Moisture in insulation of exterior wall
I ripped out some drywall in my 1965 Ranch Brick over Frame intending to install some insulation. out the small part of the wall I checked had no insulation, but the rest of the wall had faced rockwool batts that were very moist to the touch. The drywall in the insulated looks like it's stained dark from the moisture.
The uninsulated area looks like it was installed to replace a window when an expansion was added on that side of the house. There is a 1 inch gap at the top of the drywall, which I'm assuming was left to allow airflow. The gap also allows insects and Anoles to enter the wall.
What is the proper way to insulate this space? Would spray in foam insulate properly while keeping moisture out, or could I just vent the other sections and install unfaced batts? I'd rather install $60 of fiberglass than $250 of spray in foam If I have the option.
Re: Moisture in insulation of exterior wall
You vent to the outside of the house, not to the inside. If the outside of the exterior walls are tighter than the inside of the exterior side, then you will get a build up of moisture in the insulation.
I don't know of any reason to have a 1" gap at the top of the drywall, possibly the builder built the space 1" too tall and the drywall was not wide enough to fill all the space, and he didn't want to cut 1" strips of drywall to do the job right.
Around the perimeter of your house near the foundation, there should be gaps in the brick every 2' or so. This would be a vertical gap where the mortar is left out. This is done to allow the exterior to breath. Maybe someone during the energy crisis of the 70's thought that filling in these gaps, known as weep holes, would help save energy. It doesn't, in fact it wastes more energy because damp insulation looses half or more of its R-value.
Check for your weep holes and make sure they are clear. If you use faced fiberglass, put the wings over the edges of the studs, not stapled to the faces. The air gap should be next to the outside surface of the wall, not the inside. This practice of leaving a 3/4" gap on the interior side had validity when it originated, but not today.
Fiberglass insulation used to be foil backed. The foil, having a very low emissivity value gave the air gap about an R-3. That insulation was only 2 3/4" thick for an R-7, so it also had a gap on the outside as well. The practice of installing it with the air space on the interior side has held over even today and is sometimes even specified in code, but it is still wrong.
Your sheetrock should go flush to the ceiling sheetrock to keep it from sagging along the edge between the joists. The bottom piece should run about a half inch above the floor. If there is more than a 1/2" gap in the middle, it should be filled with strips of sheetrock with a 1/4" gap on each side, then taped and covered with joint compound.