Vapor Barrier placement.


Where does the vapor barrier go on an exterior framed wall assembly?

When you frame wall assemblies in a basement, and this assumes the basement is more underground than not, you install a vapor barrier on the interior side of the framed wall assembly.

Not that it (the vapor barrier) would go anywhere else in a cold region but it could be a case that you would not need one, but that would be determined by the actual wall assemblies that exist, the materials they are made of, and how cold exactly is the climate you live in and for how many days a year it remains cold at what temperature.

But, to be fair and in short, you have to address these issues from the exterior first because what you are going to do in the interior side is basically capture the ability of condensation to move, and take it out of the environment.

In cold regions, the vapor barrier goes on the interior side of the living or heated environment directly attached to the studs. This is not up for negotiation, it is based on the principles of moisture diffusion, while very complicated for those that continue to study and up-date the phenomenon, there are standard considerations.

Building Science Digest: RR-0410 Vapor Barriers Wall Design
“Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior and different climates require different approaches.”

RR-0412: Insulations, Sheathings and Vapor Retarders
“Two seemingly innocuous requirements for building enclosure assemblies bedevil builders and designers almost endlessly: keep water vapor out, let the water vapor out if it gets in. It gets complicated because, sometimes, the best strategies to keep water vapor out also trap water vapor in.”

Short answer is that in a climate with 8000 heating degree days (8000 / 72 degrees=109 days of cold weather) the vapor barrier is placed on the interior side of the room…no discussion. At 4000 heating degree days and depending on the materials that the wall was built from a vapor barrier might not, and mostly will not be used (this would be close to where I live in Grand Bay, Alabama).

As an aside, based on the perm rating of the materials I wouldn’t think that my home that is only 27ish years old (as of this writing) would have required a vapor barrier. But it does, and it is on the interior side of the framing.

We have about 3 or 4 weeks of cold weather, and this cold here isn’t below freezing cold like people experience in Northern climates. So why does it work? It works because the HVAC helps in the process. The ability of the wall assembly to dry from interior to exterior or from exterior to the interior would have been made possible by the number of heated days (by the Sun) or the HVAC. So while they got it wrong, they still got it right.

Now if we were in a tropical environment, the placement of a vapor barrier would be on the exterior of the framed assembly, often the vapor barrier will come in the form of the actual finish, like say Stucco.

But I guarantee you if you placed your vapor barrier on the exterior side of the wall assembly in a cold region, the vapor barrier would do what it is supposed to do, but the problem would be that the structure would suffer and rot, mold and decay would be the by product.

Also, if the vapor barrier is placed in the middle of a double wall assembly you are asking for trouble as well. I have to go back to my soft drink can analogy to perform this next magic trick.

Kid gets an aluminum can soft drink out of the refrigerator. Within seconds of being outside of the cold environment, the can starts to sweat. Now you think this sweat is part of the can don’t you? It isn’t, it is condensation in the air, the warm air moving towards the colder outside of the can.

Now take that analogy and place it on the inside of the middle of a wall assembly full of insulation and what happens? The gas that creates condensation when moving from the warm side to the cold side will “hit” the solid plastic sheeting, and stick to it.

When the house starts trying to dry out, the moisture will migrate back into the interior framed wall, but before it makes it inside it has to go through the insulation on this interior wall. It will diffuse on the insulation, wet the insulation and become a big problem and you will not know about it for months and months.

BSD-106: Understanding Vapor Barriers
“The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate”