Is a differential cut more trouble than it’s worth?

[billy goat]

I'm thinking a 35 degree quilt here with baffles… I've never made a differential cut but I understand it's purpose. I'm curious if anyone out there has tried it and if it's worth the effort for a fairly lightweight, warmer-weather quilt. I'm leaning towards not doing it as it would greatly simplify construction, but I'd love to hear some input from people who've done it (or decided against it).

[Rob Hubbard]

For a warmer weather quilt I would not bother personally. I did one on a below freezing sleeping bag and it was a lot of effort. I now prefer Roger Caffin's halfway house suggestion of increasing the outer material measurement lengthwise to allow space for loft. If I ever do another winter quilt that will be the method I use.

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[Bob Bankhead]

According to Nunatak, who makes the Arc quilts, a differential cut is not needed or particularily beneficial until you approach a 20°F rating.

They will use it on their warmer weather quilts if you want to pay for it, but it's basically money spent for bragging rights rather than needed functionality.

That said, Katabatik Gear uses a differential cut on all of their bags except the 40°F Chisos.

[te – wa]

i have made many, many quilts and agree that for a top quilt, small gaps can and will be present and offer very little loss to its insulation properies due to the applicable law of thermodynamics. differential is not necessary at 35 – these gaps will occur with a differential shell as well..

[billy goat]

Your post is confusing. The small gaps you refer to sound like you're talking about sew-through baffles. I'm wondering about differential cuts and how much loft I'll lose if I don't use one (from the sounds of these replies, the answer is not much and I shouldn't worry about it).

[Sam Farrington]

Cannot imagine doing a quilt or bag and not putting in diff. cut.
Without it, when any part of you pushes against the inner, it will contact the outer, creating a cold spot. Thirty-five degrees can be quite chilly.

It is relatively easy to build diff. cut into a synthetic quilt, but assume you are thinking of down, if only because of its popularity on this forum.
With the down, it is a lot of work; but so is sewing in the baffles. A number of quilts on this forum have been made without baffles, either.

This is one time where IMO the conventional wisdom is right; namely as to the need for baffles and differential cut to get the most insulative value for the weight.

If I didn't want to spend the time with the diff. cut, I would just buy one of the beatifully made products on the market that are reasonably priced. Come to think of it, that's what I did.

OK, if you want to get your quilt down to a pound or below with 10 denier fabric, you might have to make your own. In that case, I would get a good quilt or bag pattern to shortcut the time required to have baffles and diff. cut. The higher wamth to weight ratio will be worth the extra work, in the long run.

[te – wa]

the gaps i am referring to are what is found inside your bag/quilt. you will never get rid of air pockets in these places, so differential or otherwise wont make much difference there. (see below)

if you want to reduce compression of down, i would suggest that at 35° you should use 1" baffles with a 2+" (overstuffed 25%) chamber. the weight of the top quilt that fits and forms naturally around you wont be enough to compress the down if filled correctly, if it does compress the down and force it to slip away from your extreme points, (like your shoulder and hip areas) differential cut wont do much to minimize this effect. properly filled quilt does not need differential cut at that temp. that is my assertion, and the alleged consensus.

i hope this clarifies, if not, PM me. ;)

[Kevin Beeden]

Given the confusion evident in this thread, it's probably a good idea for the OP to specify what he means by 'differential cut'. Then appopriate responses can be given…

I understand it to mean cutting the outer shell bigger than the inner shell, so that the lofted bag ends up as two smooth fabric cylinders*, one inside the other, separated by a gap, with lofted insulation filling the gap between them.

* taking a very simplistic model of a sleeping bag…

Then there's 'differential fill', which I understand to mean more insulation on top than on the bottom; I've moved the inner cylinder down a bit in my 'ideal bag'.

[Javan Dempsey]

The only location where a differential cut might be necessary is where the quilt folds laterally to "tuck" around you, basically, at the arms. With low fill, (sub 3" IMHO), it's essentially useless. Although a vertical differential can save weight, when used with undersized baffles (to increase down control), since you can reduce the linear shrinkage.

Honestly, a lateral differential cut isn't really going to stop some folding and compression around the sides if your baffle material is run horizontally, a linear differential will go just as far to avoid that issue. However, bear in mind the evidence supports minor compression to not cause performance decline, only major compression.

A simple solution if you're really concerned? Run your baffles vertically instead, like we do on underquilts.

My opinion? Don't waste your time for a 3 season quilt, unless you're doing it linear to save weight from the shrink factor.

That's all I'm going to say on the matter. GL! ;)

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