Vapor Barrier Location - PDD
This seems to continue to be a confusing question for all people taking the exam. Looking at this diagram the "correct answer" is for me to build the stud, then on that metal stud with 16" gaps I am somehow expected to attach what is basically a sheet of plastic, being the vapor barrier, along that wall? Everything I have read about for this construction it should be on the winter side warm of the insulation, which would be just outboard of the stud. But also this wall detail just doesn't make sense. Wouldn't we want some form of EXT sheathing right before the rigid insulation, otherwise how is it attached? Any clarification would be appreciated.
Thanks!
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The detail is trying to cover multiple conditions in the same detail. The result is visually confusing and the detail is carried more by the text. Just focus on the vapor barrier location. You said it should be on the warm side of the stud. In a cold winter scenario that would be inside the building.
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Thanks! When I said winter side warm I was thinking that it would be on the winter warm side of the rigid insulation which would give me something to adhere it to (Sandwiched between the rigid insulation and the Batt). But your point is a good one even though I think the detail isnt great.
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winter warm side of the rigid insulation which would give me something to adhere it to (Sandwiched between the rigid insulation and the Batt)
In my experience in the PNW that is where it always goes. But the study material/exams say otherwise. I think the logic is,
1. condensation forms on the warm side of vapor barrier (important to know this well)
2. warm side is in this scenario is the interior (cold winters)
So, a vapor barrier on the exterior of the assembly would mean moisture from the interior would need to travel through the assembly. There it would be trapped inside the wall forever. Instead you put the vapor barrier on the interior, meaning the assembly is protected and the condensation, if any will be on the wall inside.
This reminds me of a separate question I remember from the exams. You'll be asked, the interior face of a wall has condensation. What can you do to remedy the condensation?
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The way you just rephrased this makes so much more sense they how I approached it. I had just been thinking of locating the barrier inside the wall construction, but you saying to find the condensation point in the assembly is a better way to look at this problem. That is the "answer" but I still do not love the detail that is shown. Thank you for breaking this down for me!
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Now I'm confused. I thought I had a good understanding of this concept, but I also wouldn't have chosen put it on the interior of the studs... If that's where your condensation will form, wouldn't that ruin the interior gypsum board? that is a porous material that absorbs moisture. I would think that promoting condensation in that way would force moisture into your gypsum, which would eventually lead to mold and mildew.
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If you’re manually heating the inside of the home, that’s going to dry the air out. The only way there would be humidity is if you introduced it on purpose with humidifiers etc. you wouldn’t purposefully humidify your home to the point there would be condensation on the walls. That would be uncomfortable.
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Many people may place it between the batt insulation and the rigid insulation—but that’s a “trap” intentionally set by the question writer. It’s a classic “gotcha” moment.
Most people know that the vapor retarder should be installed on the interior (the warm side of the insulation). However, in this particular question, because both batt and rigid insulation are used, it’s easy to make a mistake under exam pressure if you don’t carefully study the diagram.
—Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)
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In a Cold Climate (like Climate Zone 6):
- The interior is warmer than outside in winter.
- So, moisture moves outward (from warm to cold).
- If you let that moist interior air travel through the wall and it hits cold layers like rigid insulation or sheathing, it condenses.
- That’s why we place the vapor retarder on the interior side, to block that vapor from entering the wall.
Why not place it in the middle (between batt and rigid)?
- If you put it between the insulation layers, you’ve already let vapor get past the batt, and if the rigid is cold, that’s where it’ll condense.
- Now it’s trapped, and can cause mold or rot inside the wall assembly.
- This is exactly what NCARB is testing for: your understanding of how vapor moves and where condensation will form.
About the gypsum board getting wet?
- If there’s a properly placed vapor retarder, condensation won’t form on the drywall.
- You also have mechanical systems (HVAC, ventilation) to keep indoor humidity in check.
- Plus, many modern paints actually help a bit with vapor resistance. Though they’re not vapor retarders by themselves.
If you are still confused, remember the Cold Beer Analogy:
You take a cold beer glass out of the fridge and place it on the table. Almost immediately, water droplets form on the outside of the glass. Why? Because warm, humid air in the room comes into contact with the cold surface of the glass. The moisture in the air condenses into water because the glass is below the dew point temperature.
Now apply this to a building:
In winter, the inside of the building is warm and humid, while the outside is cold, just like the air around that beer glass. If the cold part of the wall assembly (like sheathing or the back of siding) is inside the insulation layer, then warm interior air can reach it. When that happens, just like on the beer glass, condensation can form inside the wall. Leading to mold, rot, and reduced insulation performance.So what’s the solution?
Just like you might wrap your cold drink in a koozie to prevent condensation, we do the same with buildings using insulation. Wrapping the building’s exterior in insulation keeps the wall components warm enough so that condensation doesn’t occur inside the wall. It pushes the dew point outside the structure.Cheers!
Rajan K.
Founder, arniko.academy
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