I am remodeling a bathroom and the exterior walls are 2x4's and I would like to add some insulation before I close the wall back up. The wall will be tiled 1/2 way up. So I was going to adhere 1" R6.5 insulation to the studs and then fasten cement board through the insulation into the studs. Is this adequate ?

Depending on your area and the local building codes you may also have to have an added vapor barrier between the wallboard and the insulation. That's the last place you want mold to grow.

Is the "aluminum foil" coating on the Dow Polyisocyanurate Rigid Foam Insulation be sufficient for a vapor barrier?

Yes and no. Over time the aluminum can oxidize if your in a high humidity area and from there vibration can caus cracking. So really it depends on your local area. Some put a sheet of plastic in between. You might just want to look around or call someone who does remodeling in your area and is familer with the local codes and ask.

Shawdog,

Sounds like you're thinking of adding the 1" panel on the studs, is that right? Why do you want to do that? The insulation goes in between the studs, in the cavity.

This areas is not a wet area, right?

Jaz

The void between the studs is filled with 3-1/2" fiberglass insulation, I want to add a bit more for the cold winters here in Northern NY. the area is in the bathroom, a "moist" environment, but not a shower wall.

Well, OK then, as long as it doesn't create other problems. If it's a simple outside wall with only 2 inside corners it should work.

Jaz