RAB Xenon X hooded jacket in a sleeping bag warmth boost?

I have one, but I’ve yet to wear it in a sleeping bag (or take it out overnight). I suspect the hood would be too thin by itself on a really cold night. The main thing I like about it over my other jackets, its wind resistance, may not be that useful in a bag.

I mean in a bag with a hood. In a hoodless bag or a quilt, yes, probably too light.

Big issues would be the insulation getting damaged and the lack of breathability.

Not the scenario you are asking about, but IMHO worth considering –

https://backpackinglight.com/forums/topic/106046/

In retrospect, it certainly could have been warmer than 32F in my shelter, but I was in a cloud, with graupel falling for a couple of hours.

My sleeping bag was dry in the morning.

YMMV

Edit: I sleep in a fleece cap that is deep enough to reach the bottom of my nose and ears. I’m a size Large, but the Xenon is a XXL (intentionally), so there is a lot of jacket to drape over the bag.

What’s the Clo value?

The Clo value will tell you how much warmth it will add because generally speaking insulation values are additive, I don’t think the breathability is an issue if you are going to be wearing non absorbent underwear either as the stuff dries off so quickly when you get out of bed in the morning and hopefully if you are warm enough to sweat you would wake up just enough to take the jacket off.

This assumes you have the room inside the sleeping bag so the bags loft isn’t compromised