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Should I omit the vapor barrier?

1110d's picture

Have a residence under construction.  The home is being built by myself and as such was originally permitted 7 years ago.  Although codes have evolved, this structure is permitted under a now obsolete building code.  The home (more of a cabin) also does not have a heat source so the use of insulation and vapor barrier are in excess of what is required by code.  The project has now advanced to the point of drywall and I'm debating if the vapor barrier should be ommitted. 

Project criteria:

  • Climate zone 7, marine.
  • The home is 3 seasons and is permitted with no heating/cooling system.
  • The home is off grid.
  • Wall assembly: Cement board siding, 3/4" air gap, Tyvek air barrier, 1/2" OSB, 2x4 studs @ 16" o/c with R-11 cavity batt

This is a primary heating climate and the use of a 6 mil vapor barrier on the inside face is standard construction.  It's not feasible to add a vapor barrier after construction (other than paint) so I'm trying to anticipate a future use.  This home will always be a 3 seasons and I don't see it being feasible to convert the home to a 4 season.  I see a wood stove or possibly a furnace as being realistic modifications made in the future.  That said, I don't see the systems being used beyond tempering a cool morning.  This would mean the vapor drive from the inside will be minimal, if present at all.

This got me thinking the primary vapor drive would actually be from the exterior.  This is a marine enviroment and as such tends to be rather cool and damp.  If an air conditioning system was added, I could see the vapor drive being reversed where the vapor is coming from the exterior to the interior.  This would mean the vapor barrier should be on the exterior where the air barrier is located.  Placing a vapor barrier on the inside face could result in moisture being trapped in the stud cavity.

I was thinking this might be a good use for a smart vapor barrier, but they are quite expensive.  The most common product is also made by a company that I've learned to not trust.  The lack of trust is the source of my hesitation over the product cost.  The only alternative product is available from only a single source and is even more expensive.

This got me to the point that maybe I'm over thinking the whole vapor barrier issue and perhaps the correct solution is to just omit it.  Again, a vapor barrier is not required by code because the home is being built without a energy source.  Omitting the vapor barrier would would present some distinct advantages in that it would permit the vapor drive to reverse.  The few times the home does see a vapor drive the wall cavity would be capable of drying to both faces.  I see the periods of vapor drive being brief with periods of system shut down between.  For example, you come up for a week vacation and run the air conditioning all week.  The vapor would drive from the exterior in, however this drive would be tempered by the home being vacant for the next month.  Omitting the vapor barrier would now allow the wall to dry in both directions so the small amount of acumulated moisture would disippate pretty quickly.

I've pretty much convinced myself that omitting the vapor barrier is the correct solution, but I'd like to solicite the opinion of others as well.  What are your thoughts on this unique installation?