# Thread: Differential cut between layers of synthetic insulation?

1. ## Differential cut between layers of synthetic insulation?

I am making an underquilt using Climashield XP. I've decided to go with two layers of combat (0.9in ea). I am planning on a differential cut in the shell (+1.75in) to avoid compressing the insulation.

My question is, should I also do a differential cut (+0.875in) between the insulation layers? Will it help with loft? Could it hurt anything?

From a mathematical perspective it makes sense that this would help loft by not stretching the outer layer. The only problem I could see with this would be gaps between layers, but that seems unlikely to me and it will be loop quilted.

2. For an underquilt, a differential cut (also known as a diff cut or diffi-cult) is probably useless complexification. You can look at a non-differential (called a space-filler cut) 2 ways: either the outside shell compresses the fill, or the fill fluffs higher to insulate you better and eliminate cold spots. In fact, the latter is what happens if you don't hang the UC too tightly so the hammock sags into it, thinning the insulation. But that can happen with a diff cut as well unless you exaggerate it a whole lot and even they some thinning will occur if you hang tight.

You don't lose anything by making a diff cut, but it is tricky with synthetic insulation. It is best done by shaping the layers (shells and insulation layers of the UC over a form and hand basting everything in place before finishing with the machine. [/I][/I]

3. The Warbonnet (Yeti precursor) synthetic UQ was a differentially cut UQ. The outer shell was larger (wider) than the inner. The innermost layer was sewn into the UQ, the outer layers could be added and removed by hand basting. The UQ had a zipper at the foot end that provided access to the inside of the quilt by turning it inside out. The Yeti-style suspension and the differential cut prevents compression of the insulation.

4. But that can happen with a diff cut as well unless you exaggerate it a whole lot and even they some thinning will occur if you hang tight.
The shock cord suspension prevents compression with a diff cut. Diff cut UQ's really are not any harder to make that a standard style IMO.

5. Originally Posted by Spock
For an underquilt, a differential cut (also known as a diff cut or diffi-cult) is probably useless complexification.... [/I][/I]
Just my opinion, I disagree. The JRB Nest & I have experienced cold spots due to either a too tight or too loose situation. With the diff cut MW have yet to experience cold spots. Differential cuts vs std cut (JRB MW3 vs. Nest) of same weight are warmer.

6. synthetic insulation won't compress as easily as down so I would say it is more important with a down UQ. Down is more free to move around in the baffle so you are much more likely to get a spot that doesn't have as much insulation where it is being compress. That isn't to say that a differential cut isn't going to benefit a synthetic UQ. It will and it will make it less idiot proof b/c you cant overtighten it. I would think about also shaping it with darts on both sides so ther aren't any air gaps. I think you are more likely to get an air gap then a compressed spot on a synthetic UQ. Shaping will also reduce the comression at the butt.

7. Originally Posted by Dutch
I would think about also shaping it...
It will be shaped, from the ends actually, to bring it about 1/2 way from rectangular to its hung/occupied shape. The remainder of shaping will come from suspension.

Regardless of what I do with the insulation itself I want to diff cut the shell. It occurred to me that if that was important perhaps diff cut between the layers of insulation would be to. I suppose another option would be to cut all the insulation to the same size but make that 1/2 of the difference between the shells.

8. I've decided to go for the differential cut. On the insulation. I'm going to use a trick Spock suggested and make a form rather than try to calculate everything. Sort of build it to fit.

So I took the pattern I developed and laid it out on a sheet of cardboard that's the finish size of the underquilt. I used an old microwave box and taped it up to get it big enough. Forgot I had packing tape at first, duct tape worked but it was overkill.

I cut out the darts along the line where the seam will be on the liner side of the underquilt.

Finally I taped it all up. The stiffness of the cardboard had the shape way off at first, didn't curve much end to end. I bent it up a little to soften it which made a huge difference. I will probably beat it up a bit more before I start laying things out.

I'm heading to the Black Hills with my family for a vacation this weekend (not a hanging one) so I won't get to start sewing for probably two weeks. I'll share my progress when I have some...

9. the insulation can compress itself.

on the old syn version of the yeti i didn't cut the insulation sheets differently, but i stepped them in by an 3/4" or so for each successive layer. doing it this way means you can only stitch the first layer to the perimeter of the shell, but it worked out nice because you could remove layers since they were just looped in. i did notice them compressing themselves before i started doing it this way though, and you have to be careful not to do it so much that they sag with air gaps between them.

10. Originally Posted by warbonnetguy
the insulation can compress itself.
Is that due to the outer surface of the insulation having slightly longer radius than the inner surface?

Also, were you laying it out on a form or flat?