To create a little slightly more spacious interior we are using 2×4 stud walls, though 2×6 seems to be more common these days. One of the advantages of the thicker 2×6 wall is the ability to stuff more insulation into that thicker cavity. So, to help bring 2×4 walls up to insulation code, our ADU will have an additional 1″ ridged foam insulation on the outside as well. Not only will this bring up the overall R-Value, but it helps to create a “tighter” space, meaning there are fewer spaces for drafty air in the walls.
R Max is the 1″ ridged insulation over the entire exterior
Over this insulation is laid the vapor water barrier, which as expected, helps control the moisture that can penetrate the exterior.
Yellow water barrier
The next layer will be the .5″ rain screen furring strips, which are essentially thin slats of wood spaced 16″ on center to which the actual siding will be nailed. The furring strips create a small airspace between the siding and the weather barrier, which if any water does get behind the siding, it will flow down and out.
The firewall side of the ADU gets its own special first layer before all the other exterior materials: sheetrock. This helps reduce the ability of fire to spread from inside to out (or vise versa) and is required by code.