NCARB PDD Exam - Topic 3 Question 14 Discussion

Actual exam question for NCARB's PDD exam

Question #: 14
Topic #: 3

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

Show Suggested Answer

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).

by Eve at May 14, 2026, 07:51 PM

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Malcolm

16 days ago

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

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