Ceiling Vapor Barriers in Post Frame Construction

A ceiling vapor barrier or no, in post frame (pole building) construction? Good question.

For most of my life I have lived where it tends to get chilly in winter. Here in Northeast South Dakota it can not only be chilly, but downright frigid. It has been drummed into my head for decades to never have a vapor barrier between a conditioned area (think heated) and an attic space above.

Well, there are some instances where, when considering proper modern construction practices, this will not be a best answer. Because I learn new stuff every day – this may contradict some prior advice I have given.

Credit to reader STEVE in DEXTER who piqued my interest when he wrote:

“I am adding a tongue and groove cedar plank ceiling to a heated room in my pole barn. I am attaching the planks directly to the bottom of trusses. Should I have a vapor barrier between the planks and the blown in insulation. I am concerned with air/dust /dirt leakage between the planks. I am considering putting up Poly or tyvek.”

Mike the Pole Barn Guru responds:

Plastic vapor barriers (Visqueen) should only be installed in vented attics in climates with more than 8,000 heating degree days. You can calculate your own heating degree days for free here: https://portfoliomanager.energystar.gov/pm/degreeDaysCalculator.  Vapor retarders (kraft faced insulation, or gypsum wallboard with 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 upon 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 (such as MemBrain™ or Intello® Plus) regardless of climate.

Controlled mechanical ventilation should be used 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 conditioned post frame buildings require controlled mechanical ventilation in order to “breathe”. This necessary air change should not happen via construction of a leaky attic ceiling (or leaky walls) and installation of attic vents. Hence requirement for an air barrier and controlled mechanical ventilation in all conditioned post frame buildings regardless of climate.

Some moisture load in conditioned post frame buildings can be relieved via diffusion (hence a vapor retarder in a ceiling rather than a vapor barrier) through a roof assembly able to handle it (example – a vented attic in a moderately cold climate or mixed climate). Understand these are climate specific recommendations. In a well-insulated attic in a severe cold climate (more than 8,000 heating degree days) not enough heat loss occurs into an attic from conditioned post frame building 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 in real cold weather does not remove moisture. We do not want any moisture in an attic in a severe cold climate for this reason. In not so miserably cold locations this changes, leading to a recommendation for a vapor barrier in severe cold climate and only a vapor retarder in most other locations.

In olden days, in severe cold climates, when attics were poorly insulated, it was okay to omit a ceiling plastic vapor barrier. Heat loss from building warmed the attic sufficiently to allow attic ventilation to remove moisture from the attic. Cold outside air was brought into attic and warmed up by escaping heat loss giving this air capacity to pick up moisture. Attic moisture was then picked up and vented to exterior. This approach worked great until large quantities of attic insulation were added. With added insulation, attic and ventilating outside air both stayed cold unable to effectively remove attic moisture. Hence a need to reduce moisture flow into the attic and add 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 these two, air leakage moves far more moisture than vapor diffusion. A vapor barrier in an attic assembly in a severe cold climate in absence of an air barrier will likely be ineffective. Conversely, an air barrier in absence of a vapor barrier can be effective. We complicate things when we install plastic in ceilings and assume it provides an air barrier. For plastic to be an air barrier it needs to be continuous.