Insulation is good. More insulation is better (although at some point, more may not be cost-effective). It reduces the amount of heat a home loses in winter or gains in summer.
You can get there by building thicker walls and putting more insulation in the cavities, or you can put insulation on the outside of the structure, as in the Perfect Wall. The photo here shows thick insulation in the cavities of a home with double-stud wall construction.
When you do it by making the walls thicker, you also change the ability of an assembly to deal with moisture because the sheathing stays colder in winter, an issue that painters discovered back in the early days of insulating homes when walls stayed wet and paint peeled off. With a 2×4 or 2×6 wall, dealing with the rain on the outside of the wall is usually enough to prevent problems (unless you’re in an extremely cold climate).
Going to thicker walls, though, especially double-stud wall construction with cavities that are 12 inches thick, makes vapor diffusion of interior moisture a bigger threat. Building Science Corporation (BSC) recently published the latest results from its study of a home in Massachusetts with double-stud walls and found a somewhat surprising result about cold sheathing getting wet.
The BSC study
BSC has been working with home builder Carter Scott of Transformations, Inc. on this Building America research project. Scott builds superinsulated homes in Massachusetts (climate zone 5A) using double-stud wall construction. The house in this study is wired up with a lot of sensors to measure temperature, moisture content, and relative humidity in various places (indoors, outdoors, and within the walls). The photo below shows the sensors in one of the walls. You can look at the full BSC report to get all the details about them.