Water
intrusion continues to be the leading cause of callbacks and
construction defects. Prescriptive versions of 2021 energy codes will
require continuous insulation in climate zones 4-8. The logic behind
this is to reduce the energy impacts of thermal bridging through framing
members. On average, homes in the U.S. have 25 percent framing factors.
This means 25 percent of the walls are wood and 75 percent are cavity
insulation. The R-value of a 2x4 is only 4.38, significantly less than
cavity insulation, which has an R-value of 13 for a 2x4 wall. Adding
exterior continuous insulation can increase the wall’s R-value by almost
50 percent.
The majority of residential buildings also have
wood-based sheathing for structural shear support and wind bracing. The
majority of continuous insulation products are low-permeability foam
sheathing that get installed over the wood sheathing. If water intrusion
gets sandwiched between the foam sheathing and the wood sheathing,
which is usually OSB, the OSB gets wet and because the foam is not
breathable, the OSB will have little chance of drying out. This trapped
moisture can create conditions for mold growth, which can lead to
structural decay of the wood sheathing in a fairly short period of time.
Needless to say, from a building science perspective, this is not
ideal.