The insulation contractor laid batts between the ceiling joists and then rolled a continuous polyethylene sheet over the top of the insulation, on the attic side of the assembly. How should the inspector treat this installation?
- AApprove it, because a vapor retarder performs the same on either face of the insulation as long as the sheet is continuous and well sealed at its laps.
Position is the whole point of a retarder. On the attic side it lets vapor pass up through the insulation and condense on the underside of the cold sheet, wetting the insulation from above.
- BApprove it, because the sheet also keeps the loose insulation from being disturbed by air moving through the attic space, which improves the assembly overall.
Holding insulation in place is not a code function, and these batts sit between joists already. The placement of the sheet is what fails, and a benefit elsewhere does not cure a retarder on the cold side.
- Reject it, because a required vapor retarder belongs on the side of the assembly that is warm in winter, meaning under the insulation against the ceiling.
- DReject it, because a vapor retarder is prohibited in a vented ceiling assembly in every climate zone the code recognizes, so the sheet has to come out.
Ceiling vapor retarders are required in the colder zones rather than prohibited. This reaches the right disposition through a rule that does not exist, and it would leave the assembly with no retarder at all.
Why C is correct
Where the code requires a vapor retarder in an assembly, it goes on the side that is warm during the heating season, which for a ceiling in a cold climate is the interior face beneath the insulation. Installed on the attic side, the sheet becomes the coldest surface in the assembly and vapor condenses against it. The correction is to remove the sheet or to rebuild the assembly with the retarder against the ceiling.
What this question is testing
Whether the candidate can apply the underlying principle of vapor retarder placement rather than reciting a table, and understands why the wrong side is worse than none at all. It also tests recognizing that the requirement is climate-zone driven rather than universal.
On the job
Poly over the top of attic insulation is a classic do-it-yourself and small-remodel mistake, often done with the honest belief that it is an air barrier. You will also see it in an attic where someone laid plastic before decking part of the floor for storage. Since climate zone drives what is required, check the zone the jurisdiction sits in and the plans before writing the correction, and be prepared to explain the physics, because the argument on site is always that plastic is plastic.
Memory technique
The retarder faces the people, not the weather.
Exam tip
Warm in winter is the phrase to hold onto; in a heating climate that is the room side of the insulation.
Where to look it up
IRC Chapter 7 for vapor retarder classes and the climate zones where each is required, together with the energy provisions for ceilings. Check the zone your jurisdiction has adopted.