Discuss NCARB PDD Exam Topic 3 Question 14

In winter conditions, when the outdoor air is below freezing and the indoor humidity is maintained at 40 percent relative humidity for 24 hours a day, ice forms on the exterior surface of the stone facing.

Which of the following elements is missing from the building wall section?

AVapor barrier

BWall insulation

CAir space

DRain screen

Suggested Answer: A

With outdoor below freezing and indoor RH ~40% continuously, interior vapor will migrate outward. If a proper interior-side vapor barrier/retarder is missing, moisture moves through the wall and can condense and freeze at the cold exterior stone, forming visible ice. Insulation, air spaces, or rain screens help heat/moisture management, but the symptom (ice due to vapor diffusion) points specifically to the lack of an interior vapor barrier in a cold-climate assembly.

PDD references: Moisture control & vapor retarder strategy in cold climates; condensation diagnostics (ASHRAE Fundamentals; ARE 5.0 PDD---Envelope moisture control).

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22 comments · newest first

Arminda 2 days ago

I feel like it could be B) Wall insulation too.

Aliza 7 days ago

I think it's A) Vapor barrier.

Stevie 12 days ago

Insulation would keep the warmth in, right?

Lewis 18 days ago

Vapor barrier makes the most sense to prevent ice.

Hermila 23 days ago

Rain screen could help too, but not sure.

Serina 28 days ago

I feel like air space is crucial here.

Elfriede 1 month ago

Definitely missing insulation.

Cammy 1 month ago

I think it's a vapor barrier.

Eleonore 1 month ago

Rain screen could also be a factor, not just vapor barrier.

Shonda 2 months ago

Totally agree, a vapor barrier is crucial!

Carla 2 months ago

Wait, ice on the outside? That sounds weird.

Amie 2 months ago

I think it's more about wall insulation.

Francis 2 months ago

Definitely a vapor barrier issue.

Natalie 2 months ago

Rain screen could help too, but I agree with vapor barrier.

Kris 2 months ago

Not sure if it's just the air space causing that.

Tamra 3 months ago

Wait, ice on the stone? That's wild!

Shannan 3 months ago

I think it's more about the insulation.

Nadine 3 months ago

Definitely a vapor barrier issue.

Kimberlie 3 months ago

I’m leaning towards the rain screen option, but I’m not entirely confident. I need to think about how it relates to moisture management.

Mindy 3 months ago

Could it be the air space? I feel like that could help with ventilation and reduce ice formation.

Vallie 4 months ago

I remember a similar question about insulation and how it affects moisture levels, but I'm not sure if that's the main issue here.

Malcolm 5 months ago

I think it might be the vapor barrier since that could prevent moisture from getting into the wall.

NCARB PDD Exam - Topic 3 Question 14 Discussion