MYOG Stretch Quilt

Unless you had this strapped tight to your pad would you realize any benefit from the stretch? I don’t think so.

elastic thread on non stretchy fabric like M50 would be no different than non elastic thread

elastic thread on something stretchy like a woven fabric would make a difference.

you can get a seam to be stretchy by using a zigzag stitch with non stretchy thread

Several manufacturers have offered stretch down bags for a number of years. It is possible and many people find some benefit in it.

Creating a stretch stitch in a woven fabric is possible (that’s how they make those bags) but may be difficult to diy.  You need a consistent way to either create many small pleates, or stretch the thread as you sew.  The pleates create the slack in the fabric necessary to elongate, or stretch. Alternatively, stretching the thread as you sew will then cause the fabric to bunch up slightly as it moves out from under the pressure foot, creating tiny pleates.

Maybe all you need to do is increase the thread tension on your machine? I don’t know if that will cause the desired bunching of fabric, or just go slack again when each stitch is completed.

I think the way to go about designing this is to cut the pattern for maximum stretch and only put the elastic thread in the seams of the inner liner. As the stitches bunch, the baffles will pull the out shell in as well. Calculate your down fill based on the non-stretched volume.  To do this accurately, you need to calibrate and control how much your elastic seams bunch up (and hence how much they can stretch).

It would also be worth your time to research all the stretch down bags out there you can find to see how the manufacturers did it and what people said about them in reviews.

I haven’t heard of any diy stretch bags, so you could be the first! No way to know but try.

Google “shirring stitch”.

Thanks everyone for your responses. I’ll do some more research. I like a challenge:)

I would try getting your hands on one and look and feel how it was made.  What is the maximum dimension of the seam and is this the same as the outer shell of the bag?  I wonder if there might be small (1/4”?) elastic sewn inside the seam?  If so, I guess you could sew the baffle to the liner and then go back and sew on the elastic.  It would be difficult to sew both at once I would think. And lastly would the stretchy seams be desirable in a quilt?  I assume the selling feature in a stretch liner would be eliminating air to be warmed by having the bag close to your body.  I would worry that with a quilt, a stretch seam would lift the liner and cause air it enter the sleeping area-most likely right at your hips if a side sleeper.