Help with flannel
 

I'm making just a cuddle throw for my 5 yo grandson. If the top is cotton and the backing is flannel, do I need to use a batting? I have some Poly batting that is light.

Can't answer your question directly .... but what I can tell you is that I have a light/summer quilt with cotton front and back, and flannelette as the batting. Not too heavy, weight or warmth wise, but still nice and cuddly and surprisingly cozy! Drapes nicely and has a nice quilty feel too it.

I use blocks from my ufo / orphan bin , and add strips and pieces of fabric that were " trim offs" of backing from other quilts. This way, the quilt is interesting to look at when folded up as well.

You don't need to use batting if you tie it. If you quilt it you could add another layer of flannel to give it a little extra body for the quilting. I am not a big fan of poly, it seems hot to me.

I would probably double up on the flannel just to give it a bit more body. probably would be fine without it, though.

I have only done one flannel quilt in my lifetime. I did not bat it because both front and back were flannel and it was a rag quilt. I discovered that was enough weight for this quilt. It was also my first strip quilt.
I would think the density of the flannel should dictate the how it is finished. Mine had a heavy quilting flannel so it did not require any batting by the time I put the flannel for the back.
IF you do put a bat in it you may want to use something light weight.

Help with flannel
 

Flannel shrinks significantly enough that it should be washed and dried at least twice. The poly batting will not shrink. My favorite batting for most quilts is Dream Cotton Request Weight - very light and drapes well, even with flannel on both sides.

I’ve only made a few flannel quilts and I didn’t use batting because they were spring/summer quilts. I did do a meander stitch to hold them together and they have been around for a few years. I definitely pre shrunk the flannel!

This was my first thought. I you plan on using batting or for that matter a second layer of flannel in the place of batting, wash all flannel first so it can do it's shrinking before you start creating your quilt.

I have made lots of flannel quilts with poly batting. I have not made a quilt without batting.

I did make a table topper without batting and I found it very hard to hold onto while quilting. I would not do that again.

Being older we get cooler than young people and lots of nights we want a regular quilt. I mainly use high loft poly batting for our quilts. Warm in the winter and not in the summer.