Commercial Building Inspector B2Building EnclosureMedium
During a final inspection of a commercial building, an inspector observes that the exterior wall assembly incorporates a vapor retarder. The building is located in Climate Zone 5A. According to IBC Section 1404.3, where should the Class II vapor retarder typically be installed within the wall assembly to be effective?
- AOn the interior side of the cavity insulation.
- BBetween the exterior cladding and the weather-resistive barrier.
- CWithin the drainage plane, exterior to the sheathing.
- DOn the exterior side of the continuous insulation.
Show answer & explanationAnswer & explanation
Correct answer: A. On the interior side of the cavity insulation.
For Climate Zone 5A (mixed-humid), the IBC generally specifies that a Class I or Class II vapor retarder should be placed on the interior side of the thermal insulation. This placement prevents moisture-laden interior air from reaching colder surfaces within the wall where it could condense.
Why the other options are wrong
- B. This location would be too far to the exterior and would likely trap moisture within the wall cavity, especially in colder months.
- C. The drainage plane handles bulk water, not vapor diffusion. A vapor retarder is distinct from the drainage plane.
- D. Placing a vapor retarder on the exterior side of insulation in a cold/mixed climate can trap moisture inside the wall cavity.
Vapor Retarder Placement (Climate Zone 5A)
The correct location of a Class I or II vapor retarder in a wall assembly for preventing moisture condensation in mixed-humid climates.
- Climate Zone 5A is a mixed-humid climate.
- Vapor retarder prevents moisture diffusion.
- Typically placed on the interior side of insulation in 5A.
Memory trick: Vapor goes where it’s warmest, retarder stops it there.