Vapor Barrier
CSI Division 07, Thermal & Moisture Protection
A material, typically polyethylene sheeting or a specially rated coating, installed within a wall, floor, or roof assembly specifically to block water vapor from migrating through it and condensing inside the assembly, where it can't dry out and causes hidden moisture damage over time.
Why it matters
Where a vapor barrier goes in an assembly (interior side vs. exterior side) depends entirely on climate: installing it on the wrong side for a given climate zone can trap moisture inside the wall instead of keeping it out, which is a more damaging mistake than installing no vapor barrier at all.
On a real project
In a cold climate, a vapor barrier is installed on the warm, interior side of exterior wall insulation, stopping interior humidity from migrating outward into the wall cavity where it would condense against the colder outer sheathing.
Who this matters most to
A Carpenter installs the vapor barrier as part of the wall assembly, typically right before drywall closes up the wall. A Quality Control Manager checks that the vapor barrier is installed on the correct side of the assembly for the project's climate zone, since the same detail that's correct in one region is a defect in another.
Where this goes wrong
A crew builds a wall assembly using the same vapor barrier placement they always use, without checking that this particular project is in a different climate zone than their usual work. Moisture gets trapped on the wrong side of the insulation, and the wall cavity develops hidden condensation and eventually mold, a problem invisible until it's serious enough to show through the finished wall.