I have the Pearl Izumi PRO Insulated Jacket (in a fluorescent "screaming" red), which is nothing more than a breathable barrier plus a Polartec Alpha insulating layer, and it's my favorite way to go at freezing temperatures. I normally wear it with a base layer and a thermal jersey.

You didn't ask, but I also have the Pearl Izumi PRO Alpha Layer, which is Polartec Alpha with no barrier, and which PI markets as a mid layer. I haven't needed it often, but I have combined it with the layers described above, and it was a little too warm at 25°F (-4°C).

In either case, I'm going to say Polartec Alpha works well for both insulation and sweat management.