Exterior insulation is on the scene and occupying a more commanding role as better building practices evolve. It offers an arguably superior opportunity to create continuous insulation of the structure. That means fewer thermal breaks and bridges. However, as the shift from theory to application occurs there are some important pieces to consider.
An initial consideration is the climate zone. Anything in Zone 6 (roughly the northern third of the US) or above requires exterior insulation. There is a seemingly intuitive conclusion to be made here. If this type of building practice makes sense where the elements are severe should I not adopt it in my zone so as to create a better built, more efficient home? Yes and no is the answer. Yes, you should build better. No, you should not assume that your friend’s bomber new house in Vermont will serve you well in San Clemente, CA.
Why not? Vapor
Exterior insulation can be useful in loads of climates but the entirety of the wall system must be considered in order to address inevitable vapor drive. In short, you must be incredibly careful not to allow condensation to happen where it can cause severe problems. You are up against moisture, heat, and cold and their very creative travel plans.
Key components are the air barrier, vapor control layer, air gap, proper (insulation) medium, and proper installation. In making these decisions there is a common goal in all climates: don’t trap moisture in a place where there is no escape route. The folks at Ecohome do a great job in simplifying this equation (and yes we don’t mind that they’ve chosen wool for their example!!):
A wool sweater, for example, is a good choice of natural insulation and will keep you warm when there is no air movement, but will allow the wind to howl right through it. A wool sweater with a raincoat will keep you warm but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to diffuse through it.
They also do a great job explaining the basics of this approach in the video here.
Healthy, high-performance homes need careful consideration relative to locale; thereafter a process can be followed to create a remarkably efficient, healthy, and comfortable space. And in our continued effort to paint a realistic picture, not bash your local GC, there is a very good chance he or she does not know how to do this properly so as ever remain incredibly vigilant, do your own homework, and don’t proceed until you are sure.