I would use both Insulbrite and cotton batting. You will be happier with the runner later. If you are still not sure, practice both options on a practice piece - maybe some extra FQ's. You can always use them later to practice other things - binding techniques, machine quilting, even inserting zippers, button holes or grommets.

I use deco bond from Pellon and one thin layer of batting. The runners lay flat and have a crisp look.

If I am know ahead of time that I am going to set things on it. I will use a thin batting. Otherwise, I use my regular batting.

I find a table runner that has something - batting or insulbrite or both - has a lot more body and lays flatter. Also, you don't have to iron it after washing, and it washes better, too. I like to make mine reversible just in case something is spilled.

If you’re intending to place dishes on the runner, you definitely will want a thin batting so it doesn’t wobble, so I’d avoid puffy, Poly or wool batting.

I use used flannel for much of my batting. It lays so well for table runners. Just completed a baby quilt using flannel for the front and back, then used flannel sheet for batting inside. It seems to make the quilt just perfect for softness and weight. The sheets are usually a little heavier flannel, often a flannel top sheet and they have been washed a lot already so no shrinkage.

I've done flannel for several runners and am not happy with the results, even though the flannel was washed and dried hot. A low-loft batting works well.

Table runners, placemats and centerpieces - I use the wine glass test - will a delicate glass get tippy? Because of that, I often just use flannel. It will give that tiny bit of cush without bulk. Exception would be if I needed it to be heat proof. Then you need to use an insulbrite type product. I am also one of those folks who does not usually prewash my fabric (I can hear the gasps now!). But, whenever I am going to use flannel in anything, I prewash all the fabrics because flannel can shrink very differently than many fabrics.

I use Pellon quilters’ fleece as well and have never had a problem.

I have used batting at times, but I really do prefer no batting in my table runners. I like them to lay flat, and if I plan to place something hot on them, I just put a pot holder under them. That's just my preference. Sometimes I want to put a small knick knack or two on the table runner, and invariably it will not stand if I use batting. Just depends on what you want to how you are going to use it.