NCARB PDD Practice Exam Question - Vapor Barrier
From what I have learned and what I know the vapor barrier should be placed on the "warm" side of the insulation (which to me would include the rigid insulation), but according to the prctice exam it is only placed on the batt insulation. This seems to be a conflict to me, does the rigid insulation not count??? 
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What is this detail? There's a roof membrane on the left side of the wall (?). Perhaps it's a roof?
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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this one also confused me. it's a wall detail. See below for question.
"An architect is designing a municipal services building in a location that has mild winters and hot, humid summers. The client insists that a sheet membrane vapor retarder of controlled permeability be specified for the exterior walls of the building.
Click on the area in the exterior wall detail to indicate where the vapor retarder should be applied."
In addition, should the plywood be next to the stud?
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A vapor barrier and a vapor retarder are not the same thing. Vapor barrier goes on the exterior side of the interior sheetrock. This is not permeable.
Vapor retarder is on the exterior side of the exterior sheathing. They are using that green zip system stuff everywhere around here. They call it a "Built-in vapor permeable water-resistive barrier". https://www.lowes.com/pd/ZIP-System-7-16-Cat-PS2-10-OSB-Sheathing-Application-as-4-x-8/1000510447
Hope this helps!
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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This detail provided in this question is a highly unusual way to build a wall, it is not code-compliant, and the answer is incorrect. Hopefully NCARB will drop the question or alter it.
For what it's worth, the most generous way of interpreting the question is to assume that a proprietary panel is being used like this one: https://www.hunterpanels.com/xci-polyiso-wall-products/exterior-wall-products/xci-ply-continuous-insulation. This panel is made of plywood and has rigid insulation (polyisocyanurate) adhered to it in the factory. The rigid insulation is installed facing the interior.
If the wall in question were constructed this way, no additional vapor control would be required -- the plywood and polyisocyanurate insulation already provide sufficient vapor control in a hot-humid climate. If the panel were made of gypsum (which is vapor open) and the rigid insulation were mineral wool (also vapor open) then the WRB ought to provide vapor control (20 perms or less ought to do it). But not only does no manufacturer make such a panel, this is not the correct answer per NCARB.
If we were to provide a vapor control membrane in the location NCARB says is correct, we would not be improving the assembly. It would also be difficult to install.
The code violation, by the way, is the single layer of WRB behind the stucco (cement plaster) cladding. Code requires two layers. The interior layer is for water and air control and the exterior layer is to serve as a bond break, preventing the stucco from adhering to the first layer. This provides a small space for drainage and drying between the two layers.
We'll be discussing this question at length on Thursday October 6 in a public (and free) Zoom session through Amber Book's 40 Minutes of Competence.
Christine Williamson
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Jun -
It helps to approach the question two ways: (1) Do I need vapor control, and (2) what is the easiest, lowest risk, lowest cost way to include it in my wall? Your confusion is related to that second question, but I think it will help to answer them both.
First, the sole purpose of providing vapor control in building enclosure design is related to durability. That's it. There's no occupant comfort consideration or energy benefit. All we're trying to do is protect the building materials themselves from damage. Damage usually occurs for two reasons: condensation (usually from interior sources of moisture - that is moisture generated by occupants cooking, cleaning, breathing, etc) and solar vapor drive (from exterior moisture - rainwater that saturates a cladding and then is driven to the interior). Both condensation and solar vapor drive occur to a certain extent in almost every single wall we construct. What we're trying to avoid - in both cases - is the introduction of more water into the wall cavity than the wall can safely tolerate before the building materials start to rot, grow mold, or otherwise decay.
Condensation control is a big concern in cold climates. This is because the moisture generated by occupants in winter finds its way through the wall and when it reaches the exterior sheathing (which is cold) condensation will occur. We deal with this in a few ways: one is to let the condensation occur and count on the wall simply drying to the exterior before anything bad happens. This works fine in milder climates. Another is to add exterior insulation so that the sheathing is no longer cold (no cold surface, no condensation). And finally, we can try to limit how much water vapor gets into the wall in the first place by providing an air and vapor control membrane on the interior. In cold climates where exterior insulation is not used, an interior vapor control membrane is mandated by code. Most of the time we install a special membrane over the cavity insulation and studs, between the cavity insulation and the interior drywall.
In sunny, wet climates, solar vapor drive can be another source of excessive moisture in our walls. Note that this is not a warm climate / cold climate issue, it's a sun and rain issue. We can and do have problems with solar vapor drive in cold climates, too. We worry about this in Boston, for example. Toronto, too. But not San Diego.
