Rigid foam over fiberglass wall batts in attic
I’m amazed at the info this site has. Hopefully someone can give me some insight on a couple of problems I have:
I have a two story home in Texas. In my garage attic, I have 60 x 10 feet of vertical walls facing the hot attic space. These walls are of 2×4 construction with vertical unfaced fiberglass batts held in place with a thin mesh.
1) The interior walls on the backsides of these attic walls are warmer to the touch than other walls in the room. There are many voids in the fiberglass batts and I can see the backsides of many of the outlet boxes.
I’d like to improve this insulation by using leftover 4×8 sheets of faced 1 inch polyiso rigd foam boards from another insulation project. I’d like to seal the outlets and then use a double layer of 1 inch 4×8 sheets over the entire 600 sq feet of walls with the reflective surface facing the attic and finishing it up with seam tape. This should add R10-12 to these walls.
My question, will I have any moisture issues using this approach?
2) My 2nd issue is that I have 8 feet of the attic walls sitting over the top of 2×12 un-insulated floor joists. Those floor joists extend 25 feet into the interior of my home. I thought there was insulation in that 8×25 space but there isn’t. The fiberglass batts in the wall above it extend past the bottom of the wall cavity and fit loosely into the cavity of the 2×12 floor joists hiding the fact that there is no insulation in that void.
Do I just roll up some fiberglass batting to block the opening of this void? Do I ignore the fact that this 8×25 foot void is directly below my 2nd story and only has sheetrock separating it from the unconditioned garage? Or is it worth it to blow insulation into this space?