Field & trade terms
Vapor barrier
Also called: vapor retarder
A vapor barrier — more precisely a vapor retarder — is a film or coating that slows the movement of water vapor through a wall, floor or ceiling assembly. Warm humid air always tries to migrate toward the cold side, and if it reaches a surface below its dew point inside the assembly it condenses, wetting insulation and framing. The retarder is placed to keep that vapor from reaching the cold surface, and which side it goes on depends on the climate, so its position is as important as its presence.
Why it matters
Put a vapor retarder on the wrong side of an assembly and it traps moisture instead of stopping it, quietly rotting framing and killing the R-value of the insulation behind it. It's also easy to compromise — a torn sheet or a seam left open lets vapor bypass the whole plane.
What an inspector looks at
The check is that the retarder is the specified material, on the correct side for the climate, continuous and lapped, and cut in cleanly around boxes and penetrations rather than left gaping. It's confirmed against the insulation and air-sealing details it works with, not on its own.
RTI is photo-gated, so the vapor retarder is shot in place before insulation and drywall bury it — seams, laps and the treatment at penetrations shown while they can still be corrected.
Standards it's verified against
Checklists that put vapor barrier to work
Free, field-ready checklists, each with the photo every item needs before it can be submitted.
Related terms
- Air barrierA continuous plane of materials that stops air from leaking through the building enclosure, sealed at every joint, seam and penetration.
- FlashingThin sheet material lapped into an assembly to direct water back out at joints, edges and openings before it can get behind the finish.
- Dry-inThe point at which a building is closed to weather — roof, wall barrier and openings sealed enough to keep water out and let interior work start.
- MEP (mechanical, electrical, plumbing)The mechanical, electrical and plumbing trades — the systems run through a building's structure that get inspected at rough-in and again at final.
- FirestoppingSealing openings where pipe, conduit or cable pass through a fire-rated wall or floor so the assembly's rating is restored.
- Backflow preventionA device or assembly that keeps used or contaminated water from reversing back into the potable supply through a cross-connection.
Knowing the term is step one. Proving it is the job.
RTI holds the submit button until every mandatory item is answered and the photos are attached, then chains the record so it can be proven untouched later.