“It’s good to omit vapor retarders in ceilings, to provide a way for moisture to leave the building.”
Some cold-climate builders believe that, while vapor retarders are useful on walls, they should never be installed on ceilings “because you have to let the ceiling breathe, so that moisture can get out of the house.” This interesting misconception contains several wrong-headed notions wrapped up in a single idea.
Most attics include ventilation. In theory (although not always in practice), attic ventilation can help remove high levels of humidity that might otherwise condense on the cold roof sheathing. However, attic moisture problems usually indicate the existence of two flaws: a wet basement or crawlspace, and a ceiling with air leaks.
Ceilings were never intended to be “moisture-relief valves” for homes. Ideally, a ceiling should be as airtight as possible, to keep warm, humid indoor air from reaching the attic. In cold climates, the ceiling should include a vapor retarder (for example, kraft facing or vapor-retarder paint) on the warm-in-winter side, to limit vapor diffusion through the ceiling.
High indoor humidity during the winter — usually indicated by condensation on windows — is rare in most homes. When it occurs, the solution is to increase the rate of mechanical ventilation. If the home lacks a whole-house ventilation system, a simple remedy for dripping windows is to leave bath exhaust fans on for 24 hours a day until the moisture problems go away.