Air barriers, vapor control and condensation · Lesson 3 of 4

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.

14 min · 11 slides · 12 questions

All lessons in this module

Slide 1 of 11

The method in nine steps

The method is a steady-state screen: it compares the temperature of the first condensing surface with the indoor dew point.

The designer of record owns each step; the diagram shows the order and where a hygrothermal analysis takes over.

The inputs below are examples for teaching, not project data.

1: Record the jurisdiction, adopted code editions and climate zone
2: Set the indoor temperature and humidity, with their basis
3: Work out the indoor dew point (the frost point if it is below freezing)
4: Set the outdoor design temperature, the average of the coldest three months
5: Find the first condensing surface and its temperature by the R ratio
6: Compare with the dew point and record the margin
7: If at or below: choose controls (warm the surface, keep air away, a suitable retarder, lower humidity)
8: Check the summer and inward case
9: Escalate special-use or inward-drive cases to a hygrothermal analysis and write the packet

One workable order of work built from the relation in Building Science Digest 163 and the limits stated by the AFS Envelope tool. The designer of record owns each step. Illustrative; the designer selects and details these layers.

Sources

Condensation Control in Walls - Cold Weather (last updated 2021-09-26) - US DOE Building America Solution Center (PNNL)
Building Science Digest 163: Controlling Cold-Weather Condensation Using Insulation (effective November 15, 2011) - Building Science Corporation (John Straube)
Design Center: Wall and roof assembly R-value, U-factor and dew point (steady-state screening tool) - Architectural Flashing Supply

Videos for this lesson

What this video shows: A long (about 90 minute) Canadian session for architects on three ways to control condensation, optional further viewing. Its stated thesis is that risk can be judged without a dew-point calculation or hygrothermal model, so it argues against the dew-point screen taught in Lesson 3 and does not teach that screen. Verified from the video’s own description.

Video: CE4: Architects: Stop Worrying About Dew Point! A Conversation with Christine Williamson by Ontario Association of Architects, on YouTube. Chosen from the video's published description.

Glossary

Steady-state screen: A calculation that compares steady temperatures through the layers of a wall with the indoor dew point. It ignores vapor permeance, moisture storage, rain, solar gain and time.
Exterior design temperature: The outdoor temperature used in a condensation screen. Building Science Digest 163 treats the average of the coldest three months as reasonably safe.
Continuous insulation: Insulation on the exterior of the framing and sheathing that raises the temperature of the sheathing. Exterior insulation decreases condensation risk in a cold-climate wall.
Margin: The difference in degrees between the temperature of the first condensing surface and the indoor dew point. A margin at or below zero is flagged for the designer.
Inward vapor drive: Vapor moving from the outside toward the inside of a wall, typically in a warm, humid climate. The Envelope tool does not assess it.