Hello PDD studiers!
During a recent virtual workshop,@coachmikeparaszczak was asked about the role of the vapor barrier in exterior wall assemblies. This topic sparks a lot of discussion and debate - in this article, I am going to discuss what you need to know about vapor mitigation for the ARE. A few important things to remember:
- The ARE is a national exam, and vapor mitigation strategies vary by climate zone.
- The ARE is designed to determine if you are a minimally competent architect. Questions are based on content found in certain references listed in the ARE Guidelines. Some are several years old. The ARE is not designed to determine if you are up to date on the latest and greatest in building science, or if you know how to design in a very specific geographic region.
- Questions on the ARE are written so they can be answered in, on average, 2.5 minutes. We all know that designing exterior wall assemblies takes a whole lot longer than that in practice. Questions about these topics are greatly simplified so that they can be answered within the allowed time.
With that out of the way - let’s talk about why we need to worry about water vapor in the first place. Water vapor exists in air - it’s an invisible gas, and is one of the three physical states that water can take - liquid, solid, and gas. Water can change its physical state depending on environmental conditions - of course, if you freeze liquid water, it becomes ice (solid), and vice versa. This is harder to visualize with water vapor, since it’s invisible, but trust me - it exists and it can changes states!
Warm, humid air contains a great deal of water vapor. Warm air wants to move towards cooler air, and when it does that, it takes its vapor with it. The issue is that the water vapor, when it cools enough, reaches the dew point - the point at which it condenses and changes state from gas to liquid. If that change occurs within your exterior wall assembly, you’ll be introducing moisture into the assembly, which can create mold, rot, and all sorts of nasty conditions, like lawsuits.
So, how does this apply to buildings? Well, if you have warm, moist air on one side of your exterior wall, and cool, dryer air on the other side, you need to take some design precautions to keep the warm air from condensing within the building envelope itself. This is where vapor mitigation comes into play - generally, try to keep water vapor from entering the assembly in the first place, and allow any vapor that does enter to dry towards the exterior of the building.
Depending on your climate zone, that warm air will be inside or outside. In many transitional climates in the US, it can be in both places, depending on the season. Where I am located in the Northeast, we generally design for cold conditions - when the interior air is warm and moist. We either use a class-II or class-III vapor retarder (which is simply latex paint) on the inside of the insulation layer of the wall. This reduces the amount of water vapor that can transport from the interior environment into the wall cavity, and it allows any vapor that does enter the wall assembly to dry towards the exterior of the building, because there’s not a vapor retarder or barrier on the exterior side.
I asked some of my colleagues here at Black Spectacles, who are located across the country, how they deal with vapor in their climate zones as well:
@kiaragalicinao , based in Oregon, noted that she’s accustomed to placing the vapor retarder on the inside of the insulation, with a WRB on the exterior. She often uses a rain screen system to provide a gap for drying, due to wind-driven rain in the PNW.
@emelytaveras , based in Virginia, said that in mixed-humid climate zones like Zone 4A, vapor control is tricky. The most important thing to consider is that the wall assembly must be carefully designed to dry toward the exterior. This means that any polyethylene or foil-faced barriers will trap moisture in the wall. For best performance, the vapor retarder should be placed on the conditioned interior side of the wall. Class II vapor retarders, such as kraft-faced batt insulation, are very effective.
All that is to say that vapor mitigation varies based on region - the type of barrier or retarder that is used differs, and although we all locate the vapor retarder on the inside of the wall assembly, that can differ as well if you’re designing in a very warm place, like Miami, where the warm side of the insulation is on the exterior.
So - how can you answer a question about this topic on the ARE, in 2.5 minutes? Generally, ARE questions will tell you that the project is based in either a hot or cold climate. As we know, this is an oversimplification, but it makes answering questions about vapor easier. If the question is in a hot climate, the warm, moist air is outside, and the opposite is true in cold climates.
A classic question will give you this information, along with probably a whole bunch of distracting information, and ask you where the vapor retarder should be placed - I saw a post recently on NCARB’s community recently that asked just that. When you see this kind of question - place the vapor retarder on the warm side of the insulation - towards the inside in a cool climate, and towards the outside in a hot climate. These questions are simply testing if you understand this concept, and if you can identify which side the warm side is. With this understanding, you can answer a question like this in about 30 seconds, and spend your extra time on more complicated questions.
I hope this was helpful, and happy studying!! I’d love to hear form folks about how they deal with vapor mitigation in their offices as well!
