and trying to find the optimum warmth:weight ratio.

What I'm confused about is how those two quilts are comparable in warmth. My opinion is that the trade-off is not worth it and there is a better way to use your materials.

I use this to spreadsheet do my calculations (this is the same one I linked earlier but I keep it as a google doc):

https://docs.google.com/spreadsheets/d/1jgIRktEQsjybNtPBBaGik67PODUBtteT4MEyFvaPZyo/edit?usp=sharing

The cross-section of a 4" wide baffle, with 2.5" height and 3" center (loft) = 11.57 in^2
The cross-section of a 4" wide baffle, with 1.5" height and 3" center (loft) = 10.71 in^2
The difference needed to fill that area is 8% (which scales to the full size of the quilt)

The rating of a 2.5" baffle quilt is 13F and the 1.5" version is 23F.

…but if you go with a 2" baffle and 2.5" loft option you also get a 23F rating…but with a cross section of 9.57 in^2

That means by leveling out the baffle:loft ratio a bit we can achieve the same warmth rating as the 1.5"/3" quilt, while using (12%) less down. The one downside to this, is that it uses a little bit more baffle material but the cost/weight of that is made up for many times over by the down that's saved. This option also has a range of warmth that is only 10F rather than 20F, so you will be much less likely to develop cold spots.

To scale this up, I plugged in the following:
48×60" size
850FP down
10% overstuff
.67 shell weight (membrane 10 or Argon67)

1.5" / 3.0" = uses 10oz of down and weights ~14.5oz
2.0" / 2.5" = uses 9oz of down and weights ~13.5oz

Both quilts estimate temp rating at 20F.

This is all "paper speculation" and not a proper test in a controlled environment but I do feel it's going to be pretty accurate in real world scenarios.

[Kevin Duvernay]

Jordo,

That is good stuff right there. Clearly your thoughts on this have evolved one step further than mine: from the idea to the math. I think we are both getting at the same point here – optimal quilt design probably involves using baffle thicknesses that are less than your final desired loft. This theory is purely observational for me, from my experience making quilts.

Question is, what is the optimal baffle:loft thickness ratio? My guess, without playing with calculators like what you have there, is it would be somewhere around 2:3, +/- a little. So, if you wanted a loft of 3 inches, 2 inch baffles would be a good place to start for your baffle thickness. I think when I threw out the 1.5 number earlier, I wasn't really thinking too deeply about it and maybe a 1:2 ratio is indeed a little too much. That said, this quilt uses practically a 1:3 ratio (in conjunction with a generous differential cut) and I really like the results. 30F under quilt at 13oz is pretty respectable. That weight includes the suspension and carabiners.

I'd also speculate this is all even more important in a karostep quilt, especially one with larger boxes, because of they are more prone to down shifting and cold spots.

Cheers
Kevin

[Alan F]

@kduvey how did you calculate the  cut?

Did you calculate the one direction like you would straight baffles?

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