I am trying to address an insulation problem that the builders of my house created. I am attaching a sketch of the area in question to hopefully make it more clear. The house is two stories and it has an unfinished basement. My office is in a corner room of the first floor and there is a large bay window area which extends out beyond the corner of the foundation. The foundation has a 90 degree bend at the corner of the house. The exterior walls are mostly directly above the foundation but right at the corner, there is a bay window area which is like 5 of the 8 sides of an octagon. These windows extend out beyond the foundation. The support for this structure is cantilevered joists under the floor of the first floor. This bay window/octagon shape thing is continued on the 2nd floor. The floor in the room goes all the way to the edge (i.e. there are no ledges or seats by the windows).

What I notice is that the floor in that corner of the room is quite cold. For example, yesterday it was 67 degrees F in the house and 52 degrees outside. The floor temperature on that corner of the room was 57 degrees while the floor temperature on the opposite corner of the room was 65 degrees. On a "cold" day (I'm in Georgia so that means 25 at night) it is much worse in that corner. Still, I want to move my desk over there and it is cold on my feet.

The top of the foundation is about 2 feet above ground in that corner of the house.

When I crawl under there what I see is that the joists which hold up that portion of the wall are just totally bare and the only thing between my toes and the outside world are a hardwood floor and the subfloor. No insulation, no vapor barrier, no nothing. To make it worse there is a floor outlet over in that corner. I see the wires totally exposed to the outside world and so there is a cold breeze right around the outlet. The outlet plate measured only about 1 degree above outside temperature and 16 degrees below room temperature.

Referring to my sketch, the places marked as "Area A" are totally exposed joists and the bottom of the subfloor is totally exposed. The only thing between Area A and the basement is a short piece of 2x10 between joists on top of the foundation. There are gaps which let cold air into the basement.

The place marked as "Area B" has a plywood cover on the bottom of the floor joists to hide the insulated flexible duct which connects to the heating/ac vent in the floor. There is no other insulation or vapor barrier in that cavity. On the basement side, there is nothing else which blocks air or insulates in between those two joists. This means not only do I have poor insulation in my office but also there is poor insulation in the basement.

In the area under the floor outlet, again the bottom of the subfloor is totally exposed to the outside of the house as are the wires.

So... how do I properly insulate this so my feet are happier? Do I use paper backed fiberglass insulation down there? Or should I use that pink 2" thick foam stuff and glue strips up between the joists? What about in that cavity around the floor vent? Spray in some of that "big stuff" expanding foam insulation? How about vapor barriers? Do I need something? If so, what and should it go right against the subfloor or should I insulate, then staple plastic on the joists over insulation and then cover in plywood?

What I didn't show in the sketch is there is also a wrap around porch on the other side of the windows so I'll have to crawl on my back way up under the porch to do whatever insulating needs doing.

Grrrrr. House is only 30 years old... Let's not even talk about the mechanics of a cantilever like this for a 2 story structure. I'm leaving that repair job to a pro.

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