This too sounds like my house...almost the exact same issues...one place I'm going to look at is the basement entry door that heads outside. Do you have one? This is my 1st year/winter in the house, so I'm going to monitor the temperature in that little entryway as the winter progresses to see if that is a possibility.
Otherwise, I may need to dig up the ground somewhere on the property...am interested in what others have to suggest as well...

I cannot think of a better place to ask this question than here in our forums.
I have a less-than-optimal setup for root cellaring. We live in a 1969 split ranch. The main part of our house was weatherized earlier this year, and all parts of it stay warm enough that it is not really appropriate for winter veggie/fruit storage (even the closets stay too warm - I have tried in past years). We have a wonderful large attached garage, but it gets cold enough to freeze hard in there, and I have learned this the hard way when I found apples and potatoes frozen rock-solid one year.
The basement has a cement floor and foundation - no dirt to dig up anywhere in there. And the basement is always too warm because of the furnace, and it's damp enough to smell musty, so I'm not interested in storing food there because I worry about what spores might be floating around in there. The ground outside is frozen solid, or I might someday consider digging an actual freestanding root cellar.
I don't have the time/energy/know-how to build something. I don't have the budget to pay someone to build something. Are there any cost-effective, creative solutions for storing root veggies and apples in a house like mine? Our extra fridge is full already with more perishable things. I have thought about storing my root veggies in coolers, perhaps styrofoam ones (those Omaha Steaks coolers seem built to last a thousand lifetimes), but I wonder about ventilation. With good air circulation I can get away with a spoiled apple not wrecking the whole containerful, but if the container was sealed and I wasn't vigilant, I imagine the spoil would spread fast.
Same for using an old chest freezer (which would be like a larger version of a cooler, right?) - the issue would be ventilation without risking too much of a drop in temp.
Still, that is the best idea I have come up with, and I'm working to collect styrofoam coolers (but it's going slowly - in our area, people are more into local grassfed beef than Omaha Steaks - a good thing, really.) Maybe I could find an old useless chest freezer to use, but I have heard those can leak freon, so I'm not sold on that idea.
Maybe at some point we can wall off a corner of the garage and insulate it to keep the temp just above freezing, but...that sounds outside the realm of my skills and budget at this time. And I don't expect the budget to change for the better.
I have my carrots in sawdust and wonder if they would be sufficiently insulated by the sawdust in a freezing garage.
(For the record, we don't store our cars in our garage, or anything else with fumes that would affect food.)
Got any quick-cheap-easy ideas?