The way solar vapor drive works is that rain will saturate a cladding, and when the sun comes out after a storm, the cladding will dry and that moisture - in vapor form - will be pushed to the interior. The problem tends to be worst when we've used a cladding that can hold a lot of water like brick or stucco (it tends to not be a problem with claddings like painted fiber cement panels, or metal panels or vinyl siding). The easiest way of avoiding this problem is to simply not use a cladding that can hold a lot of water. Or, if we have a space behind the cladding for drainage and ventilation, often the water vapor from the cladding can be flushed out before it ever gets into the wall. But it also helps if we provide some kind of vapor control on the exterior of our wall to limit how much water gets in from solar vapor drive.
This brings us to the part of the question you were having the most trouble with: what is the easiest, lowest cost, lowest risk way of providing that vapor control?
This is where the scientific question becomes a practical question. It’s best if we can use something that’s already in our wall to provide the vapor control, rather than purchase, install, and detail a brand new material. So while you’re correct that you could include a vapor barrier behind the plywood (or even behind the rigid insulation), it wouldn’t make any practical sense to buy a separate vapor control membrane and install it in either of those locations. Not only is it a cost issue, but it’s also a risk issue. Whenever you add a new material, you increase the complexity of the design and risk more mistakes.
The easiest way to provide vapor control to protect against solar vapor drive is to select a WRB that will also provide vapor control (something around 10 perms or so). This is a good rule of thumb and it’s worth remembering even though this question presents us with a highly unusual wall design. It’s also important to note that there is no code mandate for exterior vapor control to protect against solar vapor drive. Just FYI.
Finally, this is difficult to understand without images. If you want to know more about solar vapor drive you can just google it and some good articles will pop up. I just did a post on it on Instagram (@buildingsciencefightclub).
Christine Williamson
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This thread has really gotten off track.
The exam question posted by in the original comment actually asked about vapor retarder, not a barrier. And it's unclear WHERE this section was cut. There's a membrane on one side and "exterior" on the other side (?). In fact, after seeing several threads with questions from these new exams NCARB posted, and doing some of them myself, I am advising my clients not to use them. There are too many errors and unclarities. The demonstration exam has been curated and revised... I suggest skipping these new exams that were just posted by NCARB until all the kinks (and mistakes!) are worked out.
Know the difference between a vapor retarder and vapor barrier. Vapor barriers go on the warm side (interior side, on outside face). A vapor retarder goes under the exterior siding.
Don't get caught up in recent trends in building science. The ARE tests your knowledge of tried and true, basic building detailing. It doesn't address odd detailing or cutting edge tech. As someone who has taken and passed the ARE 5.0, that's my best advice. Many of the experts folks are listening to have idea what is on the exam.
Hope this helps & keep studying!
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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Hi Christine,
Thank you so much for taking the time to ask my question here!!! The answer is very detailed and I am much more clear with this. I'm a fan and had been following you on Instagram. Those diagrams and topics there are awesome. However, your comprehensive explanation here is even better!
I'd like to share the link here for others who may want to follow. https://www.instagram.com/buildingsciencefightclub/
Again, thank you, Christine!
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Jun - You're welcome! Glad this helped.
Rebekka is surely correct about not getting caught up in this question. I am certain NCARB didn't intend to include a question that was so complicated using such an atypical (and non code-compliant!) example. Had I come across this question in the exam I would have wasted a bunch of time trying to figure it out AND gotten the answer wrong. And building science is my specialty! I'm sure it's more productive to take a holistic approach to passing the exams.
The rule of thumb we all learned in school will probably serve you pretty well for the purposes of the exam ("vapor control on the warm side of the insulation").
That said, I really hate for architects-in-training to see questions like this and think *they're* the ones who are wrong. You're not wrong. This was just a bad question. Test writers are humans and can make mistakes, just as we can (and do!) in professional practice.
Christine Williamson
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Hi Rebekka,
THANK YOU for your suggestions too! Your answer is simple and clear. Noted it already.
Deeply appreciate your kind help and support around this forum!!! Without you and others who are here and keep answering and helping, this forum is basically dead. lol
I also tried not to think deeply about vague exam questions. However, just like what others experienced, my last PDD versions felt like completely new study material. I was almost passed in my second time and then feel nowhere to start again. Trying to post those questions, and the discussion here basically gives me back more confidence. It is not my way of studying is wrong... Will keep going to get this done.
Thank you very very much for being here and helping!!!!
Jun
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Jun,
You're welcome! ❤️
Keep in mind, you can skip vague/odd/unclear questions that are a time waste. It's a skill you need to learn...
Good luck & keep studying!
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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