It adds the warmth not from thickness but from trapping in heat. But it only does this when it's actually acting as a liner and not standalone.

It's like the Patagonia Houdini windbreaker. By itself, it gives a little bit of warmth but not a whole lot. It's thin so while it cuts down on wind, it doesn't insulate for shit. But when you have it zipped up and under a puffy, it's toasty as hell. Sometimes too warm.

I'll routinely use my Reactor Compact Plus under my Teton Sports TrailHead, and it makes a noticeable difference. Certainly not the 20º advertised, but at least 10º.