Ever more stringent energy code requirements have necessitated increased insulation levels for non- combustible (i.e. steel stud, CMU, concrete) walls, and in many cases this includes adding exterior insulation. Additional insulation thickness and changes to the insulation location require reconsideration with regards to vapor diffusion and condensation control.

Varying vapor permeability of different insulation products, membranes and other building materials introduces significant complexity to wall assembly design. Some insulation materials, like mineral wool and fiberglass, are vapor permeable, while others, like XPS, EPS, polyisocyanurate and closed-cell spray foam, are relatively impermeable.

Energy codes are silent on the issue and building codes can be confusing as it relates to exterior insulation selection and vapor diffusion control for walls.

This bulletin clarifies and provides guidance on vapor diffusion and condensation control in these new wall assemblies.

An exterior wall physically separates the outdoor and indoor environments. The differences in temperature, moisture, and air pressure between the outdoors and indoors result in loads that the wall must control or accommodate. Insulation is used to control heat flow, an air barrier is installed to control air flow, and claddings, flashings and sheathing membranes are used to control water penetration. Selection of a particular arrangement of material layers, and the vapor permeance of those layers, will control vapor diffusion.