The two most abused terms in show business (aka building envelope design) are air barrier and water resistive barrier.

The air barrier is the system (yes, a system, not a mere material…there is no can-o-air-barrier) that attempts to keep the conditioned inside air inside the building.

The Water Resistive Barrier (or the WRB) is part of a system that provides the environmental separation that keeps the outside out (weather and air). Seeing how the WRB materials are almost never “waterproof”, I really wished we called it a “weather resistant barrier”. Hell, we could still call it a WRB. At any rate, the WRB delineates the drainage plane and when properly tied into the other components of the exterior building envelope, provides the environmental protection intended by the IBC.

Easy enough, right? Oh no.

For some reason (that I cannot explain), we follow different rules for residential construction than those we use for commercial construction (and we have products that claim to be cans-o-air-barrier #ugh).

Let’s take commercial first (because it is easy!) In almost every project that we review (and the Zero/Six crew does a powerful amount of 3^rd party peer reviews for projects across the USA) the air barrier and the WRB are the same. Praise the lord! We build commercial buildings like the space shuttle. The exterior building envelope is developed to be continuous (sealed tight), all outside air comes through the ventilation system, and the buildings are pressurized such that unconditioned migrant air is pushed away from any breaches. #glorious

In residential construction the air barrier and the WRB are typically two separate systems. The air barrier is formed by the wall, floor, and ceiling finish systems (commonly referred to as “paint to paint”). Since there are a thousand holes (outlets, lights, etc.) in these systems, the quality of the air barrier is dependent on sealing those holes and there are complete systems for improving the quality of this inboard barrier. The WRB in residential construction is typically a house wrap that is not tied into all other building envelope systems (for example the roof). Additionally, the building envelope is not continuous; it is interrupted by vents (eave and ridge vents) that invite air beyond the limits of the exterior building envelope.

You can probably see I have a preference, but before we go too much further understand that “you can do you, buddy”; I am not here today to judge…BUT, considering that residential structures are almost never positively pressurized (pressurised for UK Barry), we better be damn good at keeping the moisture away from our moisture sensitive structures if we use the inboard barrier. When our inboard (air) barrier is tight and our outboard (water) barrier is “vented,” the chances that humid air is going to enter our wall and ceiling assemblies (due to negative building pressure) goes up exponentially.  

Bottom line is we see a ton of material failures (from migrant air) around windows and other penetrations in structures where the air barrier is inboard. What is scary is that air tightness testing does not address the phenomenon that I am describing. We have had projects pass blower door testing in year 1 and fail three years later due to material degradation of in the air barrier system. That talk about sustainability and resiliency needs to be revisited.

My point? The exterior building envelope is not that complicated, but it is a series of systems that must be coordinated by someone that has a general idea of how the systems (including HVAC) work together. While specialists (roofing, windows, air barrier) are quite common, finding a generalist can be a challenge…and you need a generalist.

I often use the conductor of the orchestra to describe the envelope consultant. The idea is that there are just too many instruments to know how to play them all. The conductor’s job is to make sure they all work together. No conductor and things get noisy.