First, what part of the country are you in? In Utah (where I last saw a root cellar) had a dirt floor, so humidity from the ground could help keep the vegetables humidified. If you are in Alaska, I’d assume that your root cellar would look different than the ones in Utah. Anyway, the Utah root cellar had concrete walls and dirt behind that. I don’t remember what the ceiling of the root cellar was made of, or if the house sat on top of it. If the house sits on top of it, the ceiling should be insulated as well. It shouldn’t have any heat going to it, and it needs air flow from the outside. The root celler I saw had an insulated door in the basement, but many enter from the outside. http://www.organicgardening.com/learn-and-grow/building-root-cellar-your-home

Thanks @snowberry.

If your door enters from the outside, you’ll want to make sure it’s animal proof. No point in a root cellar if you’re feeding the rats. ;D