Carter's bunkhouses were, I believe, recently renovated in the same style as Lonesome Lake's. This means that they are much better protected from wind. What this also means is that the little rooms in both locations will get soaking wet inside for lack of air circulation. Crack a window if you stay in a bunkroom at Carter or Lakes. It's no fun to have condensation dripping on you from the ceiling in the middle of the night.
(old-timers will recall when guests were specifically asked to do this because otherwise the forming frost would set off the fire alarms)
This is not an issue with Zealand due to the bunkrooms being much, much larger, with more air leaks.
I've slept at Carter and it was -12°F outside, -11.9°F inside. It's no problem with a good bag, and let's not forget the right clothes to sleep in. You will want long sleeves and long underwear. I recommend a turtleneck if you can stand the feeling. And bring a hat. You don't need much, but you will want something, even just a poly or silk liner. Some good sleeping bags will wrap over your head like a hood, but I still find it more comfortable to have something between the bag and my head.
The plastic mattresses at the huts are wicked cold, and will conduct that cold right into your sleeping bag, since of course under you the bag is compressed and not insulating as well. I always bring a closed-cell foam pad (z-rest or ridgerest) to put between the bag and mattress as insulation.
Unless it's an attached bathroom like Zealand or Cardigan, guys, bring a pee bottle and leave your modesty at home. Ladies, uh, sorry.
Also, remember that the big will be incredibly cold when you get into it. Throwing in a tightly-closed Nalgene with hot water will help out with this, especially in heating up the toe area. However ... this might make you too warm when you first get in the bag, resulting in leaving it a little unzipped, leaving your arms out, etc. Then later, when the bottle has gone cold, you'll wake up freezing and have to close yourself into the bag. I think it's better to skip the hot water bottle and just get in the bag and seal myself in.
Speaking of sealing and moisture, keep in mind that bag is designed to allow the vapor from your body out, but don't put your head inside the bag. That's too much, and it will end up freezing inside the bag, both making it heavier and also reducing its insulating abilities.
Finally, don't forget to keep everything that shouldn't freeze, as well as your electronics, in the bag with you. I tend to put my camera, headlamp, and spare batteries into one of my water bottle cozies and shove that down into the bag. That way, it's a soft, padded container and neither hurts the equipment nor my feet.