Question: Adding fabric length to accommodate loft with insulated clothing/gear?

Poly, Nylon, etc pretty much do not shrink. Apex is a poly based insulation. So, I would not worry too much about shrinkage with any synthetics.

Any good pattern will be laid out ignoring shrinkage. ie, it assumes stuff does NOT shrink. So, if you are designing a jacket, to continue your example, you need to allow for the thickness of the insulation. Normally, both sides are allowed for, but this can get a bit tricky at zippers, pockets, collars, and hems. It will move the zipper to the front or back or middle of the garment. I would just add 1/2 the thickness, as you have mentioned, to both sides so everything goes to the center. I am not really great at sewing, though.

Sewing sleeping bags has taught me that you need to allow for the stuffing somewhere. Ha, especially the width. Don’t ask . . .

Using cotton, wool or other natural fibers where shrinkage is expected, the normal procedure is to wash the fabric first. Then iron it out and lay out a pattern. Note that it could still continue to shrink. But, normally the pre-wash will cover about 80% of shrinkage. Two pre-washes will do like 90%. Three will do around 95%…etc… This is a just a standard Gaussian curve and in statistics is called a standard deviation. You can look it up for exact numbers, but they probably wouldn’t apply to a specific fabric, anyway. They do have tables on the web somewhere for shrinkage. I use the Gaussian curve only as a general guideline, only.

circumference of a circle is 2 * pi * radius

If your Apex is 1.2 inches loft, then the outside shell fabric should be about 7.5 inches bigger around to accommodate the loft.

Theoretically, maybe a differential cut would be better – outside shell 7.5 inches bigger, but totally unnecessary – just make both layers 7.5 inches bigger and the inside layer will wrinkle to take up the slack, you won’t notice.

+1 to James – nylon and poly don’t shrink

I just made a vest out of 2.5 ounce Apex – 4.5 ounces – works really good.  Warm down to about 32 F.  When it was 25 F I put on another vest over it, down, which made me toasty.

I have a 5 oz Apex vest but I don’t like it so much.  Too bulky in pack.  Too heavy.  Warmer than needed.  Before it was a vest, it had arms – even worse.  I removed the arms and made booties which are good.  Plus, the Michelin Man effect.  Maybe the arms should be 2.5 ounce?

oh, and make it 3.75 inches longer to accommodate the shoulders.

Sweet, Differential cut. That makes sense and there are some good threads that show up with that new vocabulary.

Would I do a differential cut for the apex as well? It makes me think that I should judge the apex by half its loft,0.6″ because it is the average of its thickness. But that is just the gut feeling, whether I should really do the full 1.2″ differential, or not one at all, still is a bit hazy for me.

So 2*PI*0.6=    3.77″ wider for the insulation in a differential cut of 0.6″.

theoretically, the inside surface of the Apex should be nominal width, the middle of the Apex should be 3.75 inches, and the outside surface of the Apex should be 7.5 inches wider

however, I don’t think it’s worth it, just make everything 7.5 inches wider.  The outside shell fabric will be big enough around so the Apex can fully loft.  The inside fabric and Apex will fold as needed.

if you make a sleeping bag with 5 inches loft, then yeah, a differential cut might be worth it.

Make everything larger and let the inside layer fold up as needed.
That’s pretty simple – and very effective. A good idea.

Cheers

Sweet, thanks for the help Jerry.