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.

If you can pull it off, a differential cut is a nice feature.

There is something to be said for keeping things simple, though.  If this is your first sleeping gear project, most likely you will be making another if you enjoy the first one.  If this turns out to be the case, many of the small design changes you’ll want to make can be built into V2.

I had another thought.  What if I cut both layers of insulation the same size but made that half way between the differential on the shells?  The outer surface of the inner layer and the inner surface of the outer layer would be layer would be the “ideal” length.  You’d have a little bunching of the inner layer and a little stretching of the outer layer.  Would that be better than cutting the insulation to just match the inner shell?  (for that matter would it be better to cut it to match the outer shell?)