Air barriers, vapor control and condensation
Why moisture condenses in a wall, how air leakage and diffusion carry it there, vapor retarder classes and air leakage limits in the codes read here, a worked dew-point screen, the defects it catches and the evidence a reviewer needs.
4 lessons · about 56 min · 12 diagrams · 2 videos
Version 1.0.0
Builds on these modules
Lessons
0 of 4 complete
Lesson 1 · 13 min · 9 slides · 11 questions
Air barriers, vapor control and condensation: principles and terminology
What condensation needs, how moisture moves through a wall by air and by diffusion, why leaks usually matter more in cold weather, the special cases where they do not, and who is responsible for each decision.
You will be able to: Define dew point, frost point, vapor drive, diffusion and air transport, say why air leakage usually causes cold-weather condensation, separate an air barrier from a vapor retarder, and state who owns each decision.
Not startedStart →Lesson 2 · 16 min · 12 slides · 14 questions
Air barriers, vapor control and condensation: materials, standards and design constraints
What a perm measures, the three vapor retarder classes and the test method behind them, what the 2021 residential and building code text asks for, the air leakage limits in the 2021 IECC, and which kind of analysis fits which question.
You will be able to: Read a perm rating and its test method, place a material in a vapor retarder class, read the vapor retarder and air leakage provisions of the codes this module reads, and choose between a dew-point screen, a code table and a hygrothermal analysis.
Not startedStart →Lesson 3 · 14 min · 11 slides · 12 questions
Air barriers, vapor control and condensation: methods and worked application
A nine-step screen from design conditions to a job packet, worked on two example walls with every input stated, how much exterior insulation keeps the sheathing warm, the summer and inward case, and when to escalate.
You will be able to: Apply a documented first-condensing-surface screen to a wall: record the inputs and their sources, compute the indoor dew point and the sheathing temperature, record the margin, choose among the control options, and say what the screen cannot show.
Not startedStart →Lesson 4 · 13 min · 10 slides · 11 questions
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.
Not startedStart →
Practical task
Produce a documented condensation-risk job packet for a constructed cold-climate office wall: record the design conditions and their basis, compute the indoor dew point and the sheathing temperature with and without exterior insulation, mark up a section, and list the defects a reviewer should check.
Hand in
- An assumptions table: jurisdiction, adopted code editions and climate zone (stated as unknown if not given), indoor temperature and humidity with their basis, outdoor design temperature with its basis, and every layer R-value with its source.
- The indoor dew point worked out with the Magnus form (for 70 °F air at 35% RH, 41.1 °F) and the temperature of the back of the sheathing by the R-ratio relation, for the wall as drawn and for one alternative.
- A marked-up wall section showing the air control layer, the vapor retarder class and its test method, the first condensing surface and the dew-point line, with the margin in degrees.
- A one-page defect analysis: which of the six defects in the lesson could apply, and the corrective options the design team could weigh.
- A list of open questions for the engineer, including the summer and inward case and whether a hygrothermal analysis is needed.
Rubric
| Criterion | Meets the standard when |
|---|---|
| Inputs and units | Every input has a unit and a source, example values are marked as examples, and the jurisdiction and edition are recorded or marked unknown. |
| Dew point and surface temperature | The dew point and the sheathing temperature are computed correctly, the margin is stated in degrees, and a frost-point note is made if the indoor dew point is below freezing. |
| Layers and classes | The air control layer and vapor retarder are shown as separate layers, any perm value is given with its ASTM E96 method, and no metal flashing is labelled an air barrier or vapor retarder. |
| Limits and escalation | The packet says the screen is steady-state, names what it leaves out, and states whether the summer case or a special use calls for a hygrothermal analysis by the designer. |
| Defects and ownership | At least one defect is diagnosed with a plausible corrective option, and the decisions are assigned to the design team rather than to AFS. |