The Trouble with Cantilevers
I see a lot of houses with cold rooms, of drafty spots. When I ask homeowners where the problem is worst, nobody ever points to the center of a blank wall. Houses are uncomfortable where something “interesting” is going on. Corners, windows, joints, seams, overhangs, staircases, etc.. You could think of a house as a series of stacked blocks. Problems are more likely to occur where the blocks don’t line up. A cantilever is just such a place. It’s a floor that extends beyond the foundation to hang over open space. It’s also one of the usual suspects when there’s a comfort problem in a house.
To visualize why cantilevers can be troublesome, let’s imagine what the house looked like while it was under construction, before the drywall and siding was installed. We’re looking at the “bones” of the house. Walls are made of 2x4 lumber, perhaps, with a 2x4 at the top (called the top-plate) and another at the bottom of the wall (the bottom plate). On top of the first floor walls, they’ll build the second floor out of 2x10’s, perhaps. 
That means, in this example, that the floor is almost 10” thick. You could install a 9 or 10 inch batt of fiberglass into the space between the floor joists. There’s NO NEED to insulate the floor of the second story, though, because it’s separating the living room and the bedroom! A wall or a floor that separates two indoor spaces doesn’t need insulation. But what if it’s cantilevered out, and the second floor is bigger than the first?
In this example, we’ll need to insulate the overhang. If it’s a two-foot overhang, the joists are simple extended two feet out beyond the wall below. They probably used 2’ pieces of fiberglass batt insulation as well. The floor is now insulated where it’s an external surface of the structure (the “building envelope”). In an ideal world, this is where our tale would end. Unfortunately, theoretical “laboratory-perfect” construction doesn’t exist, especially in an older house. Houses are not built to be as air-tight as the bottom of a boat -- different materials expand and contract with the change of seasons, and insulation both settles with time, and allows airflow. What does this mean for your heating bills?
In the wintertime, cold air can leak into your overhang and take up residence in your floor cavity. The fiberglass batt does little to slow-down the airflow. This cold air can now travel all the way through your floor to the far side of the house, making your downstairs ceilings cold, and your upstairs floors cold. The furnace now runs overtime to keep up… and when it runs at maximum capacity, perhaps it can’t produce enough heat to keep that farthest bedroom warm on that coldest day in February (<>). In the summertime, the HOT air gets in there, and you’ve got a radiant heated floor… NICE! (no, not really that nice, is it?) Folks with Cape Cod style houses can relate to this – they’ve got the same problem. Instead of an overhanging floor, they have open floor-joists connected to an attic – but it causes the same issues.
To fix this, we need to stop the air infiltration, keep the cold air from traveling through the joist cavities towards the rest of the house, and “beef-up” the insulation in the overhanging portion of the floor. I’ve seen this done from inside the house and from outside the house. I’ve seen it done with injection foam, rigid foam, spray foam, and dense-packed cellulose. The best solution for your particular home will depend upon a number of factors. Ask your Energy Auditor or Home Comfort Advisor how to solve the cantilever problem in YOUR home.