During my recent section hike, I became very aware of wind chill.
My underquilt it a clone of a JRB. a close enough copy that people often ask if it is one. I have slept comfortable down to mid 20's with it. Other nights, I haven't been comfortable. I always attributed the variance to the tightness of the hang.
One night, on Chestnut Knob, it was very windy. (I mean the reknown "howling" mountain winds). I finally had to add my sleeping pad to survive. Real temps were above freezing.
I finally thought of wind chill and began to sleep in the shelters on cold nights. With both the down bag and the underquilt on top, I was very warm and cozy, if a bit uncomfortable.
I had brought my dri-ducks poncho to which I had added corner loops and end lanyards to adjust fit (same as the underquilt has) . I think this is similar to the "weathershield" that JRB markets. I had never tested it for this use.
Finally, week or so later, I remembered I was so equipped and rigged it on a windy night, The difference was amazing. I was quite comfortable, even with the wind whistling in.
Just saying that it worked for me.