This article originally appeared on M. Stephen Doggett, PHD’s LinkedIn page. Pairing insulation permeability with WRB permeability – what is the preferred solution for hybrid walls with inboard WRBs and foam insulation? Although the WRB may be inboard of the insulation, it is still the primary drainage plane. Water will find it and remain resident there for a period of time. This is why water-resistance of WRBs is more important than ever. But vapor transport is also important. When pairing permeable WRBs and impermeable facers, this interstitial vapor is transported inward through the membrane and into the sheathing substrate. It does not release effectively to the exterior. This can be a problem in cold climates where hybrid walls are typically configured with batt-filled stud cavities and an interior vapor retarder. Pairing the same assembly with a permeable facer – or a permeable facer that is perforated – allows vapor to migrate bi-directionally to the interior and exterior. The alternative approach is to achieve sufficient R-value to the exterior side while also employing an impermeable WRB. This method has significant shortfalls as other factors such as insufficient R-value, poor insulation continuity, thermal bridging, and WRB continuity can jeopardize what otherwise looks like a perfect design. About M. Steven Doggett, PHD We at Performance Pro Supply are big fans of the work of M. Steven Doggett, Ph.D., LEED AP.  The founder and Principal Scientist of Built Environments, Inc., Dr. Doggett has over 20 years of experience with environmental consulting, building forensics and building envelope design. Though his expertise centers on moisture control, he also regularly consults on hygrothermal (heated water) performance, indoor air quality, and improved building service life.