Every since the first winter after we moved into our house 6 years ago, we knew there was some sort of insulation problem along one wall. It was always on the "to do" list to figure it out, but it never seemed to be a priority. The plan was to tackle it when we re-did the second floor space that is closest to the wall. However, today I was working on a project that adjoins the problem wall on the first floor and the issue hit me in the face, literally.

I had just cut a hole for a recessed can and was greeted by a constant blast of cold air emanating from the new orifice. After a little investigation, I found that outside air had unfettered access to the joists between the first and second floors. Further, since sheathing was not installed all the way down a second floor wall, the cold air had access to that wall, too.

Here is a drawing of the cross-section I am dealing with. The problem is I am not sure about the "correct" way to remediate this issue:

Key:

A:

Air infiltration via the soffit. Note that this roof only covers the portion shown (about 2' of ceiling on the first floor) and it does not have any sort of vent.

B:

Sheathing on second floor wall does not extend to sill plate. Outside air flows up wall.

C:

There are no barriers to stop the air from passing through the loose fiberglass that is in the joists between the two floors. I am not 100% sure, but I think these joist bays pass completely through the house and, at the other end, are constructed exactly the same. Thus, air can enter one side of the house, cross through the joists, and exit the other side. These ceiling joists cross over a conditioned space (14') and the garage (20'). For reference, the length along C is about 2'.

D:

This is the ceiling where I was installing recessed lights (ATIC). I had not planned on pulling any of this drywall down, but I don't see any other way of attacking this problem. So, I'll probably pull down 2'-3' along the wall to have access.

Thoughts:

I see two options for attacking these problems. In both options, I would glue and screw sheathing along B to prevent air infiltration into the 2nd floor wall, replace the loose insulation at F with batt, and re-drywall at D. The first option would have me installing blocking at E. The problem I have with this is air can still get into those first two feet of ceiling. The other option I can see is to work in pieces of sheathing through the joists, up into the dead space and sheath the top of the joists along C. For any spots that I can't get sheathing into (eg, along the roof joists), use blocking and a closed-cell foam to seal. This would keep outside air from coming into the joists (at least from this end). I would have to work in short pieces (16" - 32") in order to be able to screw them down from above or I would have to figure out a way to secure it from below (blind nuts, bolts, and angle iron on the joists?)

Any comments or suggestions?

Thanks!