I have a cold room behind the kitchen. When the cold weather arrives, we seal it off -- completely -- from the rest of the house. I stuck a thermometer in there this year. It read 40 deg F or lower while the kitchen on the other side of the wall read 65 deg F. It's colder because it has horrible windows (double panes of bare, bulky glass which will make great cold frame covers when we replace the windows next year -- they slide in wooden grooves and are quite drafty) and not much insulation anywhere. The wall between it and the kitchen is insulated and we've created a barrier + door in the 4 foot wide opening between the two.
Our basement is small and the presence of the gas furnace down there keeps it too warm to keep many root cellar crops for long. Squash does well. There's a corner where I can put a bucket of apples. But potatoes, onions, and carrots invariably sprout.
I had a large plastic box with an attached deep lid, left over Reflectix from the door, and left over insulation from the attic. I constructed a false floor in the box and stuffed it with insulation. More insulation went in the lid and sides and was covered with the Reflectix. I placed the finished box towards the kitchen wall side of the room.
I put in onions, red onions, potatoes, sweet potatoes, carrots, mandarins and grapefruit in mid-December. The last of the mandarins were used (and still in excellent shape) the last week of January. We went away the first two weeks of February, turning down the heat in the house to 60 deg F. When we came back, the carrots were frozen -- they were in plastic and in the section of the box furthest from the wall. But sweet potatoes in plastic in the middle of the box were moldy (they went to the compost). The rest of the produce was fine. A key point coming out of this is to not store items in plastic. Those in cloth or heavy paper bags were fine.
We'll see how things go the rest of February.