Explain Vapor Retarder

Vapor control is the part of an assembly that gets decided by habit more than any other, usually as "the plastic goes on the warm side". This explains what the layer is actually doing, in the assembly you have, so the decision can be made rather than repeated.

Inputs the command needs

1. The question, or the assembly. Either works. With an assembly, the explanation is specific; without one, it is general and says so.
2. Climate zone, and its source, if you have it.
3. Code text, quoted, if a verdict is wanted rather than an explanation.

Workflow

Step 1 — Establish what is being asked

Separate the three questions that arrive as one:

• What does a vapor retarder do? (mechanism)
• Where should it go in this assembly? (position)
• Which one is allowed here? (code)

Answer them in that order and say plainly that the third is not answered from memory.

Step 2 — Explain the mechanism, and separate it from air

Diffusion: water vapor moving through a material because there is more of it on one side. Slow, steady, and driven by the vapor pressure difference.

Air leakage: air carrying water through a hole. Fast, and usually responsible for far more moisture than diffusion.

State the separation explicitly. A perfect vapor retarder with a gap around it is an air leakage path, and the gap wins.

Step 3 — Explain the class system as a system

Explain that retarders are graded by how much vapor they let through, and that the grades run from nearly closed to fairly open. Explain what "open" buys you: the ability to dry in that direction.

Do not state the numeric boundaries between classes and do not assign a material to a class. Say that the boundaries are published in the standard the user's code references and that products state their own class and permeance.

Step 4 — Put it in the assembly, if one was given

Walk the layers named and say:

• Which layer is the lowest-perm one, and where it sits relative to the insulation.
• Which direction the assembly can therefore dry.
• Whether any layer other than the intended retarder is acting as one. A foil facing, a membrane, an impermeable exterior cladding: these often do the job by accident.
• Whether there are two low-perm layers with insulation between them, which leaves the assembly no direction to dry in.

State these as observations. Whether the arrangement is acceptable depends on arithmetic and code, neither of which is here.

Step 5 — Raise the code question without answering it

If a climate zone was given, say what has to be checked: which class is permitted for that zone and that assembly type, and whether continuous exterior insulation changes the answer. Name the check; do not perform it from memory.

If code text was pasted, hold the assembly against the quoted text and say which part of the text applies.

Step 6 — Close

Every run ends with:

• Air leakage is not diffusion, and it moves more water.
• The arithmetic has not been done here. condensation-check does it.
• The class permitted in your zone has not been recalled.

Rules

• Never state a permeance figure or a numeric class boundary.
• Never assign a product or material to a class.
• Never recall what a climate zone requires.
• Answer mechanism, then position, then code, in that order.
• Separate diffusion from air leakage in every run.
• Quoting a code clause found by web search is not quoting the code. Search identifies the document; it does not verify the text.

Anti-patterns

• "The vapor barrier goes on the warm side." That is one climate's rule of thumb stated as physics.
• Naming a class for a common material because it is well known.
• Explaining vapor control without mentioning air leakage.
• Giving a verdict on an assembly when no code text was supplied.
• Treating a low-perm exterior cladding as irrelevant because it is not called a vapor retarder.
• Running condensation-check from inside this command. It is a pointer.