Air barriers, vapor control and condensation · Lesson 4 of 4

Air barriers, vapor control and condensation: defects, verification and case study

Six common defects and how each is recognized, the evidence a specifier checks before accepting a design, where the sheet metal sits among the layers, and a constructed case study of an indoor pool.

You will be able to: Diagnose a plausible condensation defect, explain the corrective options the design team can weigh, and assemble the evidence a reviewer needs: product data, code check, calculation record and continuity checkpoints.

13 min · 10 slides · 11 questions

All lessons in this module

Slide 1 of 10

Defect: a gap in the air control layer

Building Science Digest 104: in cold climates an interior air barrier controls the exfiltration of indoor air, often moisture-laden.

In Building Science Insight 049’s photograph the frost has accumulated on the backside of the sheathing.

It is not in the middle of the cavity insulation. The photograph shows where the frost collected; it is not tied to a particular gap.

Sources

Building Science Digest 104: Understanding Air Barriers (2006) - Building Science Corporation (Joseph Lstiburek)
Building Science Insight 049: Confusion About Diffusion (May 2011 rev.) - Building Science Corporation (Joseph W. Lstiburek)

Glossary

Exfiltration: Indoor air leaking outward through the enclosure. In a cold climate it can carry humid air to cold sheathing; an interior air barrier controls it.
Reservoir cladding: A cladding, such as brick veneer, that can absorb water in a rainstorm and release it later, complicating wall design.
Double vapor barrier: Vapor barriers on both sides of an assembly, which leave no drying path. Building Science Digest 106 recommends avoiding it.
Calculation record: The written record of a condensation screen: indoor temperature and humidity with their basis, the dew point, the surface temperature, the margin and the tool used.
Approved design: In the code text this module reads, a design using accepted engineering practice for hygrothermal analysis, permitted as an alternative to the vapor retarder tables.