Ceiling vapor barrier debate

"Should you put a vapor barrier in an insulated ceiling or not? I'm in a cold climate location, where many longtime builders swear that you shouldn't put a ceiling vapor barrier in. The reasons go something like "Because you have to let the moisture escape" or "Because the house has to breathe out the top." What do the experts say?

To hell with the experts, this is my answer.

Plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days. Vapor retarders (kraft faced insulation or latex ceiling paint) should be used in all other climates except hot humid or hot dry climates. In hot humid climates, attics should not be vented and vapor retarders should not be installed on the interior of assemblies. In hot dry climates a vapor retarder should also not be installed, but attics can be vented.

All vented and unvented attics should have an air barrier regardless of climate.

Omitting a ceiling vapor barrier by arguing that you have to let the moisture escape or because the house has to breathe out the top is actually, sorta, kinda, correct. It is also sorta, kinda, incorrect. Oh boy what an answer. Now, I am a real fan (ha, ha) of controlled mechanical ventilation to control and limit interior moisture levels in severe cold, cold and mixed climates, as well as to limit other interior contaminants in all climates. In other words, all houses require controlled mechanical ventilation in order to "breathe". It is also my view that this necessary air change should not happen via the construction of a leaky attic ceiling (or leaky walls) and the installation of attic vents. Hence the requirement for an air barrier and controlled mechanical ventilation in all houses regardless of climate.

Having said that, I do not have a problem with relieving some of the moisture load in the house via diffusion (hence a vapor retarder in a ceiling rather than a vapor barrier) through a roof assembly that can handle it (i.e. a vented attic in a moderately cold climate or mixed climate). Understand that this is a climate specific recommendation. In a well insulated attic in a severe cold climate (more than 8,000 heating degree days) there is not enough heat loss into an attic from the house to allow attic ventilation to remove much moisture. Attic ventilation requires heat loss to remove moisture from attics. Cold air can't hold much moisture. Ventilating a heavily insulated attic with outside air when it is real cold does not remove moisture. We do not want any moisture in an attic in a severe cold climate for this reason. This changes in locations where it is not so miserably cold. Hence, the recommendation for a vapor barrier in severe cold climate and only a vapor retarder in most other locations.

In the old days, in severe cold climates, when attics were poorly insulated it was okay to omit a ceiling plastic vapor barrier. The heat loss from the house warmed the attic sufficiently to allow attic ventilation to remove moisture from the attic. Cold outside air was brought into the attic and warmed up by the escaping heat loss giving this air the capacity to pick up moisture. Moisture in the attic was then picked up and vented to the exterior. The approach worked great until we added large quantities of attic insulation. With the added insulation the attic stayed cold and the ventilating outside air stayed cold unable to effectively remove attic moisture. Hence the need to reduce moisture flow into the attic and the need for a vapor barrier.

One other complication needs to be stated. Vapor moves two ways, by diffusion through materials, and by air leakage through gaps and holes in building assemblies. Between the two, air leakage moves far more moisture than vapor diffusion. A vapor barrier in an attic assembly in a severe cold climate in the absence of an air barrier will likely be ineffective. On the other hand, an air barrier in the absence of a vapor barrier can be effective. We complicate things when we install plastic in ceilings and assume it is also an air barrier. For plastic to be an air barrier it needs to be continuous, like a condom - nuff said.